Energy balance, greenhouse gas emissions, and profitability of thermobarical pretreatment of cattle waste in anaerobic digestion Academic research paper on "Chemical engineering"

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Waste Management

journal homepage: www.elsevier.com/locate/wasman

Energy balance, greenhouse gas emissions, and profitability of ■. Cros,

thermobarical pretreatment of cattle waste in anaerobic digestion

Jörn Budde3'*, Annette Prochnowa,b, Matthias Plöchla, Teresa Suärez Quinones3, Monika Heiermann3

aLeibniz-Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany b Humboldt-University of Berlin, Faculty of Life Sciences, Invalidenstraße 42, 10115 Berlin, Germany

article info

abstract

Article history: Received 8 July 2015 Revised 1 December 2015 Accepted 5 December 2015 Available online 18 December 2015

Keywords: Assessment

Lignocellulosic feedstock Maize

Methane yield GHG mitigation LCA

In this study modeled full scale application of thermobarical hydrolysis of less degradable feedstock for biomethanation was assessed in terms of energy balance, greenhouse gas emissions, and economy. Data were provided whether the substitution of maize silage as feedstock for biogas production by pretreated cattle wastes is beneficial in full-scale application or not.

A model device for thermobarical treatment has been suggested for and theoretically integrated in a biogas plant. The assessment considered the replacement of maize silage as feedstock with liquid and/ or solid cattle waste (feces, litter, and feed residues from animal husbandry of high-performance dairy cattle, dry cows, and heifers). The integration of thermobarical pretreatment is beneficial for raw material with high contents of organic dry matter and ligno-cellulose: Solid cattle waste revealed very short payback times, e.g. 9 months for energy, 3 months for greenhouse gases, and 3 years 3 months for economic amortization, whereas, in contrast, liquid cattle waste did not perform positive replacement effects in this analysis.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license

(http://creativecommons.org/licenses/by/4XI/).

1. Introduction

Anaerobic digestion is a mature and proven technology providing a versatile renewable energy carrier (Browne and Murphy, 2013). The growing number of biogas installations and the rising

Abbreviations: AHO, annual hours of operation; ADO, annual days of operation; P1, biogas plant 1; P2, biogas plant 2; CO2-eq., carbon dioxide equivalent; CMC, carbon dioxide mitigation costs; CHP, combined heat and power plant; constr, construction; C, costs; m3, cubic meter; d, days; "C, degree celsius; q, density; dig, digestate; $, dollar; DM, dry matter; ECAM, Economic amortization mass; ECAP, Economic amortization period; g, efficiency; el, electric; E, Energy; EPBM, Energy payback mass; EPBT, Energy payback time; €, Euro; ct, Euro-cent; FS, feedstock; FM, fresh matter; g, grams; GHG, Greenhouse gas; GPBM, greenhouse gas payback mass; GPBT, greenhouse gas payback time; h, hours; in, input; K, Kelvin; kg, kilograms; kJ, kilojoules; km, kilometers; kW, kilowatts; kW h, kilowatt hours; LCA, life-cycle assessment; LCM, liquid cattle manure; l, liters; LHV, lower heating value; MT, maintenance; MS, maize silage; m, mass; Mg, megagrams; PMC, methanation capacity; lN, norm liters; OM, organic matter; out, output; %, percent; P[n]Y[n]M, period in years and months; P, power; s, seconds; Tset-point, set-point temperature; SCM, solid cattle manure; SLCM, solid and liquid cattle manure; cp, specific heat capacity; spr, spreading; T, temperature; th, thermal; TBH, thermobarical hydrolysis; V, volume; WL, wheel loader; a, years; Y, yield.

* Corresponding author. Tel.: +49 331 5699 315. fax: +49 331 5699 849.

E-mail address: jbudde@atb-potsdam.de (1. Budde).

demand for higher methane output have led to an increasing request for effective feedstock and hence for a shift from liquid animal waste to solid energy crops and residue from agricultural production (Plöchl et al., 2009). Therefore, the biogas sector is not only encouraged to deploy new and untapped biomass resources (Zhang et al., 2013), but also to use advanced and innovative technologies to improve biogas production and process efficiency, as well as to increase cost effectiveness (Appels et al., 2008; Ward et al., 2008).

Solid manure from livestock represents a huge still unexploited potential as feedstock for conversion processes. More than 30 million tons of solid animal manure accrued in Germany in 2009 (Schultheiß et al., 2010). However, it contains considerable amounts of straw, which is almost indigestible under usual conditions of anaerobic digestion (Grabber, 2005). Furthermore, lignocellulosic feedstock tends to float and agglomerate to an almost irreversible floating layer in the digester. Therefore, the complex structure of straw requires proper pretreatment enabling efficient fragmentation of the less digestible material.

Previous studies comparing different methods such as mechanical, thermal, chemical and/or biochemical pretreatment have identified thermobarical pretreatment (also called liquid hot water or thermal pressure treatment) as an appropriate method for breaking down fibrous, high-molecular substances into their

http://dx.doi.org/10.1016/j.wasman.2015.12.003 0956-053X/© 2015 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

constituents, thus making them available for a more rapid and extensive anaerobic digestion resulting in higher methane yields (Budde et al., 2008; Carlsson et al., 2012; Hendriks and Zeeman, 2009; Schumacher, 2008; Weiß and Brückner, 2008). Nevertheless, unsuitable pretreatment conditions can result in decreased methane yields due to formation of inhibitors and non-digestible substances. In principle, high temperatures and related saturated water vapor pressures (range 140-250 °C and 3.6-39.7 bar) are used to hydrolyze high-molecular substances (i.e. lignin, cellulose, hemicellulose) and thus anticipate the biological step. Budde et al. (2014) demonstrated in extensive lab-scale experiments that thermobarical treatment significantly enhances the methane yield of dairy cattle waste. Thermobarical treatment temperatures in these lab-scale experiments were 140-220 °C in 20 K steps for a 5-min duration. Methane yields could be increased by up to 58% at a treatment temperature of 180 °C. At 220 °C, the abundance of inhibitors and other non-digestible substances led to lower methane yields than those obtained from untreated material.

Recently, few studies have been published dealing with pre-treatment of biomasses and residues derived from agriculture for subsequent biomethanation in full-scale application (Cano et al., 2014; Elbeshbishy et al., 2011; Mönch-Tegeder et al., 2014; Shafiei et al., 2013). The authors investigated the impact of mechanical pretreatment using a cross flow grinder on horse manure, steam explosion on wheat straw and cow manure, and ultra-sonication on hog manure. Results were given in changes in energy or methane output in any case and often economic data are given whereas changes in on-farm procedures, e.g. waste handling, spreading procedures, or transportation of biomasses were disregarded.

Thermobarical pretreatment is to be considered as a variant with moderate electric and high thermal power consumption, hence appropriate for biogas plants with attached combined heat and power plant (CHP) as often available in agricultural context. Low to moderate installation and maintenance costs and no hazardous risk but a strong effect on biomethanation are further advantages of this pretreatment option. As no pumps or comminution are needed it is highly appropriate for common agricultural biomasses and residues.

The implementation of this technology should preferably take place in existing biogas plants (Menardo et al., 2011), mostly equipped with continuous stirred tank reactors as digester (Weiland, 2008). However, pretreatment of various feedstocks challenge sustainable production in terms of greenhouse gas (GHG) mitigation, efficient energy conversion, or profitability (European Commission, 2010). The GHG emissions of biogas production are mainly determined by the type of feedstock and its origin (Fritsche, 2007; Seeberg-Elverfeldt, 2010). Nothing yet is published about the energy balance, the GHG emissions or the profitability of thermobarical hydrolysis (TBH).

The objective of this paper is to assess the feasibility of TBH in a modeled full-scale application, considering balances of energy and GHG as well as profitability. A thermobarical hydrolysis device is therefore theoretically integrated in an already existing biogas plant. The exhaust gas from a combined heat and power plant (as part of the biogas plant) is used for provision of thermal energy. Feedstocks (solid and liquid cattle manures and mixtures of these from two origins) were pre-treated at conditions of 140-180 °C and associated saturated water vapor pressures (3.6-10.0 bar). The assessment considers the entire changes caused by that integration, e.g. production and operation of the device, changes in feedstocks and feedstocks composition, changed storage conditions, and changes in plant operation.

2. Material and methods

2.1. System boundaries, scenarios and functional unit

Full scale application of thermobarical pretreatment is assessed in terms of energy efficiency and greenhouse gas emissions as well as economic performance. The assessment of GHG emissions follows an attributional life cycle assessment (LCA) approach according to ISO 14044 (2006). The explicit assessment method used here is explained in detail in the following sections. Input data derived by other studies were either given directly in the text or presented in Table 1. The sources of the respective data are given for each value separately and values are introduced in their genuine units. All calculations were presented explicitly and applied to spread sheet analysis for computation. In the following, boundaries necessary to perform this assessment are set up for the system before and after implementation of a TBH.

The system before the implementation of TBH is shown in Fig. 1a (Budde, 2015). In this case maize silage is delivered to a plant site as biogas plant feedstock. The feeding to the biogas plant is accomplished by a wheel loader. Liquid cattle manure (LCM) from stable is stored for an intermediate period (several hours up to several days) and then pumped to the biogas plant. Solid cattle manure (SCM) from cattle breeding and digestate from biogas plant are stored for up to several months until they are spread on the field as fertilizer. The heat of combined heat and power generation from biogas is partly used for maintaining mesophilic conditions in the digester. Electricity, as the most valuable product, is sold. The plant presented here is exemplarily for the majority of biogas plants in Germany. Among 63 biogas plants analyzed all over Germany approximately 95% are using maize silage (FNR, 2010). LCM is used by 60% but only 22% are using SCM as well and only at a share of 2 mass-percent whereas LCM accounts for 24% (w/w). Approximately 90% of these plants are equipped with digesters for wet fermentation (continuous stirred tank reactors) and only 10% with digesters enabling utilization of SCM without pretreatment, so called dry fermentation. The majority of plants (33%) are using 250 kWel CHPs, 17% 300 kWel CHPs, and 25% 500 kWel CHPs. It is assumed that the feeding and spreading procedures used in this model are current practice in biogas plants of that size.

Fig. 1b displays the alteration of the system by a retrofit of a TBH device. The alterations are regarded in scenarios named after the types of cattle wastes (see Section 2.3) and are further distinguished by the treatment temperature(T = 140,160,180 °C)or lack of treatment respectively (Table 2):

SCM: Pretreatment of solid cattle manure.

LCM: Pretreatment of liquid cattle manure.

SLCM: Pretreatment of a mixture of solid and liquid cattle

manure.

The following process alterations due to changes in feedstock and TBH treatment are regarded:

- Changes resulting from substitution of maize silage.

- Changes in the feeding process (on-site process).

- Parameters of decomposition of the different feedstock for GHG balance.

- Changes in manure and digestate spreading procedure (off-site process).

Energy equivalent shares of maize silage are substituted by a fixed volume of one of the three feedstocks. Thus a unit volume of LCM, SCM or SLCM substitutes different gravimetric shares of

Table l

General input-data.

General data

Reference

Density of methane at 0 "C Lower heating value of methane Molar mass of methane Molar mass of carbon dioxide Specific heat capacity of water Electricity costs

Federal electricity-mix production costs Emissions from federal electricity-mix

Combined heat and power plant - nominal data

Nominal electric power

Nominal thermal power

Electric efficiency

Thermal efficiency

Combined heat and power plant - exhaust gas Mass flow

Specific heat capacity Input temperature Output temperature Thermal power

Thermobarical hydrolysis device -

Dosing feeder

Vessel

Stirring device Thermal-oil facility Associated heat exchangers

Thermobarical hydrolysis device Stainless steel provision Drawing of tubes Rolling of sheets

Thermobarical hydrolysis device Stainless steel provision Drawing of tubes Rolling of sheets

components masses

energy for construction

emissions from construction

0.72 kg m-3 50.01 MJ kg-1 16 g mol-1 44 g mol-1 4.20 kJ kg-1 K-1 11.00 ct kW h-1 5.80 ct kW h-1 0.611 kg CO2-eq. kW h-1

330 kW 400 kW 0.387 0.469

1878 kg h-1 1.125 kJ kg-1 K-1 500 °C 230 °C 158.46 kW

4500 kg 3500 kg 1000 kg 4000 kg 1000 kg

24.3 kW h kg-1 1.3 kW h kg-1 3.1 kW h kg-1

5.7123 kg CO2-eq. kg-1 0.39233 kg CO2-eq. kg-1 0.39272 kg CO2-eq. kg-1

VDI (1991)

Beitz and Grote (1997)

VDI (1991) Own assumption Stenull and Raab (2010) Vogt (2008)

GE Jenbacher GmbH and Co OHG (2006)

GE Jenbacher GmbH and Co OHG (2006) Unpublished engineering report

Unpublished engineering report

Ecoinvent (2007); cumulative energy demand

Ecoinvent (2007); CML 2001 GWP 20a

maize silage. LCM and SCM are pretreated by TBH and then fed to the biogas plant. LCM and SCM exceeding the daily load of the TBH device are not regarded in this analysis and remain in the usual process chain. The SCM is loaded by a wheel loader and LCM pumped through pipes to the TBH vessel. Thermobarical pretreatment is driven by heat from CHP.

Scenarios SCM and LCM considered feedstock from two different biogas plant sites (P1 and P2) and therefore differing feedstock compositions (Budde et al., 2014). Scenario SLCM was based on a mixture of solid and liquid cattle manure from biogas plant 1 only as feedstock. The utilization of untreated SCM was not regarded in the scenarios SCM and SLCM due to lesser digestibility and the risk of formation of floating layers.

Environmental assessment in this study, based on methodology described in ISO 14044 (ISO 14044, 2006), focuses on energy efficiency and GHG emissions and comprises

- feedstock supply,

- transport of feedstock on an agricultural biogas plant site,

- storage of waste from dairy cattle farming (solid and liquid cattle manure) and of digestate from biogas process,

- thermobarical pretreatment of solid and liquid cattle waste and its mixture,

- their conversion to electricity and heat via biomethanation and combined heat and power plant,

- manure spreading with or without pretreatment and biomethanation.

All parameters are related to (theoretical) electricity output, the main product of the overall process with the functional unit 1 kW hel as it allows the comparison with other bioenergy and

non-renewable energy systems and pretreatment options related to them (Cherubini and Stremman, 2011). The electricity output as functional unit is directly linked to the depletion of non-renewable energy resources.

The values related to time period are given as per year. An integration over the year is necessary as the amount in terms of fresh matter (FM) pretreated per year is a fixed value but the methana-tion capacity is influenced by the respective methane yield and by the organic matter (OM) content of feedstock.

Although the parameterization in per Mg implies that a size independent scaling would be possible - disregarding the fact that the amount of feedstock that can be treated in a given biogas plant is limited - values are also given in per Mg FM, thus allowing to compare this study with other studies giving mass related values only.

2.2. Principle construction and operational design of a full-scale thermobarical device and its retrofit to an existing biogas plant

The schematic flow chart of the thermobarical device and its integration into the biogas plant is shown in Fig. 2. The TBH device is linked to the CHP via a thermal-oil circuit. Regarding feeding of the biogas plant, it is interposed in the pipeline of liquid cattle waste transport from stable to biogas plant. If liquid waste is unavailable, a recirculation pipeline from/to digester is to be retrofitted.

The main components of this TBH device are a dosing feeder, a high temperature/high pressure vessel and a stirring device. The production of these components is assumed to be comparable to the rolling of stainless steel sheet. The thermal-oil facilities and the associated heat exchanger consisting mainly of stainless steel

digestate storage

System boundary

spreading

feedstock provision

(maize silage) production, transport, storage, processing

solid cattle manure storage

liquid cattle manure storage

plant feeding by wheel loader

biogas plant (biomethanation)

combined heat and

power plant (CHP)

dairy cattle husbandry

CAPTION

E: electric energy F: fuel

GHG: greenhouse gases H: thermal energy

- -energy flow

-material flow

•---system boundary

--|>GHG

)-{>GHG

digestate storage

System boundary

spreading

feedstock provision

(maize silage) production, transport, storage, processing

solid cattle manure storage

liquid cattle manure storage

plant feeding by wheel loader

GHG<--

thermobarical hydrolysis (TBH)

plant feeding by wheel loader

biogas plant (biomethanation)

combined heat and power plant (CHP)

dairy cattle husbandry

CAPTION

E: electric energy F: fuel

GHG: greenhouse gases H: thermal energy

- -energy flow

-material flow

----system boundary

<3-E -■>GHG

d-->GHG

Fig. 1. Flow scheme of mass, energy, and fuel and GHG emissions of (a) the reference system before and (b) the modified system after implementation of a TBH device.

tubes are assumed to be comparable to cold drawn tubes. The Heat uncoupled from CHP exhaust gas is transported to the TBH

masses of the respective components are summarized in Table 1. device vessel via a thermal-oil circuit by a thermal-oil pump with a The entire facility is designed for a lifetime of 20 years. nominal electric power of 4.0 kWel. The actual consumption of

Table 2

Overview of the different scenarios in terms of raw material and pretreatment temperature.

Raw material Pretreatment set-point temperature (°C)

w/o 140 160 180

P1-LCM X X X X

P1-SCM X X X

P1-SLCM X X X

P2-LCM X X X X

P2-SCM X X X

LCM - Liquid cattle manure; P1 - Plant 1; P2 - Plant 2; SCM - Solid cattle manure; SLCM - Solid and liquid cattle manure; w/o - without pretreatment.

electricity depends on the running hours of the consumer loads. The amount of heat available depends on CHP power and the resulting temperature and mass flow of exhaust gas. To control the heat input into the feedstock, the vessel inlet and outlet temperatures of the thermal-oil are measured, as well as the temperature of the feedstock and the inner-vessel pressure. The heat-flow is controlled by using a bypass to the exhaust gas heat exchanger.

The solid feedstock is brought to a receiver tank of 12 m3 once a day where it can be mixed with liquid feedstock. A feedstock volume of 3 m3 is delivered four times per day from the receiver tank to the vessel where the pretreatment takes place. Charging can be realized by a dosing feeder, equipped with a scale for measuring the amount of feedstock. The pipe between dosing feeder and vessel is blocked by a valve during pretreatment. Different mixing elements are possible, but to avoid clogging it is recommended that a mixing stirrer agitating near the vessel inside wall should be installed. Stirring power is approximately 1 kWel Mg"1 substrate at the beginning, and decreases with increasing temperature and ongoing hydrolysis to 0.3 kWel Mg"1 substrate for temperatures above 100 °C (unpublished engineering report).

The time for heating up feedstock to 100 °C is estimated to be 1.61 h, and 3.22 h for reaching set-point hydrolysis temperature. This period of 4.83 h is used for all designated set-point temperatures. The electric mixing power for SCM is then calculated to be

1.0 kWel. For further calculations it is assumed that the power for mixing LCM and SLCM equals that of SCM (unpublished engineering report).

After pretreatment the heated feedstock is released through a bottom drain valve using the water vapor pressure inside the vessel as driving force. Simultaneously liquid feedstock is pumped through a pipeline that is linked to the outlet pipe. The mixtures of both treated feedstock and liquid feedstock are then delivered to the digester of the biogas plant.

2.3. Feedstock

The main characteristics of the feedstock are displayed in Table 3 (for full characteristics see Budde et al. (2014)). SCM used for dry cows and heifers consisted of less straw in the case of P1 than SCM from P2. Although LCMs of P1 and P2 show differences, they both include parts of the litter and feed residues fallen through the slatted floor of high-performance dairy cattle. Methane yields of raw materials as well as alteration in methane yields due to pretreatment are gained from lab-scale experiments (Budde et al., 2014). The farm-based raw materials have been treated in a 600 ml stainless steel reactor of company Parr Instruments at temperatures of 140-220 °C and related saturated water vapor pressure (3.6-23.2 bar) in 20 K steps for a 5-min duration at setpoint temperature. A detailed description of the methods applied in lab-scale is given in Budde et al. (2014). Calculations of methane yields refer to a fresh matter base, as the daily amount of feedstock to be pretreated is limited by the TBH device vessel-volume. The fresh matter volume flow to be pretreated is 12 m3 d"1 for any feedstock. Mass flows were calculated using (bulk) densities of 1000 kg m"3 for LCM and 800 kg m 3 for SCM (Ruhlmann, 2000). The density of SLCM was set to 1000 kg m"3, assuming that the spaces in the solid cattle manure are completely filled with liquid. The fresh matter mass fraction of SCM in SLCM is 0.327. The resulting mass flows of the different feedstocks are 0.1389 kg FM s"1 in the case of LCM and SCLM and 0.1111 kgFMs"1 in the case of SCM. The annual average temperature of fresh solid and liquid cattle manure is set to 15 °C.

Liquid-soiid-mixture to digester

Fig. 2. TBH facility. Schematic flow chart of the entire TBH facility (simplified) including the heat uncoupling from CHP and the feeding device (based on DIN EN ISO 10628).

Table 3

Parameters of untreated and treated feedstock (from Budde et al., 2014) and number of wheel loader trips.

Raw Set-point Average methane ODM OM Mass fraction of methane Fresh matter

material temperature yielda Y30 from SCM in methane related

from SLCM methane

yield of

LCM SCM MS SCM

(°C) (lNkg-'OM) (% FM) (lN kg- 1 FM) (m3 d-1 ) (trips d-1)

P1-LCM Untreated 203.37 6.37 7.17 - - - - - -

P1-LCM 140 305.80 6.37 7.17 - 7.3 0.0 3.3 0.0 -4

P1-LCM 160 310.57 6.37 7.17 - 7.7 0.0 3.4 0.0 -4

P1-LCM 180 234.72 6.37 7.17 - 2.2 0.0 1.0 0.0 -1

P1-SCM Untreated 167.53 15.00 15.67 - - - - - -

P1-SCM 140 186.09 15.00 15.67 - 0.0 29.2 10.4 12.0 1

P1-SCM 160 187.38 15.00 15.67 - 0.0 29.4 10.5 12.0 1

P1-SCM 180 216.41 15.00 15.67 - 0.0 33.9 12.1 12.0 -1

P1-SLCM Untreated 225.66 7.36 8.09 0.467 - - - - -

P1-SLCM 140 296.08 7.36 8.09 0.393 4.8 9.4 2.5 4.5 2

P1-SLCM 160 291.08 7.36 8.09 0.391 4.6 9.2 2.5 4.5 2

P1-SLCM 180 289.37 7.36 8.09 0.495 2.1 11.6 2.0 4.5 3

P2-LCM Untreated 224.76 5.41 6.05 - - - - - -

P2-LCM 140 258.77 5.41 6.05 - 2.1 0.0 0.9 0.0 -1

P2-LCM 160 261.67 5.41 6.05 - 2.2 0.0 1.0 0.0 -1

P2-LCM 180 245.28 5.41 6.05 - 1.2 0.0 0.6 0.0 -1

P2-SCM Untreated 161.69 16.34 16.92 - - - - - -

P2-SCM 140 231.58 16.34 16.92 - 0.0 39.2 14.0 12.0 -2

P2-SCM 160 255.35 16.34 16.92 - 0.0 43.2 15.4 12.0 -4

P2-SCM 180 176.64 16.34 16.92 - 0.0 29.9 10.6 12.0 1

LCM - Liquid cattle manure; MS - Maize silage; ODM - Organic dry matter; OM - Organic matter (sum of ODM and volatile organic acids); P1 - Plant 1; P2 - Plant 2; SCM -Solid cattle manure; SLCM - Solid and liquid cattle manure; Y30 - Methane yields from lab-scale experiments. a Mean of three replicates (LCM and SLCM from plant 1 were not tested in repetition).

b Number of wheel loader trips needed for transport of the respective feedstock (LCM, SCM, or SLCM) minus number of wheel loader trips necessary to transport substituted maize silage.

Volume flow Number of wheel of loader tripsb

Maize silage is chosen as reference feedstock. It is the most frequent crop feedstock in anaerobic digestion in Germany today due to its high methane yield per hectare (DBFZ, 2010). Characteristics of maize silage are a bulk density of 300 kg m3 (Koppen, 2002) and a fresh matter methane yield of 90 lN kg-1 FM (calculated according to data from KTBL; KTBL, 2010a).

2.4. Full-scale biogas plant

A modular design has been projected to ensure wide-range applicability of the TBH. It allows a retrofit to almost all kinds of existing biogas plants. Therefore, the effect of thermobarical pre-treatment is calculated without considering the particular biogas process in detail.

All calculations assume that the heat necessary to maintain mesophilic temperatures (Tdig = 40 °C) is provided either directly from CHP to digester and/or via TBH. Heat losses during pretreat-ment in comparison to losses in the heating system of the biogas plant digester are not regarded. It is assumed that hydraulic retention time is not considerably decreased and viscosity not changed by the additional feedstock per day necessary to provide the same mass flow of methane (compared to maize silage). Reduction in volume of feedstock due to biomethanation is not considered. A possible change in feedstock degradation or degradability and stirring power is not regarded.

A wheel loader with a shovel-volume of 1 m3 is used for transport of solid feedstock to the feeding device of the biogas plant or the receiver tank of the TBH device. It is assumed that the distance between the storage location and the feeding device is 300 m, that the distance is covered with an average speed of 20 km h-1 and that both charging and discharging are done within 15 s. One charging process lasts 2.3 min under these assumptions.

LCM - also if used without pretreatment in biogas plant - does not require transport by wheel loader. But it could save an extra amount of maize silage if pretreatment enhances the fresh matter

related methane yield. It could therefore save wheel loader trips that would otherwise have been necessary for transporting maize silage equivalent to the additional methane yield. SCM has a lower fresh matter related methane yield than maize silage, but a higher bulk density. Hence maize silage substitution changes the number of trips of the wheel loader necessary to provide the same amount of methane after biomethanation of SCM. In the case of the SLCM, the necessary extra trips and the saved trips corresponding to the respective mass fractions of LCM and SCM in SLCM are calculated. The fresh matter volume-flow of feedstock and maize silage substituted and the numbers of additional or reduced wheel loader trips per day are shown in Table 3. They are calculated according to

V V YCH4.FM.FS • qFS n N

V FM.MS = V FS w--UJ

' CH4.FM.MS • qMS

The nomenclature for this and all following equations is presented in Table 4.

Heat is the main energy needed for TBH. It is provided by a combined heat and power plant. The CHP used for these calculations is a 330 kWel biogas-otto-engine (for further data see Table 1).

The maximal set-point temperature rset-point for TBH process is 180 "C. The temperature of uncoupled heat needs to be considerably higher to obtain a heat-flow that allows feedstock to be heated in a reasonable time. Therefore, the exhaust gas output temperature is set to 230 "C. The thermal power available for the TBH process is calculated to be 158 kJs-1 from the characteristics of the exhaust gas of the CHP in the example. It is assumed that CHP (and biogas plant) are in full operational state for ADO = 350 days or AHO = 8400 h per year respectively, for all following calculations (FNR, 2010).

2.5. Energy balance

The energy balance considers the energy needed for construction materials Econstr as well as the electricity input (energy per

Table 4

Nomenclature for symbols in equations in the order of their appearance.

Equation number

Parameter

Nomenclature

V FM,MS VFS

Ych4,fm,fs Ych4,fm,ms Pfs Pms

Pth,FS

:J>,H2O

Tset-point

Tfs AHO

eth,Fs

1 set-point

Pth ,process m fs Cp,H2O 1 set-point Tdig AHO

eth,process cp, H2O 1 set-point Tdig

YCH4,FM,LCM wOM,LCM Y30,LCM,T Y30,LCM,w/o

YCH4,FM,SCM

wOM,SCM

Y30,SCM,T

wCH4,FM,SCM,T

wFM,SCM

Y30,SCM,T

wOM,SCM

Y30,LCM,T

wOM,LCM

Ych4,fm,(s)lcm,t

wCH4,FM,SCM,T

wCH4,FM,SCM,w/o

Y30,SLCM,T

Y30,SLCM,w/o

wOM,SLCM

YCH4,FM,S(L)CM,T WCH4,FM,SCM,T Y30,SLCM,T wOM,SLCM

YcH4,FM,FS

LHVCH4

AHO EPBT

Econstr Pel,out Pel,in

Econstr eel,out eel,in

GHGnet GHGconstr

GHGel,in dGHGSCM

Fresh matter volume-flow of maize silage substituted (m3 d-1)

Fresh matter volume flow of feedstock (m3 d_1)

Methane yield of feedstock on a fresh matter basis (lN CH4 kg-1 FM)

Methane yield of maize silage on a fresh matter basis (lN CH4 kg-1 FM)

Bulk density of feedstock (kgm-3)

Bulk density of maize silage (kgm-3)

Thermal input to feedstock for reaching set-point temperature (kW h a-1) Mass flow of the respective feedstock (kgFM s-1) (temperature-independent) specific heat capacity of water (kJ kg-1 K-1) Set-point temperature of the pretreatment (°C) Feedstock temperature (°C) Annual hours of operation (h a-1)

Thermal input to feedstock for reaching set-point temperature (kW h Mg-1 FM) (temperature-independent) specific heat capacity of water (kJ kg-1 K-1) Set-point temperature of the pretreatment (°C) Feedstock temperature (°C)

Thermal input serving as process energy for the biogas plant (kW h a-1) Mass flow of the respective feedstock (kgFM s-1) (temperature-independent) specific heat capacity of water (kJ kg-1 K-1) Set-point temperature of the pretreatment (°C) Temperature of digester content (°C) Annual hours of operation (h a-1)

Thermal input serving as process energy for the biogas plant (kW h Mg-1 FM) (temperature-independent) specific heat capacity of water (kJ kg-1 K-1) Set-point temperature of the pretreatment (°C) Temperature of digester content (°C)

Methane yield of LCM assigned to the TBH on a fresh matter basis (lN CH4 kg-1 FM) Mass fraction of organic matter in fresh matter of LCM

Methane yield of LCM pretreated at the respective temperature 1 (lN kg-1 OM) Methane yield of untreated LCM (lN kg-1 OM)

Methane yield of SCM on a fresh matter basis (lN CH4 kg-1 FM)

Mass fraction of organic matter to fresh matter of SCM

Methane yield of SCM pretreated at the respective temperature 1 (lN kg-1 OM)

Ratio of methane from SCM to methane from LCM within SLCM at the different pretreatment temperatures T Fresh matter mass fraction of SCM in SLCM

Methane yield of SCM pretreated at the respective temperature T (lN kg-1 OM) Mass fraction of organic matter to fresh matter of SCM Methane yield of LCM pretreated at the respective temperature T (lN kg-1 OM) Mass fraction of organic matter in fresh matter of LCM

Methane from LCM within SLCM (lN kg-1 FM)

Weighted ratio of the respective methane yields of feedstock pretreated at the respective temperature T

Weighted ratio of the respective methane yields of untreated feedstock

Methane yield of SLCM (In kg-1 OM)

Methane yield of untreated SLCM (lN kg-1 OM)

Mass fraction of organic matter in fresh matter of SLCM

Methane from SCM within SLCM (lN kg-1 FM)

Weighted ratio of the respective methane yields of feedstock pretreated at the respective temperature T

Methane yield of SLCM (In kg-1 OM)

Mass fraction of organic matter in fresh matter of SLCM

Methanation capacity (kW h a-1)

Methane yield of feedstock on a fresh matter basis (lN CH4 kg-1 FM)

Density of methane (kg m-3)

Lower heating value of methane (MJ kg-1)

Mass flow of the respective feedstock (kgFM s-1)

Annual hours of operation (h a-1)

Energy payback time (P[n]Y[n]M)

Energy needed for construction materials (kW h)

Electric energy output (kWha-1)

Electricity input for operation of the TBH device (kW h a-1)

Energy payback mass (Mg FM)

Energy needed for construction materials (kW h)

Electric energy output (kW h Mg-1 FM)

Electricity input for operation of the TBH device (kW h Mg-1 FM)

Net GHG balance (kg CO2-eq. kW h-1) Emissions from TBH construction (kg CO2-eq. kW h-) Emissions from TBH operation, (kg CO2-eq. kW h-1) Emissions from SCM storage (kgCO2-eq. kW h-1)

Table 4 (continued)

Equation number

Parameter

Nomenclature

GHGnet

GHGconstr

GHGeMn

sghgScm dGHGMs

GHGel,in Pel,in GHGgrid Pel,out

GHGkin eel,in

GHGgrid

dGHGSCM

Mco2 mm CH4 , SCM

MCH4 Pel,out

mco2 MCH4

ych4 ,FM ,SCM PCH4

eel,out

Profit

Cconstr

Cel,in

SCspreading

dCel,out

Profit1

spreading

SCspr,dig,FS

V FM , FS Pfs qdig Cspr,V

Pel,out

d spr, dig , FS

V FM , FS Pfs Pdig

Cspr,V

SCspr,SCM,w/o

V FM ,SCM pSCM Cspr,m

Pel,out

spr, SCM ,w/o

*-spr,m SCsprea

Emissions from maize supply (kg CO2-eq. kW h" ) Net GHG balance (kgCO2-eq. Mg"1 FM) Emissions from TBH construction (kgCO2-eq. Mg"1 FM) Emissions from TBH operation, (kg CO2-eq. Mg"1 FM) Emissions from SCM storage (kgCO2-eq. Mg"1 FM) Emissions from maize supply (kg CO2-eq. Mg"1 FM)

Emissions from TBH operation, (kg CO2-eq. kW he"l1) Electricity input for operation of the TBH device (kW h a"1) Emissions balance of grid electricity (kg CO2-eq. kW he"l1) Electric energy output (kW h a"1)

Emissions from TBH operation, (kg CO2-eq. Mg"1 FM) Electricity input for operation of the TBH device (kW h Mg"1 FM) Emissions balance of grid electricity (kg CO2-eq. kW h"!1)

Emissions from SCM storage (kgCO2-eq. kW h"1)

Molar mass of CO2 (gmol"1)

Mass flow of methane attained from SCM (kg s"1)

Molar mass of CH4 (gmol"1)

Electric energy output (kW h a"1)

Emissions from SCM storage (kgCO2-eq. Mg"1 FM) Molar mass of CO2 (g mol"1) Molar mass of CH4 (gmol"1)

Methane yield of SCM on a fresh matter basis (lN CH4 kg"1 FM) Density of methane (kg m"3)

Costs of electricity production using maize silage (€ Mg"1 FM) Costs of electricity production using maize silage (ctkWhü1) Electric energy output (kW h Mg"1 FM)

Profit (ct kW h"1 )

Costs of purchasing the TBH device (ctkWh"1)

Costs of electricity consumption of the TBH device (ct kW h"1)

Costs of maintaining the TBH device (ct kW h"1 )

Costs of wheel loader employment (ct kW hs1 )

Costs of changes in spreading-procedure (ct kW h" )

Revenues from a bonus within German feed-in tariff structure of electricity sales (ct kW h"1) Costs of electricity production using maize silage (ctkWh"1)

Profit (€ Mg"1 FM)

Costs of purchasing the TBH device (€ Mg"1 FM)

Costs of electricity consumption of the TBH device (€ Mg"1 FM)

Costs of maintaining the TBH device (€ Mg"1 FM)

Costs of wheel loader employment (€ Mg"1 FM)

Costs of changes in spreading-procedure (€ Mg"1 FM)

Revenues from a bonus within German feed-in tariff structure of electricity sales (€ Mg"1 FM) Costs of electricity production using maize silage (€ Mg"1 FM)

Costs of spreading digestate from the respective feedstock (ct kW hä1) Fresh matter volume flow of the respective feedstock (m3 d"1) Bulk density of feedstock (kg m"3) Density of digestate (kgm"3)

Costs of transport and spreading of LCM or digestate (€ m"3) Annual days of operation (d a"1) Electric energy output (kW h a"1)

Costs of spreading digestate from the respective feedstock (€ Mg"1 FM) Fresh matter volume flow of the respective feedstock (m3 d"1) Bulk density of feedstock (kgm"3) Density of digestate (kgm"3)

Costs of transport and spreading of LCM or digestate (ctm"3) Mass flow of the respective feedstock (kg FM s"1)

Costs of spreading untreated SCM (€ kW h"1)

Fresh matter volume flow of SCM (m3 d"1)

Bulk density of SCM (kg m"3)

Costs of transport and spreading of SCM (€ Mg"1)

Annual days of operation (d a"1)

Electric energy output (kW h a"1)

Costs of spreading untreated SCM (€ Mg"1 FM) Costs of transport and spreading of SCM (ctMg"1) Costs of changes in spreading-procedure (ct kW h"1 )

(continued on next page)

Table 4 (continued)

Equation number Parameter Nomenclature

dCspr,SCM,w/o Costs of spreading untreated SCM (ct kW h—l1)

dCspr,dig,SCM Costs of spreading digestate from SCM (ct kW h—l1)

dCspr,dig,MS Costs of spreading digestate from maize silage (ct kW h—l1)

28 spreading Costs of changes in spreading-procedure (€ Mg—1 FM)

spr , SCM ,w/o Costs of spreading untreated SCM (€ Mg—1 FM)

spr , dig , SCM Costs of spreading digestate from SCM (€ Mg—1 FM)

dCspr , dig , MS Costs of spreading digestate from maize silage (€ Mg—1 FM)

29 dcel,ut Revenues from a bonus within German feed-in tariff structure of electricity sales (€ Mg—1 FM)

eel,out Electric energy output (kW h Mg—1 FM)

30 ECAP Economic amortization period (P[n]Y[n]M)

Investment Investment (€)

Profit Profit (€ a—1)

Depreciation Depreciation (€ a—1)

Interest Interest rate (€ a—1)

31 ECAM Economic amortization mass (t FM)

ECAP Economic amortization period (P[n]Y[n]M)

m FS Mass flow of the respective feedstock (kg FM s—1)

ADO Annual days of operation (d a—1)

32 CMC CO2 mitigation costs (€ Mg—1 CO2-eq.)

CEP,TBH Electricity production costs (€ kW he—l1)

CEP,grid Grid electricity-mix production costs (€ kW h—1 )

GHGgrid Emissions balance of grid electricity (kg CO2-eq. kW h—i1)

GHGnet Net GHG balance (kg CO2-eq. kW h—1)

time or per mass) for operation of the TBH device Pelin or eelin and compares these inputs with the energy output, electric (Pel,out or eel,out) as well as thermal (Pth,out or ethout), attained from pretreated feedstock via methane. Furthermore, the heat balance of heating up feedstock through TBH (PthFS or eth,FS) and heat transfer into digester (Pth,process or eth,process) is presented, as well as savings from energy needed for maize supply (dPMS or deMS). The energy payback time or payback mass (EPBT or EPBM) is calculated in order to assess the energy efficiency.

The respective energy input for employment of agricultural machines for TBH device or biogas plant feeding and manure or digestate spreading was neglected. Calculation revealed values lying in most cases below 0.91% of the entire energy input or output of the system, and never more than 2.45%.

The energy for construction and provision of raw material Econstr in kW h has to be taken into consideration as part of an energy converting facility. The total energy demand for provision of stainless steel and for construction of a TBH device £constr is calculated as 374,600 kWh. Dependent on the lifetime of the TBH facility and on feedstock density the mass related values econstr are either 4.5 or 5.6 kW h Mg"1 FM.

The electricity demand Pel,in or eel,in of the largest consumer loads of the TBH device, the stirrer and the thermal-oil pump, adds up to 42,000 kWhela"1 or dependent on feedstock density either 10 or 12.5 kWhel Mg"1 FM.

In idealized calculation, thermal energy necessary to heat feedstock is affected by feedstock temperature TFs, set-point temperature of the pretreatment rset-point, and the density of the feedstock (the specific heat capacity of water and the volume flow of respective feedstock are assumed to be fixed values). The thermal input to feedstock Pth,FS in kW h a"1 for reaching set-point temperature during thermobarical hydrolysis is

Pth,FS = mFS • cp,H2O • (Tset-point - TFs) • AHO

Related to fresh matter the thermal input eth,FS in kW h Mg-1 FM is calculated according to

eth,FS = cp,H2O • (T

set-point

- Tfs) • 3.6-

After thermobarical hydrolysis, the heated feedstock is discharged into the biogas plant. It is assumed that there is no loss in thermal energy during transport of feedstock from the TBH device to the biogas plant. The heat input into the downstream biogas plant is determined by the temperature difference between feedstock (TFS) and digester content (Tdig) and the mass flow of the respective feedstock. The thermal energy input Pth,process, given in kW h a"1 or eth,process, given in kW h Mg"1 FM, serves as process energy for the biogas plant: pretreatment and heating of feedstock contribute positive energy to the system while the untreated variants make a negative contribution. Process energy in kW h a"1 is calculated

Pth .process — mFS • cp,H2O • (Tset-point — Tdig) • AHO

Related to fresh matter the process energy in kW h Mg 1 FM is calculated according to

: cp,H2O • (Tset-point — Tdig) • 3.6

In the case of no treatment, the term rset-point is to be replaced by TFS. The required heating of 'cold' untreated feedstock to mesophilic digester temperatures of 40 °C leads to losses of 122,500 kWhth a"1 in the case of LCM and SLCM and 98,000 kW hth a"1 in the case of SCM. When related to fresh matter, it is 29.2 kW hth Mg"1 FM in any case.

It is assumed that the methane achievable from the respective feedstock in biogas plant equals the methane yield determined in lab-scale experiments.

The processing of LCM as well as the methane attainable from untreated LCM are assigned to the biogas plant, as it is assumed that LCM is used or can be used in continuous stirred tank reactors without any pretreatment.

YCH4,FM,LCM — Wom.LCM • (Y30.LCM,T — Y30 ,LCM ,w/o)

The methane available from SCM is completely allotted to the TBH, as it is not suitable for utilization in conventional continuous stirred tank reactors without pretreatment.

YCH4 .FM .SCM — wOM . SCM • Y30 .SCM.T (7)

The shares of methane from LCM and SCM within SLCM have to be related to the respective feedstock. It is expected, that the difference in methane yield of the respective feedstock - SCM or LCM - due to pretreatment corresponds with that in the mixture of both - SLCM. The ratio of methane from SCM to methane from LCM within SLCM at the different pretreatment temperatures T is then calculated according to

Y30 .scm .t • wom .scm

wch4 ,FM ,SCM ,T = wfm .scm

Wfm ,scm

Y30 ,LCM,T • Wom ;LCM

Y30 ,scm,t • Wom ,scm

Y30 ,LCM,t • Wom ,lcm

(1 — wfm ,scm)

The methane from LCM within SLCM YCH4,FM,(S)LCM,T in lN kg-1 FM is

Ych4 .FM.(S)LCM — [(1 — Wch4 .FM .SCM.T ) • Y30 .SLCM.T

— (1 — Wch4 .FM .SCM .w/o) • Y30 .SLCM .w/o] • Wom .SLCM (9)

The methane attainable from SCM within SLCM is

Ych4 ,fm ,s(l)cm = Wch4,fm ,scm,t • y30 ;slcm,t • Wom ,slcm

Table 3 shows the values of untreated LCM and SCM of biogas plant 1 allowing this calculation.

The methanation capacity PMC in kW h a—1 is calculated from the respective methane yields (Table 5)

Pmc = Ych4,fm ,fs • qCH4 • LHVch4 • mfs • AHO

The methane is converted to electricity and heat by the CHP. The term Pel,out or eel,out is the electricity in kW hel a—1 or kW hel Mg—1 FM achieved from the respective feedstock (Table 5).

The thermal output Pth,outor ethoutinkW hth a—1 orkW hth Mg—1 FM is calculated by the thermal efficiency gth of the CHP (Table 1).

The energy saved by substitution of maize silage (dPMS), needed for the entire supply chain of maize silage, is in any case 0.162 kWhkWh— (Isermeyer et al., 2007). The values related (deMS) to fresh matter are displayed in Table 5.

The energy payback time or payback mass respectively is the time or FM mass needed for providing just as much net energy as is needed for construction of the power generation plant regarded. Assuming that only electric energy is used for production and processing of steel and semi-finished products, as is usual in modern steel plants, the energy payback time in (P[n]Y[n]M) adds up to

EPBT =

Pel .out Pel .i

The energetic payback mass in Mg FM is calculated according to

Econstr

EPBM =

eel out — eel in

2.6. Greenhouse gas balance

The net GHG balance GHGnet in kg CO2-eq. per kW hel or per Mg fresh mass (symbols indicated by a superscript double plus are related to fresh matter) results from emissions from TBH construction GHGconstr and operation GHGel,in, saved emissions from SCM storage dGHGSCM, and saved emissions from maize supply dGHGMS (Table 6):

GHGnet — GHGconstr + GHGel .in + dGHGscM + dGHGM

according to

GHGnet — GHGconstr

+ GHG', in + dGHGScM + dGHGM.

The GHG payback time or payback mass (GPBT or GPBM) is the time or fresh matter needed for saving the same amount of GHG that accrued during construction of, in this case, the TBH device

or GHG

It is assumed that the emissions which result from wheel loader employment for feedstock processing and from the entire process chain of spreading the agricultural manures can be neglected. Calculations revealed values below 0.88% in most cases and never more than 2.27%. Possible emission savings from avoiding LCM storage in open liquid manure storage facilities are assigned completely to the biogas plant and not considered here. It is further assumed that thermobarical pretreatment releases feedstock constituents which, without pretreatment, would not have contributed to methane emissions during storage of untreated LCM because of their lower degradability.

Emissions from TBH construction GHGc

or GHG

calculated based on parameters of semi-finished products, as explained for the energy input. The emissions sum up to 85,468 kg CO2-eq., whereby the masses of raw materials and the emissions from provision as shown in Table 1 are taken into consideration.

One has to consider the emissions balance of grid electricity GHGgrid or GHGgrid, as under German conditions, due to the difference in purchase prices of electricity and the granted feed-in tariff for electricity from biomass it is advantageous to use electric energy from the grid for the internal processes of a biogas plant (Table 1). Related to the electric energy produced the emissions GHGel,in in kg CO2-eq. per kW hel are calculated according to

GHGel in

Related to fresh matter the emissions GHGel in in kg CO2-eq. per Mg FM are calculated according to

GHC^l in — eel .in • GHGgrid (17)

SCM is usually stored in heaps on a concrete surface. During storage the carbon within the SCM is aerobically reduced to CO2 by microorganisms. It is expected that otherwise all carbon in CH4 that is composed by microorganisms during anaerobic digestion would have led to CO2 emissions. Therefore the emissions due to utilization of SCM are calculated according to

dGHGscM — — MO2 • • 31.536 • 106

Related to fresh matter these emissions are calculated according to

dGHGScM — — MT02 • YCH4 .FM .SCM • qCH4

The emissions of CH4 and N2O during rotting process are disregarded.

In the case of SLCM only the emissions from SCM are taken into account.

The emissions from cultivation, harvest, transport and ensiling of maize silage from whole crop GHGMS are set to 0.144 kg CO2-eq. kW he—l1 (Isermeyer et al., 2007). It is further assumed that 25% of the ensiled maize used is grown on former grassland. This land use change causes emissions of 0.0695 kg CO2-eq. kW h— (Isermeyer et al., 2007; Meyer-Aurich et al., 2012). Thus the use of maize silage results in total emissions of 0.2135 kg CO2-eq. kW h—l1. The values related to fresh matter are displayed in Table 6.

The GPBT or GPBM respectively is the period or the fresh matter of the respective feedstock needed to save the same amount of emissions deriving from constructing the TBH device.

Related to fresh matter the net emissions GHGnet are calculated

Table 5

Energetic assessment including energy payback time.

Raw Set-point Pconstr Pel,in Pth,FS Pth,process Pmc Pel,out Pth,out ¿Pms EPBT

material temperature LCM SCM LCM SCM

("C) (kWh a-1) (P[n]Y [n]M)

P1-LCM 140 18,730 42,000 612,500 490,000 308,498 - 119,389 - 144,685 19,341 P4Y10M

P1-LCM 160 18,730 42,000 710,500 588,000 322,857 - 124,945 - 151,420 20,241 P4Y6M

P1-LCM 180 18,730 42,000 808,500 686,000 94,436 - 36,547 - 44,290 5,921 -

P1-SCM 140 18,730 42,000 490,000 392,000 - 980,265 - 379,362 459,744 61,457 P1Y1M

P1-SCM 160 18,730 42,000 568,400 470,400 - 987,062 - 381,993 462,932 61,883 P1Y1M

P1-SCM 180 18,730 42,000 646,800 548,800 - 1,139,986 - 441,175 534,654 71,470 P11M

P1-SLCM 140 18,730 42,000 612,500 490,000 202,105 394,995 78,214 152,863 280,040 37,435 P2Y

P1-SLCM 160 18,730 42,000 710,500 588,000 193,797 386,303 74,999 149,499 272,067 36,369 P2Y1M

P1-SLCM 180 18,730 42,000 808,500 686,000 87,794 486,492 33,976 188,273 269,340 36,004 P2Y1M

P2-LCM 140 18,730 42,000 612,500 490,000 86,406 - 33,439 - 40,524 5417 -

P2-LCM 160 18,730 42,000 710,500 588,000 93,778 - 36,292 - 43,982 5879 -

P2-LCM 180 18,730 42,000 808,500 686,000 52,138 - 20,177 - 24,453 3269 -

P2-SCM 140 18,730 42,000 490,000 392,000 - 1,316,902 - 509,641 617,627 82,562 P10M

P2-SCM 160 18,730 42,000 568,400 470,400 - 1,452,106 - 561,965 681,038 91,038 P9M

P2-SCM 180 18,730 42,000 646,800 548,800 - 1,004,477 - 388,733 471,100 62,975 P1Y1M

econstr eel,in eth,FS eth,process eMC eel,out eth,out deMs EPBM

LCM SCM LCM SCM

(kW h Mg- 1 FM) (MgFM)

P1-LCM 140 4.5 10.0 145.8 116.7 73.5 - 28.4 - 34.4 4.6 20,330

P1-LCM 160 4.5 10.0 169.2 140.0 76.9 - 29.7 - 36.1 4.8 18,968

P1-LCM 180 4.5 10.0 192.5 163.3 22.5 - 8.7 - 10.5 1.4 -

P1-SCM 140 5.6 12.5 145.8 116.7 - 291.7 - 112.9 136.8 18.3 3731

P1-SCM 160 5.6 12.5 169.2 140.0 - 293.8 - 113.7 137.8 18.4 3702

P1-SCM 180 5.6 12.5 192.5 163.3 - 339.3 - 131.3 159.1 21.3 3153

P1-SLCM 140 4.5 10.0 145.8 116.7 48.1 94.0 18.6 36.4 66.7 8.9 8321

P1-SLCM 160 4.5 10.0 169.2 140.0 46.1 92.0 17.9 35.6 64.8 8.7 8621

P1-SLCM 180 4.5 10.0 192.5 163.3 20.9 115.8 8.1 44.8 64.1 8.6 8729

P2-LCM 140 4.5 10.0 145.8 116.7 20.6 - 8.0 - 9.6 1.3 -

P2-LCM 160 4.5 10.0 169.2 140.0 22.3 - 8.6 - 10.5 1.4 -

P2-LCM 180 4.5 10.0 192.5 163.3 12.4 - 4.8 - 5.8 0.8 -

P2-SCM 140 5.6 12.5 145.8 116.7 - 391.9 - 151.7 183.8 24.6 2691

P2-SCM 160 5.6 12.5 169.2 140.0 - 432.2 - 167.3 202.7 27.1 2421

P2-SCM 180 5.6 12.5 192.5 163.3 - 299.0 - 115.7 140.2 18.7 3630

econstr - Energy for construction of thermobarical hydrolysis device; eel,in - Electricity consumption of thermobarical hydrolysis device; eel,out - Electric energy attainable from the respective feedstock; eMC - Methanation capacity; EPBT - Energy payback time; EPBM - Energy payback mass; eth,Fs - Thermal power necessary for heating feedstock; eth, out - Thermal energy attainable from the respective feedstock; eth process - Thermal energy flow from feedstock to biogas plant; LCM - Liquid cattle manure; P1 - Plant 1; P2 -Plant 2; Pconstr - Energy for construction of thermobarical hydrolysis device; Pel,in - Electricity consumption of thermobarical hydrolysis device; Pel,out - Electric energy attainable from the respective feedstock; PMC - Methanation capacity; Pth,FS - Thermal power necessary for heating feedstock; Pth,out - Thermal energy attainable from the respective feedstock; Pth,process - Thermal energy flow from feedstock to biogas plant; SCM - Solid cattle manure; SLCM - Solid and liquid cattle manure.

2.7. Profitability

The assessment of the economic performance of TBH is based on profitability, amortization period and CO2 mitigation costs (CMC).

Costs arise from purchasing (Cconstr or Cj;onstr) and operating (Cel, in or Cel in and CMT or CMT) the TBH device. Changes in wheel loader employment dCWL or dCWL can save as well as cause costs, whereas changes in spreading-procedure ¿Grading or dC^preading save costs in any case. Revenues in this analysis result from a bonus within the German feed-in tariff structure for electricity sales dCel,out or ^ out. The agricultural wastes used instead of maize silage are assumed to be available free of charge. All energy related cost factors related to TBH are balanced to a base of 10.06 ct kWh"1, derived from the costs of electricity production using maize silage dCMS considering an average price of 35 € per ton of fresh maize silage (Bonewitz, 2007). Related to fresh matter the costs in € Mg"1 FM of electricity production using maize silage are calculated according to

¿CMs = ¿Cms • out (20)

Hence, profit in ct kW he"l1 or in € Mg"1 FM is obtained as

Profit = Cconstr + Cel ,in + CMT + dCWL + dCspreading + dCel ,out + ¿Cms

Profit' — Cjonstr + Cel, in + CMt + dCWL + dCspreading + dCel,out + dcmS

Investment for TBH is estimated to 250,000 € (unpublished engineering report). Fixed costs of 17,500 € a"1 comprise depreciation (life time 20 years, operation time 8400 h a"1) and interest (at a rate of 4%). Variable costs include electric energy consumption Cel in or C^ in (4620 € a"1, own calculation), repair and maintenance CMT or CMT (4200 € a"1; KTBL, 2010b) and labor costs from wheel loader employment.

Changes in costs due to alteration in feedstock comprise costs of wheel loader and spreading. Investment of the wheel loader is 73,000 €. Fixed costs include depreciation (lifetime 10.3 years), interest (at a rate of 4%), taxes and insurance (50 € a"1). Variable costs comprise diesel fuel (0.7 € l"1; 6.5 l h"1), lubricants (2 € l"1; 0.065 l h"1), repair and maintenance (1.41 € h"1). The costs of common labor are 13 € h"1 (Hanff et al., 2010; KTBL, 2010b). These cost items are summed up to 36.73 cent per minute of wheel loader employment. Specific costs in cent per kW hel or in € per Mg FM are calculated depending on the volume of the respective feedstock to be transported for substitution of maize silage and the electric energy attainable or the FM mass flow of the respective feedstock.

Table 6

Greenhouse gas emissions and greenhouse gas payback time.

Raw material Set-pnint temperature GHGcQnstr GHGel,in SGHGSCM SGHGms GHGnet GHGnet GPBT

(°C) (kg CO2-eq. kW h"1) (kgCO2-eq. a-1) (P[n]Y[n]M)

P1-LCM 140 0.0358 0.215 - -0.2135 0.037 4446 -

P1-LCM 160 0.0342 0.205 - -0.2135 0.026 3260 -

P1-LCM 180 0.1169 0.702 - -0.2135 0.606 22,133 -

P1-SCM 140 0.0113 0.068 -0.512 -0.2135 -0.646 -245,112 P4M

P1-SCM 160 0.0112 0.067 -0.512 -0.2135 -0.647 -247,019 P4M

P1-SCM 180 0.0097 0.058 -0.512 -0.2135 -0.657 -289,928 P4M

P1-SLCM 140 0.0185 0.111 -0.271 -0.2135 -0.355 -81,954 P1Y1M

P1-SLCM 160 0.0190 0.114 -0.273 -0.2135 -0.353 -79,173 P1Y1M

P1-SLCM 180 0.0192 0.115 -0.347 -0.2135 -0.425 -94,560 P11M

P2-LCM 140 0.1278 0.767 - -0.2135 0.682 22,796 -

P2-LCM 160 0.1178 0.707 - -0.2135 0.611 22,187 -

P2-LCM 180 0.2118 1.272 - -0.2135 1.270 25,628 -

P2-SCM 140 0.0084 0.050 -0.512 -0.2135 -0.666 -339,568 P3M

P2-SCM 160 0.0076 0.046 -0.512 -0.2135 -0.672 -377,504 P3M

P2-SCM 180 0.0110 0.066 -0.512 -0.2135 -0.648 -251,906 P4M

GHGcnnstr GHGkta sghgscm SGHGMs GHGnet GPBM

(kg CO2-eq. Mg- -1 FM) (MgFM)

P1-LCM 140 1.0 6.1 - -6.1 1.1 -

P1-LCM 160 1.0 6.1 - -6.4 0.8 -

P1-LCM 180 1.0 6.1 - -1.9 5.3 -

P1-SCM 140 1.3 7.6 -57.8 -24.1 -73.0 1172

P1-SCM 160 1.3 7.6 -58.2 -24.3 -73.5 1163

P1-SCM 180 1.3 7.6 -67.2 -28.0 -86.3 991

P1-SLCM 140 1.0 6.1 -18.6 -11.7 -23.2 3678

P1-SLCM 160 1.0 6.1 -18.2 -11.4 -22.5 3800

P1-SLCM 180 1.0 6.1 -22.9 -11.3 -27.1 3154

P2-LCM 140 1.0 6.1 - -1.7 5.4 -

P2-LCM 160 1.0 6.1 - -1.8 5.3 -

P2-LCM 180 1.0 6.1 - -1.0 6.1 -

P2-SCM 140 1.3 7.6 -77.6 -32.4 -101.1 846

P2-SCM 160 1.3 7.6 -85.6 -35.7 -112.4 761

P2-SCM 180 1.3 7.6 -59.2 -24.7 -75.0 1140

GHGconstr and GHGconstr - Emissions from construction of thermobarical hydrolysis device; GHGelin and GHG^ in - Emissions from electricity consumption of thermobarical hydrolysis; dGHGSCM and dGHGSCM - Emissions from storing LCM; dGHGMS and dGHGMS - Emissions from substitution of maize silage; GHGnet and GHGnet - Sum of all emissions; GPBT - Greenhouse gas payback time; GPBM - Greenhouse gas payback mass; LCM - Liquid cattle manure; P1 - Plant 1; P2 - Plant 2; SCM - Solid cattle manure; SLCM - Solid and liquid cattle manure.

The agricultural wastes would have led to costs for transport and spreading if not used for biomethanation. Costs that arise from transport and spreading of LCM or digestate sum up to Cspry =6.21 € perm3. Costs Cspr,m of 8.46 € Mg—1 SCM result from transport and spreading of SCM (Hanff et al., 2010; Schindler, 2009; own calculation). Those costs are decreased by the saved spreading of digestate from feedstock to be substituted, in our case maize silage. The bulk densities of feedstock before biomethanation are qSCM = 800 kg m—3 in the case of SCM and qMS = 300 kg m—3 in the case of maize silage. After biomethana-tion the density of digestate qdig is 1000 kg m—3 in any case. The costs in ct kWh—1 arising from spreading digestate are

spr,dig,FS

VFM ,FS • qg • CSpr,V ■ ADO

Related tn fresh matter the costs in € Mg 1 FM arising from spreading digestate are calculated according tn

spr dig FS

V fm ,fs • qg •C m fs

- •8640~

As mentioned above, any processing of LCM is assigned to the biogas plant. As a result of increased methane yields through TBH, a concordant share of digestate from maize silage is replaced.

The saved spreading of SCM reduces costs, but the higher amount of digestate arising from SCM compared to digestate from maize

silage causes additional costs for spreading. The costs in ct kW hel1 from spreading untreated SCM are calculated according to

spr,SCM,w/o :

Vfm ,scm • qSCM • Cspr,m • ADO

Related to fresh matter the costs in € Mg—1 FM from spreading untreated SCM

Cz — C

spr.SCM .w/o — L spr .m

Costs in ct kW h—l1 or in € Mg—1 FM of spreading SLCM are compartmentalized according to its volumetric fractions.

The overall costs in ct kW h—1 for any feedstock sum up to

dCspreading — — dCspr.SCM.w/o + dCspr.dig.SCM — dCspr.dig.MS (27)

Related to fresh matter the overall costs in € Mg—1 FM for spreading are calculated according to

dCspreading — — dCspr.SCM.w/o + dCspr.dig.SCM — dCspr.dig.MS (28)

Main income from biogas production is determined by feeding electricity into the grid with a given feed-in tariff. Despite a new Renewable Energy Sources Act that came into force in 2014, the following calculations are in accordance with the Renewable Energy Sources Act of 2012 (German Government, 2012a) as the TBH device - designed as modular concept - is aimed for retrofitting already existing biogas plants. The basic feed-in tariff for electricity generated from biomass amounts to 12.3 ct kW h—1. If only

renewable resources such as energy crops are used, an additional 6.0 ct kW h" can be obtained. Another 2.0ctkWhel1 can be attained if animal waste or other agricultural residues are used. These 2 cents are calculated with fixed methane yields for SCM of 53 lN kg"1 FM and for LCM of 17lNkg"1 FM (German Government, 2012b). These additional 2 ct kW he"l1 are perpetuated in the new Renewable Energy Sources Act of 2014 (German Government, 2014) and calculated with the same fixed methane yields named above. As the basic fee is allotted to the biogas plant, only the additional fee for using agricultural wastes can be charged to the TBH, thus it is calculated by the shared contribution of feedstock constituents to generated electricity. The fee attainable is set at a fixed 2 cent per kW hel. Related to fresh matter mass the fee attainable in € Mg"1 FM is calculated according to

dCel ,oUt — 0.02€ • kWh—l1

The economic amortization period (ECAP) or amortization mass (ECAM) is the time in years and months (P[n]Y[n]M) or FM mass in Mg pretreated until investment for the entire TBH facility and all running expenditures - except for depreciation and interest rate - is paid back. In order to assess ECAP or ECAM, total investment is divided by profit. Depreciation and interest rate (in sum 17,500 € a"1) are added to profit as CTBH, as part of profit already contains investment for such a TBH device in the form of depreciation and interest rate

ECAP =

Investment

Profit + Depreciation + Interest

ECAM — ECAP • mFs • ADO • 86.4

The CO2 mitigation costs in € per Mg of CO2-eq. (in addition to that of the downstream biogas plant) are deduced from the electricity production costs CEPTBH (Profit without dCMS) compared to the grid electricity-mix production costs CEP,grid ( ;

Institut, 1998; Stenull and Raab, 2010).

EP , grid

GHGgrid — GHG,

2.8. Sensitivity analysis

Sensitivity analyses are performed by alternating the following input parameters by ±50%:

- stirring power exemplary both for changes in demand of electric energy of TBH and as parameter influenced by feedstock viscosity,

- investment for displaying changes in fixed costs,

- organic matter content of feedstock as example for changes in feedstock composition and for presenting the impact of changes of organic loading rate on the model output.

The output parameters chosen are:

- GHG as a parameter sensitive to changes in feedstock parameters and energy demand of the device but not influenced by changes of economical parameters,

- CMC as it is displaying the environmental impact and the costs occurring if using TBH in combination with an agricultural biogas plant as mitigation strategy,

- ECAP as it is sensitive to all input parameters that lead to changes in economic viability but disregards the environmental impact.

3. Results and discussion

3.1. Energy balance of thermobarical pretreatment

The energy input for TBH construction is 18,730 kWhel a"1 (Table 5; Budde, 2015) and accounts for between 2.15% and 3.40% of the overall energy demand (thermal as well as electric). The operation of the TBH demands much more energy, electric as well as thermal. The annual electric energy demand amounts to 42,000 kW hel a"1 and is needed for stirring and the thermal-oil pump, independently of the kind of feedstock. Related to fresh mass values are either 10 or 12.5 kWhel Mg"1 FM depending on feedstock bulk density of 1000 or 800 kg m"3 respectively. The electric energy demand presented here is comparable to values reported by Monch-Tegeder et al. (2014) who needed 11.3 kW hel Mg"1 FM for mechanically pretreating horse manure in full-scale application and by Elbeshbishy et al. (2011) who calculated 13 kW hel Mg"1 FM for ultrasonic pretreatment. It accounts for between 4.83% and 7.63% of the total energy demand of the TBH device. Thermal energy input required for achieving the treatment temperature also depends on the kind of feedstock, expressed in different densities. It ranges from 490,000 to 808,500 kW hth a"1 or 145.8 to 192.5 kW hth Mg"1 FM, accounting for roughly 90% of the overall energy demand. Between 392,000 and 686,000 kW hth a"1 or 116.7 to 163.3 kW hth Mg"1 FM of this energy are available as process energy to maintain mesophilic conditions in the digester. The difference in energy values of TBH heat input and output to digester is caused by the different temperatures of feedstock before TBH treatment and substrate in the digester (15-40 °C). Consequently, the thermal energy actually required for hydrolysis is negligible.

The energy output of the TBH treatment of the different feedstocks is expressed as methanation capacity and is therefore independent of the overall conversion route of the methane. It can be divided into electric and thermal output, as well as losses if particular CHP is considered as conversion. The methanation capacity depends on feedstock and methane attainable as a consequence of differences in pretreatment. In general it can be stated that SCM from plant 2 pretreated at 160 °C displays best and LCM from plant 2 pretreated at 180 °C always shows lowest output. Values for methanation capacity range from 52,138 to 1,452,106 kWh a"1. Considering a model CHP (Table 1), an electric energy output of 20,177-561,965 kW hel a"1 can be achieved. Thus, TBH-treated feedstock would contribute 0.7 to 20.3 % of the electric output of the CHP. The thermal output of the CHP from pretreated feedstock ranges from 24,453 to 681,038 kWhth a"1. A (surplus) energy output from 4.8 to 167.3 kW hel Mg"1 FM and 5.8-202.7 kW hth Mg"1 FM can be gained via the conversion route methane - CHP. Thus, the electricity input for thermobarical treatment accounts for 7% of the electricity output in best case and 208% in worst case.

An increase in uncoupled thermal energy from CHP is possible by cooling down the exhaust gas to 180 °C, as during the first period of heat-up the heat input into feedstock is limited by the available thermal power. Above a feedstock temperature of approximately 110 °C, the heat input into feedstock is limited by the heat transfer that is lower than the thermal power available. It should be possible to considerably enhance the thermal power used by operating two devices time off-set in parallel during this second period. Thus, the share of alternative feedstock can be significantly enhanced.

The electric energy possibly gained depends on feedstock and changes in digestibility as well as on the organic matter (OM (in % FM)) of feedstock. Feedstock with high OM and hence higher FM-related methane yield is preferred as the fixed vessel volume

limits the fresh matter volume to be pretreated per batch. In the case of P1-LCM pretreated at 180 °C and all variants of P2-LCM, the electricity consumption is higher than the electricity produced. But compared with alternative pretreatment methods the electric energy consumption is very low: e.g. an extruder sized for the same mass flow will need approximately 20 kWel and will cause much higher costs for maintenance and spare parts (Weiß and Brückner, 2008).

From an energetic point of view, it is possible to obtain more thermal energy from pretreated feedstock than is necessary for pretreatment, e.g. P2-SCM pretreated at 140 and 160 °C deliver 617,627 and 681,038 kW hth a—1, but need only 490,000 and 568,400 kW hth a—1 respectively. From the exergetic point of view, the balance may be inversed. Only 96.3 kW (24.1% of nominal thermal power) are transferred to feedstock through TBH from 158 kW thermal power (that is 39.5% of the nominal thermal power of the cHP) uncoupled from cHP exhaust gas.

Substituting maize silage as biogas feedstock can save between 3269 and 91,038 kW h a—1, thus enhancing the "fuel" output (PMC) by 6.3% in any case.

The lowest energy payback time is 9 months for P2-SCM pretreated at 160 °C. For LCM it is possible to reach an EPBT of 4 years 6 months (P1-LCM, 160 °C) although only the extra amount of methane is included in the calculation. Compared with other renewable energy converting facilities, the EPBTof TBH can be very short. Wind turbines for example need 3-6 months, photovoltaic cells 2-5 years to feed in the electricity needed for construction (Lübbert, 2007). Related to fresh mass (EPBM) 2,421 (P2-SCM, 160 °C) to 20,330 Mg FM (P1-LCM, 140 °c) are to be pretreated in order to achieve the same amount of energy that was spent for construction of the TBH device. The annual volumetric throughput of 4200 m3 FM equals 3360 or 4200 Mg FM, depending on bulk density. The energy payback time or mass in total is at least that of the biogas plant as the TBH device is only a pretreating unit attached to a biogas plant (including the CHP) that is the actual renewable energy converting facility.

3.2. Greenhouse gas emissions balance of thermobarical pretreatment

Thermobarical hydrolysis can save a considerable amount of greenhouse gases of up to 0.672 kg C02-eq. kW h—1 or 377,504 kg C02-eq. per year (best case P2-SCM, 160 °C) (Table 6). LCM does not save GHG emissions but emits from 0.026 to a maximum of 1.270 kg C02-eq. kW h—l1.

The emissions from construction and operation of a TBH device related to electric energy output are affected by the fresh matter related methane yields of respective feedstock while the absolute emissions are independent of feedstock and treatment temperatures. The emissions from construction are 0.0076 (P2-SCM, 160 °C) to 0.2118 kg C02-eq. kW h—l1 (P2-LCM, 180 °C), accounting for 14% of all emissions (without regarding emissions mitigated) in any case. The complementary 86% of emissions arise from grid electricity used for operation of the device and amount to 0.046 (P2-SCM, 160 °C) to 1.272 kg C02-eq. kW hel1 (P2-LCM, 180 °C).

The mitigation potential of using SCM is 0.512 kgC02-eq. kWh—l1 in any case, as both the saved C02 emissions and the electricity generated are directly dependent on methane production. It accounts for 71% of the overall mitigations. The mitigation potential of SLCM derives from the share of SCM and is between 0.271 and 0.347 kg C02-eq. kW h—l1, accounting for between 56% and 62%. Although significant emissions of CH4 and N20 during the rotting process are to be expected they were not regarded here. These emissions depend strongly on the kind of SCM and the actual storing conditions. It is unknown how farmers are handling the material although best practice suggests to regularly turning the material for keeping aerobic conditions and by this preventing emissions that

occur only under anaerobic storing conditions. A rough calculation revealed that approximately 200 kg CO2-eq. Mg—1 FM (17 g C m—3 FM d—1 as methane and 0.411 g N m—3 FM d—1 as nitrous oxide) (Sneath et al., 2006) can be avoided within an assumed storing period of 180 days. These emissions are more than doubling the direct emissions of CO2. Thus, integrating the avoided emissions from other greenhouse gases than CO2 in this model will lead to a further considerable reduction of net emissions.

LCM does not mitigate GHG emissions, as it is assumed that thermobarical pretreatment releases feedstock constituents that - without pretreatment - would not have contributed to methane emissions during storage of untreated LCM because of their lower degradability.

When maize silage is substituted, further GHG emissions of 0.2135 kg CO2-eq. kW h—1 can be avoided. These include the entire supply chain as well as direct land use changes. This avoidance contributes to the overall mitigation by 100% in the case of LCM, 29% in the case of SCM, and 38-44% in the case of SLCM. The differences in relative mitigation are mainly attributable to the differences in SCM storage and their emission mitigation.

The net emissions of TBH treatment can be as low as 7% of the total mitigated emissions (P2-SCM, 160 "C), or exceed them by 695% (P2-LCM, 180 "C). SCM and SLCM save emissions in any case. The highest value of emissions is 11% of mitigated emissions for SCM and 27% for SLCM. LCM always emits more GHG than are mitigated. The relative values range from 112% to 695%.

The differences between the various scenarios become even more obvious if mitigation effect per year is considered. The values range from —377,504 (P2-SCM, 160 "C) to 25,628 kg CO2-eq. a—1 (P2-LCM, 180 "C). For example, P1-SLCM and P2-SCM both treated at 160 "C differ in energy-related GHG emissions from —0.353 kg CO2-eq. kW h—1 to —0.672 kg CO2-eq. kW h—, while the annual emissions vary more than 4-fold with values of —79,173 kg CO2-eq. a—1 and —377,504 kg CO2-eq. a—1 respectively. The difference in energy related emissions is mainly caused by mitigation potential of the various feedstocks while the time-related value is further amplified by the much higher fresh matter related methane yield of SCM compared with SLCM or LCM.

Treatment temperature further controls the mitigation potential. Appropriate treatment can lead to 50% higher avoided GHG emissions per year, e.g. SCM from P2 pretreated at 160 "C with a daily input of 12 m3 saves 377,504 CO2-eq. per year, whereas when pretreated at 180 "C, with even a higher methane yield than the untreated SCM, it saves only 251,906 kg CO2-eq. a—1. In most cases, 160 "C is the optimal pretreatment temperature.

The net GHG emissions described above directly influence the greenhouse gas payback time. The GPBT of a TBH device using SCM is very short, between 3 and 4 months. Pretreating SLCM leads to a GPBT of between 11 and 13 months. LCM does not have any GPBT because emissions always exceed mitigation. Related to fresh mass a substantial amount of between 22.5 and 112.4 kg CO2-eq. Mg—1 FM can be omitted by implementing a TBH device in an already existing biogas plant. The GPBM ranges from 761 (P2-SCM, 160 "C) to 3800 Mg FM (P1-SLCM, 160 "C).

3.3. Profitability and mitigation costs of thermobarical pretreatment

The annual profits range from 3763 € a—1 for P1-SLCM (180 "C) to 60,253 € a—1 for P2-SCM (160 "C) whereas LCM generates losses of up to 23,199 € a—1 (Table 7).

Depreciation and capital costs for the TBH device are responsible fortwo thirds of the overall costs and range from 3.11 (P2-SCM, 160 "C) to 86.73 ct kW h—1 (P2-LCM, 180 "C). Each, costs of electricity consumption and costs of maintenance and spare parts, account for a sixth of the overall costs, ranging from 0.75 (CMT; P2-SCM, 160 "C) to 22.90 ct kW h—1 (Cd,in; P2-LCM, 180 "C).

Table 7

Economic assessment including economic amortization period and CO2 mitigation costs.

Raw Set-point Cconstr Cel,in CMT dCWL dCspreading dCel,out dCMs Profit Profit ECAP CMC

material temperature

(°C) (ct kW h—i) (€ a—1) (P[n]Y[n]M) (€ Mg—1CO2-eq.)

P1-LCM 140 14.66 3.87 3.52 —0.99 —1.80 —2.00 —10.06 -7.19 —8584 P28Y 200

P1-LCM 160 14.01 3.70 3.36 —0.95 —1.77 —2.00 —10.06 -6.28 —7848 P25Y11M 180

P1-LCM 180 47.88 12.64 11.49 —0.81 —1.78 —2.00 —10.06 -57.36 —20,964 - 114,151

P1-SCM 140 4.61 1.22 1.11 0.08 —3.78 —2.00 —10.06 8.83 33,488 P4Y11M —36

P1-SCM 160 4.58 1.21 1.10 0.08 —3.77 —2.00 —10.06 8.87 33,870 P4Y10M —37

P1-SCM 180 3.97 1.05 0.95 —0.07 —3.50 —2.00 —10.06 9.67 42,644 P4Y2M —43

P1-SLCM 140 7.57 2.00 1.82 0.26 —1.94 —2.00 —10.06 2.36 5450 P10Y11M 20

P1-SLCM 160 7.80 2.06 1.87 0.26 —2.00 —2.00 —10.06 2.07 4656 P11Y3M 23

P1-SLCM 180 7.87 2.08 1.89 0.40 —1.87 —2.00 —10.06 1.69 3763 P11Y9M 25

P2-LCM 140 52.33 13.82 12.56 —0.88 —1.75 —2.00 —10.06 -64.01 —21,404 - -

P2-LCM 160 48.22 12.73 11.57 —0.81 —1.80 —2.00 —10.06 -57.85 —20,994 - -

P2-LCM 180 86.73 22.90 20.82 —1.47 —1.94 —2.00 —10.06 -114.98 —23,199 - -

P2-SCM 140 3.43 0.91 0.82 —0.12 —3.27 —2.00 —10.06 10.29 52,438 P3Y7M —47

P2-SCM 160 3.11 0.82 0.75 —0.21 —3.13 —2.00 —10.06 10.72 60,253 P3Y3M —50

P2-SCM 180 4.50 1.19 1.08 0.08 —3.72 —2.00 —10.06 8.94 34,748 P4Y9M —37

Cz constr C* Cel , in C* CMT dCWL dCspreading dCkout dCMs Profit* ECAM

(€ Mg— 1 FM) (Mg FM)

P1-LCM 140 4.17 1.10 1.00 —0.28 —0.51 —0.57 —2.86 -2.04 117,764

P1-LCM 160 4.17 1.10 1.00 —0.28 —0.53 —0.59 —2.99 -1.87 108,790

P1-LCM 180 4.17 1.10 1.00 —0.07 —0.16 —0.17 —0.88 -4.99 -

P1-SCM 140 5.21 1.38 1.25 0.09 —4.27 —2.26 —11.36 9.97 16,475

P1-SCM 160 5.21 1.38 1.25 0.09 —4.29 —2.27 —11.44 10.08 16,352

P1-SCM 180 5.21 1.38 1.25 —0.09 —4.60 —2.63 —13.21 12.69 13,966

P1-SLCM 140 4.17 1.10 1.00 0.14 —1.07 —1.10 —5.54 1.30 45,751

P1-SLCM 160 4.17 1.10 1.00 0.14 —1.07 —1.07 —5.38 1.11 47,390

P1-SLCM 180 4.17 1.10 1.00 0.21 —0.99 —1.06 —5.32 0.90 49,381

P2-LCM 140 4.17 1.10 1.00 —0.07 —0.14 —0.16 —0.80 -5.10 -

P2-LCM 160 4.17 1.10 1.00 —0.07 —0.16 —0.17 —0.87 -5.00 -

P2-LCM 180 4.17 1.10 1.00 —0.07 —0.09 —0.10 —0.48 -5.52 -

P2-SCM 140 5.21 1.38 1.25 —0.18 —4.97 —3.03 —15.26 15.61 12,011

P2-SCM 160 5.21 1.38 1.25 —0.35 —5.24 —3.35 —16.83 17.93 10,803

P2-SCM 180 5.21 1.38 1.25 0.09 —4.31 —2.31 —11.64 10.34 16,077

Cconstr and cConstr - Costs of construction of thermobarical hydrolysis device; Cei,in and C^ in - Costs of electricity consumption of thermobarical hydrolysis device; dCelout and dC^j out - Costs of selling electricity; CMC - CO2 mitigation costs; dCMS and dCMS - Costs of maize silage; CMt and CMT - Costs of maintenance of thermobarical hydrolysis device; dCspreading and dC*preading - Costs of spreading manure or digestate; dCWL and dCWL - Costs of feedstock transport by wheel loader; ECAP - Economic amortization period; EcAm - Economic amortization mass; LCM - Liquid cattle manure; P1 - Plant 1; P2 - Plant 2; SCM - Solid cattle manure; SLCM - Solid and liquid cattle manure.

Additional wheel loader employment compared with the reference scenario costs up to 0.40 ct kW h"1 (P1-SLCM, 180 °C). These costs have a 1-3% share in the overall costs. Up to 1.47 ct kW h"1 (P2-LCM, 180 °C) can be saved if wheel loader employment can be decreased, thus contributing 9% to the revenues.

The electricity production costs can be reduced by a maximum of 3.78 ct kW h" (P1-SCM, 140 °C) by changing the spreading procedure from solid manure to liquid digestate. The reduction in costs following substitution by LCM is up to 1.94 ct kW h"l1. Saved spreading costs amount to 12-13% of the total revenue for LCM, 20-24% for SCM, and 13-14% for SLCM.

Although methane yields of the feedstock examined differ from the statutory values, the fee attainable is fixed at 2 cent per kW hel in each case. In general, it can be stated that a methane yield higher than the statutory level is assumed to lower the share of energy crops in total feedstock to achieve the same electric power and therefore increases the fee attainable and consequently the income. This applies for LCM from plant 1 pretreated at 140 and 160 °C only. The income assigned to this bonus contributes 13-14% to the total revenues.

If related to fresh mass, the revenues are between 0.65 (P2-LCM, 180 °C) and 22.42 € Mg"1 FM (P2-SCM, 160 °C) disregarding additional income from feed-in fees according to the Renewable Energy Sources Act (German Government, 2012a) which range from 0.10 (P2-LCM, 180 °C) to 3.35 € Mg"1 FM (P2-SCM, 160 °C).

The overall electricity production costs, disregarding saved expenditures for maize silage and additional income due to Renewable Energy Sources Act, range from 1.34 ct kW he"l1 (P2-SCM, 160 °C) to 127.04 ct kW h" (P2-LCM, 180 °C). These values allow the feedstock regarded to be assessed under differing legal or economic circumstances. They demonstrate that TBH pretreat-ment is feasible as long as feedstock substituted costs more than 1.34 ct kW h"1. Considering all possible revenues, the total electricity production costs would be negative and amount to "0.66 ct kW h"1 in the best case (P2-SCM, 160 °C). Related to the targeted electricity production costs of 10.06 ct kW he"l1 - that are the costs of maize silage - these overall costs are within a very wide range: between "107% for P2-SCM (160 °C) and 1243% for P2-LCM (180 °C). The LCM variants have a negative profit and are not economically viable. The profits of the other variants range from 1.69 for P1-SLCM (180 °C) to 10.72 ct kW h" for P2-SCM (160 °C). Moreover, the energy-related values are here amplified by the methanation capability of the respective feedstock. The annual profits are then between 3763 for P1-SLCM (180 °C) and 60,253 € a"1 for P2-SCM (160 °C). If related to fresh mass, the total costs amount for 2.24 (P2-SCM 160 °C) to 6.10 € Mg"1 FM (P2-LCM 180 °C). Cano et al. (2014) presented costs for construction of a steam explosion facility for pretreating, e.g. cow manure from slaughterhouse of 3.33 € Mg"1 FM, considering a mass flow of 30,000 Mg FM a"1. These costs are higher than the construction of

A P1-LCM

• P1-SCM

♦ P1-SLCM

X P2-LCM

■ P2-SCM

140°C

160°C

180°C

m U M V-

U C ü --1

-10% 0% 10% relative change of stirring power

Fig. 3. Sensitivity analysis of the influence of changes in (a) stirring power on GHG emissions, (b) OM content on GHG emissions in terms of kg C02-eq. kW h—l1, (c) OM content on GHG emissions in terms of kg C02-eq. a—', (d) OM content on C02 mitigation costs, (e) stirring power on the economic amortization period, (f) OM content on the economic amortization period, and (g) investment on the economic amortization period.

1 nn nnn

-600.000

915 1,039

A P1-LCM

• P1-SCM

♦ P1-SLCM X P2-LCM ■ P2-SCM

140°C 160°C 180°C

relative change of OM content

Fig. 3 (continued)

▲ P1-LCM

• P1-SCM

♦ P1-SLCM X P2-LCM ■ P2-SCM

140°C 160°C 180°C

—240-

relative change of TBH stirring power 180

lllil 60!»!! 1--I

7,214 193,964

A P1-LCM

• P1-SCM

♦ P1-SLCM X P2-LCM ■ P2-SCM

140°C 160°C 180°C

Fig. 3 (continued)

a TBH device - disregarding capital costs and depreciation - that is 2.98 € Mg—1 FM at a mass flow of4200 Mg FM a—1 and although lower at a mass flow of 3360 Mg FM a—1 (3.72 € Mg—1 FM) it is assumed that they are higher in any case if regarding the economy of scale.

The ECAP lies between 3 years and 3 months in the best case (P2-SCM; 160 "C) and 28 years in the worst case (P1-LCM; 140 "C). Most feedstocks, but none of the LCM variants, are economically feasible as the device is designed for a service life

480 18,057 3,327

A P1-LCM

• P1-SCM

♦ P1-SLCM X P2-LCM ■ P2-SCM

140°C 160°C 180°C

Fig. 3 (continued)

of 20 years. The fresh mass to be pretreated for regaining the entire investment (ECAM) is between 10,803 for P2-SCM (160 °C) and 117,764 Mg FM for P1-LCM (140 °C).

If the additional electricity production costs according to this study are lower than the grid electricity production costs, the additional CO2 mitigation costs become negative. The CO2 mitigation costs of the process step regarded would result in -50 € Mg-1 CO2-eq. in the best case, thus reducing the CO2 mitigation costs of electricity in biogas plant. These costs usually range between 350 € Mg-1 CO2-eq. for biogas plants using a substantial share of LCM and 600 € Mg-1 CO2-eq. for biogas plants using energy crops only (Scholz et al., 2011).

3.4. Sensitivity analysis

Sensitivity analyses were performed to assess the viability of the model against hardly determinable parameters, to justify simplifications and to assess the impact of variations of selected input parameters on representative output parameters.

GHG emissions per unit energy increase with increasing stirring power (Fig. 3a), decrease with increasing organic matter content (Fig. 3b) and are indifferent to changes in investment (not shown here).

The increase in GHG emissions with increasing stirring power is due to the assumption that the electricity derives from the national grid with its fossil and nuclear resources. The alteration of stirring power from -50% or +50% leads to changes in GHG emissions of maximum 0.127 kg CO2-eq. kW h-1 for P2-LCM (180 °C).

OM content of the particular feedstock has a considerably stronger impact on GHG emissions than the changes in stirring power. The alteration of OM content of -50% leads to an increase of maximum 1.484 kg CO2-eq. kW h-1 (P2-LCM; 180 °C) while the increase in OM content by 50% leads to a decrease of maximum

0.495 kg CO2-eq. kW h-1 (P2-LCM; 180 °C). Owing to the increasing amount of maize silage displaced with increasing OM content, the GHG emissions of LCM scenarios decrease. Increasing OM content in SCM scenarios omits surplus emissions from storage and hence decreases the net GHG emissions.

Although the effect of OM content of a particular feedstock is non-linear and almost doubles the GHG emissions per unit energy with a decrease of 50% in the OM content of LCMs, the impact on GHG emission per year is negligible for LCMs (Fig. 3c). On the other hand, the GHG emissions per year from SCM scenarios decrease considerably with increasing OM content of the feedstock. These differences reflect the increasing yield in surplus energy deriving from feedstock with higher OM content like SCMs.

CO2 mitigation costs are significantly decreased by increasing OM content of feedstock (Fig. 3d) but are almost unaffected by changes in stirring power (not displayed here). Investment is directly influencing linearly the CMCs (not displayed here).

Scenarios with high electricity production costs and high energy related emissions compared to that of grid electricity in base scenario are strongly influenced by changes in OM content. As shown above, increase in OM content reduces the emissions related to electric energy and reduces the costs for producing that electric energy as well. Especially the CMCs of LCM scenarios are therefore significantly decreased by increased OM content. SLCM also shows a sixfold increase with decrease of the OM content of feedstock by 50%. The SCMs are slightly reduced by decrease of OM content and almost unaffected by increase.

Sensitivity analyses for the ECAP display a baseline (x-axis) at the specified lifetime of the TBH facility of 20 years or 240 months (Fig. 3e-g).

Stirring power only slightly influences the ECAP (Fig. 3e). The maximal change in the ECAP within lifetime of the TBH facility is

frnm 135 mnnths fnr the nriginal scenarin tn 130 mnnths fnr -50% and tn 138 mnnths fnr +50% stirring pnwer. That is a change nf 5.7% nnly.

In cnntrast, changes in OM cnntent strnngly influence the ECAP (Fig. 3f). The SCMs shnw an increase nf the ECAP nf 60 mnnths in average with decreasing OM cnntent by 50% and a decrease nf 18 mnnths in average with an increase nf the OM cnntent nf 50%. In the case nf LCM frnm P1 pretreated at 140 °C, 15 € per year nnly can be gained fnr paying nff the investment if the OM cnntent is decreased by 50%. That leads tn an irrelevantly high ECAP nf approximately 16,000 years. On the nther hand, hnwever, an increase in OM cnntent by 50% leads tn an amnrtizatinn nf investment after 172 mnnths which is within the specified lifetime nf the TBH facility.

ECAP nf variants with already high prnfits are mnre nr less unimpaired by changes in investment as it is given fnr the SCM sce-narins (Fig. 3g). In cnntrast, variants with nnly lnw profit are strnngly influenced: A surplus in investment nf 50% causes a dnu-bling nf ECAP fnr P1-SLCM. Especially fnr variants with profits abnve the interest rate and belnw the repayment rate as fnr P1-LCM pretreated at 140 and 160 °C prohibitively high values nccur here as well: The ECAP fnr P1-LCM pretreated at 140 °C is 18,057 mnnths (apprnx. 1500 years) if investment is increased by 50%. In cnntrast, lnwering the investment by 20% will make LCMs ecnnnmically viable as well.

The perfnrmed sensitivity analyses cnnfirm the assumptinn that the nverall results are influenced nnly marginally by the accuracy nf estimating the stirring pnwer.

The energy per time unit attainable frnm SCMs is high in cnn-trast tn the changes in energy related nutput parameters. In nppn-sitinn tn the SCMs, nnly slight changes in nutput parameters nccur if related tn time fnr the LCMs as the energy per time unit is lnw. Hnwever, if related tn energy there are substantial changes nf the nverall results.

4. Conclusions

Thermnbarical hydrnlysis is feasible fnr many feedstncks such as snlid cattle wastes and mixtures nf snlid and liquid cattle wastes. The additinnal benefits exceed the additinnal expenses in terms nf energy, greenhnuse gas emissinns and profitability. Ther-mnbarical hydrnlysis treatment nf liquid cattle waste dnes nnt prn-vide sufficient advantages cnmpared with untreated liquid cattle waste. Therefnre substituting maize silage with treated liquid cattle waste is nnt recnmmended.

In general, it is preferable tn use a feedstnck with a high fresh matter related nrganic matter cnntent and lignn-cellulnse that is usually unsuitable fnr anaerobic digestinn withnut pretreatment. Thermnbarical hydrnlysis nf such material will have shnrt tn very shnrt energetic and greenhnuse gas payback time and ecnnnmic amnrtizatinn perind.

Acknowledgement

The wnrk underlying this publicatinn was suppnrted by the European Cnmmissinn FP 6, Cnntract Nn TREN/06/FP6EN/S07. 64183/019884.

Appendix A. Supplementary data

Supplementary data assnciated with this article can be found, in the nnline versinn, at http://dx.dni.nrg/10.1016/j.wasman.2015.12. 003.

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