Scholarly article on topic 'Green conversion of municipal solid wastes into fuels and chemicals'

Green conversion of municipal solid wastes into fuels and chemicals Academic research paper on "Chemical sciences"

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Electronic Journal of Biotechnology
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{"Anaerobic digestion" / Biogas / "Conversion of waste to energy" / "Environmental problems" / Ethanol / Gasification / Hydrogen / "Public concerns" / Pyrolysis / "Uncontrolled release of greenhouse gases" / "Waste disposal"}

Abstract of research paper on Chemical sciences, author of scientific article — Leonidas Matsakas, Qiuju Gao, Stina Jansson, Ulrika Rova, Paul Christakopoulos

Abstract Presently, the society is facing a serious challenge for the effective management of the increasing amount of produced municipal solid wastes. The accumulated waste has caused a series of environmental problems such as uncontrolled release of greenhouse gases. Moreover, the increasing amount of wastes has resulted in a shortage of areas available for waste disposal, resulting in a nonsustainable waste management. These problems led to serious public concerns, which in turn resulted in political actions aiming to reduce the amount of wastes reaching the environment. These actions aim to promote sustainable waste management solutions. The main objective of these policies is to promote the recycling of municipal solid waste and the conversion of waste to energy and valuable chemicals. These conversions can be performed using either biological (e.g., anaerobic digestion) or thermochemical processes (e.g., pyrolysis). Research efforts during the last years have been fruitful, and many publications demonstrated the effective conversation of municipal solid waste to energy and chemicals. These processes are discussed in the current review article together with the change of the waste policy that was implemented in the EU during the last years.

Academic research paper on topic "Green conversion of municipal solid wastes into fuels and chemicals"

Accepted Manuscript

Green conversion of municipal solid wastes into fuels and chemicals

Leonidas Matsakas, Qiuju Gao, Stina Jansson, Ulrika Rova, Paul Chris-takopoulos

PII: S0717-3458(17)30004-0

DOI: doi:10.1016/j.ejbt.2017.01.004

Reference: EJBT 224

To appear in:

Electronic Journal of Biotechnology

Received date: Accepted date:

1 October 2016 18 January 2017

Please cite this article as: Matsakas Leonidas, Gao Qiuju, Jansson Stina, Rova Ulrika, Christakopoulos Paul, Green conversion of municipal solid wastes into fuels and chemicals, Electronic Journal of Biotechnology (2017), doi: 10.1016/j.ejbt.2017.01.004

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Electronic Journal of Biotechnology EJBT-D-16-00150 R1 Review Article Received: October 10, 2016 Accepted: January 16, 2017

Areas: Chemical Engineering; Industrial Biotechnology

Green conversion of municipal solid wastes into fuels and chemicals

Leonidas Matsakasa*, Qiuju Gaob, Stina Janssonb, Ulrika Rovaa, Paul Christakopoulosa

a Biochemical Process Engineering, Division of Chemical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Lulea University of Technology, 971-87 Lulea, Sweden

b Department of Chemistry, Umea University, 901 87 Umea, Sweden Corresponding author: leonidas.matsakas@ltu.se Abstract

Society is facing a serious challenge for the effective management of the increasing amount of produced municipal solid wastes. The accumulated waste has caused a series of environmental problems such as uncontrolled release of greenhouse gases. Moreover, the increasing amount has resulted in a shortage of areas available for waste disposal, resulting in a non-sustainable waste management. These problems led to serious public concerns which in turn resulted in political actions aiming to reduce the amount of the waste ending in the environment. These actions aim to promote sustainable waste management solutions. The main objective of these policies is to promote recycling of municipal solid waste and also the conversion of waste to energy and valuable chemicals. These conversions can be performed by using either biological (e.g. anaerobic digestion) or thermochemical processes (e.g. pyrolysis). The research efforts during the last years have been fruitful and many publications demonstrate the effective conversation of municipal solid waste to energy and chemicals. These processes are discussed in the current review article together with the change of waste policy that was implemented in EU during the last years.

Keywords: anaerobic digestion; biogas; conversion of waste to energy; environmental problems; ethanol; gasification; hydrogen; public concerns; pyrolysis; uncontrolled release of greenhouse gases; waste disposal.

1. Introduction

The rising environmental problems, involving greenhouse gases and air and water pollution together with the energy crisis and resource scarcity will become more imminent in the coming decades, dictating that we need to take actions towards a more sustainable society. Energy and chemicals are mainly produced by using fossil resources which causes the release of CO2 in the atmosphere together with other toxic compounds (such as volatile organic compounds and nitrogen oxides). It is estimated that 90% of the global emission in CO2 (which reached approximately 34

billion tons of greenhouse gases in 2011) was derived from the combustion of fossil fuels [1]. Moreover, as these raw materials are finite, processes utilizing them as feedstock are not sustainable and hence security of supply will be an important issue for humanity.

Another important challenge is the rising accumulation of wastes in the environment. The waste accumulation in the environment has raised the public awareness due to the problems the amount disposed into the environment causes. On the other hand, the huge volumes that are produced globally alongside with the diversity that these waste present makes them ideal candidates to be used for high value applications [2].

Conventional methods that are widely used for the treatment and management of municipal solid waste (MSW) include, landfilling, incineration, composting, etc., present some negative characteristics. Waste disposal by landfilling cause severe environmental issues such as uncontrolled release of methane in the atmosphere, a gas that has 20 to 23 times higher GHG (greenhouse gas) potential than CO2, production of leachate which contaminates the soil and the ground water, unpleasant odors and spread of pathogenic microorganisms [3,4]. For example, more than 95% of food waste (FW - estimated to be between 25% and 70% of MSW) are ending up in landfills, which has a catastrophic impact on the climate due to the release of methane and other GHGs (which is calculated to be 125 m3 of gas per ton of landfilled FW) [5,6]. Incineration is done for the production of heat and energy. Although it is a valuable method, especially to more remote areas, it could result in air pollution (as dioxins and similar persistent organic pollutants (POPs) can be produced) and the chance to extract valuable chemicals from the waste is lost [6,7]. Moreover, incineration presents high capital and operating cost [8] and special care has to be taken for the safe disposal of the produced fly ash during the process. Composting is the process where organic materials are stabilized using indigenous microorganisms and can be used as an environmental friendly fertilizer. On the other hand, the process of composting should be carried out properly, otherwise composting could result in problems such as strong odors and possible generation of GHGs [6]. Nevertheless, the potential of valorizing the wastes to produce valuable fuels and chemicals is an attractive alternative solution that has gain the interest of both scientific and public opinion. Our previous research has even showed that refuse-derived fuels (RDFs) based on MSW showed improved fuel properties and lower emissions of POPs during combustion when MSW from areas with extensive separation of FW in the households was used [9,10].

Although MSW could be an excellent substrate for upgrading to valuable products and energy, especially considering its availability, their handling presents some challenges. For example, MSW present a high variability in both regional and seasonal composition and volumes, which may e.g. affect the emissions generated during incineration [11]. Another challenge is the high water content which makes them prompt to microbial contaminations. This could also potentially result in public health issues since pathogens can also contaminate MSW. The high moisture content also results in increased volumes and weight of MSW making drying an important step to reduce the cost of transportation. Drying and transportation are energy and cost demanding and in turn impact the total process cost. The fact that the generation of MSW is spread in virtually every residual area can offer a solution

to avoid both drying and long-distance transportation. A decentralized system can be introduced where low-volume facilities locally use MSW [12].

2. Municipal waste in Europe

The amount of MSW produced worldwide was estimated to 2 billion tons per year with a projection of increasing to 9.5 billion tons per year by 2050 [5]. In Europe, according to Eurostat, the amount produced at 2014 was 475 kg/capita in average, which is a reduction compared to 2000 (523 kg/capita) [13]. The highest production of municipal waste is observed in Denmark (759 kg/capita) and Cyprus (626 kg/capita), whereas the lowest in Poland and Romania (272 kg/capita). The amount of waste produced in Sweden at that time was 438 kg/capita.

In 2001, the European Council adopted the first EU Sustainable Development Strategy (SDS) following the commitments taken during the earth Summit in Rio de Janeiro (Brazil) in 1992 [14]. The aim of SDS was to support actions that will improve the quality of life of current and future generations [14]. It is well established in the union that environmental issues, especially effective waste management and resource efficiency, are key priorities of European Commission as demonstrated in the Roadmap to a resource efficiency in Europe [15] and the 7th Environmental Action Programme [16]. On the field of MSW management, EU has implemented different policies setting targeting member states to fulfill concerning recycling and deflecting MSW from ending in landfill sites; with a target of 50% recycling of the household waste by 2020 [17,18]. The main policy of EU concerning the waste management is demonstrated in Figure 1 as established in the Waste Framework Directive 2008/98/EC [15,16]. According to this scheme, which reflects the policy followed by member states of EU, the highest priority is the prevention of the production of new wastes followed by their reuse and recycling. Practices like landfilling should be restricted to absolutely necessary.

Prevention

Disposal (e.g. Landfill)

Fig. 1. The EU waste management hierarchy scheme [18,19].

According to Eurostat [20] during the period 2000-2014 it was observed that there was a decline (from 523 kg/capita to 475 kg/capita) in the generated municipal wastes in EU27 (Figure 2). What is more interesting about the data collected by Eurostat is the change over time in the distribution between the different management practices of MSW. For example, the problematic practice of landfill significantly decreased from 288 kg/capita to 131 kg/capita which is equal to a 54.5% reduction over a period of 14 years. On the other hand, incineration of MSW increased by approximately 60%, recycling by 59% and composting and digestion by 53%. Over these years, landfilling of MSW dropped from 55% to 27.6% of the total waste and from a dominant practice became the second most used, after recycling (27.8% of the total municipal wastes). European Environmental Agency reports that there is an increase of the number of countries that recycle (including material recycling, compost and anaerobic digestion) more than 25% of the total municipal waste, i.e. from 11 countries to 16 during the period 2001-2010 and a decrease of the countries that landfill more that 75% of municipal waste, i.e. from 17 to 11 countries for the same period [19]. According to another report of the same agency, in Austria, Denmark, Norway, the Netherlands, Belgium, Sweden and Germany virtually no MSW is ending in landfills, which is very promising for the future of waste management in Europe [21]. As a result of the implemented MSW management policies by EU, the net emissions of CO2 reduced from 67 Mt in 2001 to 29 Mt in 2010 [19]. Although the results are very promising there is a significant variation between the results of different countries, with some countries to perform very well and other to underperform or even reduce the amount of wastes recycled [19]. For example, on 2010, 5 countries were recycling more than 50% of their total wastes, whereas 7 other countries were recycling less than 10%.

1 400 £ C ro

n 300 'EL

]jf 200 100 0

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Other ■ Composting and digestion

■ Material recycling ■ Total Incineration (Including energy recovery)

■ Landfill

Fig. 2. Distribution of municipal waste treatment in EU27 according to the data extracted from Eurostat [20].

Sweden has a long history in environmental protection which covers a wide range of actions, taking special care on the sustainable management of waste. For this reason

a number of legislation has been imposed, covering a wide range of waste management topics during the last two decades [22]. The two main waste legislations according to the same webpage are the Chapter 15 of the Environmental Act (1998:808) and the Waste ordinance (2001:1063). The Swedish Environmental Protection Agency (EPA) is responsible for guiding and supervising the authorities responsible for the inspection and enforcement and then the municipalities are responsible for implementing the waste legislation. Finally, county administration boards are responsible for the major waste treatment plants [22].

In Sweden, according to the report of the European Environment Agency, recycling and incineration of municipal waste account for the 49% each of the total amount, whereas only 1% of the total municipal wastes end in landfill [23]. According to the same report, the landfill tax that was imposed on 1st January 2000 and was raised in 2002, 2003 and 2006 (overall increase of 74%) played a vital role in the reduction of the amount of the wastes resulting in landfill. Two more important milestones in the reduction of the amount landfilled were the landfill ban on sorted combustible waste on 2002 which on 2005 was expanded to include all the organic waste with minor exceptions - Forordning (2001:512) om deponering av avfall [23,24].

All the actions that were taken in Sweden, had a result on the gradual decrease of GHGs emissions during the period 1990 to 2013, with the lowest level to be presented in 2013, although the emissions created in other countries as a result of Swedish consumption increased at the same time [25]. It can also be noted that already from the 1995, prior to the implementation of EU legislation about waste management, landfill in Sweden accounted for 35.2%, whereas the main practice for waste managements was incineration (38.3%) (Figure 3, Data extracted from Eurostat, [20]). It is also worth notice that after 2009, the amount of waste ending in landfill is very low accounting for less than 6 kg/capita. Finally, over the period 19952014 the proportion of recycling materials in the total municipal waste increased from 20.2% to 33.3% and the proportion of compost and digestion increased from 6.2% to 16.4%.

# / / # # / ^ ^ # ^ f / / ^ / # ^ ^

Material recycling Landfill

I Composting and digestion Total Incineration (Including energy recovery)

Fig. 3. Distribution of municipal waste treatment in Sweden according to the data extracted from Eurostat [20].

3. Biotechnological process

Biotechnological conversion of MSW involves the use microorganisms in order to convert the organic fraction of the MSW to different molecules. Among the most common processes of microbial conversion of MSW the production of liquid fuels (ethanol) and gaseous fuels (methane and hydrogen). The composition of MSW varies, depending on the source of the waste and this has a great impact on the yield of the subsequent processes as well. Generally, high composition in food and vegetable waste is more desirable as these waste are easier degradable and often result in high yields compared to other wastes such as papers, cardboards, etc. The effective decomposition of complex polymeric molecules (such as cellulose and proteins) to simple molecules (such as sugars and amino acids) is often considered a rate-limiting factor of the process. In order to improve the solubilization of the organic materials of the MSW, different research groups have evaluated a range of pretreatment processes such as thermo-chemical hydrolysis [26], hydrothermal [27] enzymatic hydrolysis [28] and combinations of chemical, thermal and enzymatic treatments [29]. Enzymatic hydrolysis (or saccharification) can involve a wide range of enzymes targeting in different molecules, such as cellulases and hemicellulases for lignocellulose, a-amylases, p-amylases and glucoamylases for hydrolysis of starch, lipases for fats and oils and proteases for proteins. The enzymes or mixtures of them that will be used depend on the nature of the raw material as well as with the targeted compound and the ability of the used microorganisms to hydrolyze these compounds.

3.1 Ethanol

Ethanol is considered as one of the most important liquid biofuel and has already been used as vehicle fuel since 1896 when Henry Ford designed his first car which was running on pure ethanol [30]. Ethanol can be used in vehicles after blending with gasoline at different ratios from 5% (E5) to 100% (E100 or pure ethanol). An advantage of using ethanol as vehicle fuel is that it can be directly used in conventional cars with blends up to E10, whereas when the proportion of ethanol increases some modification (such as fuel pump) are required, with blends higher than E25 to require also engine modifications [31]. Other benefits of using ethanol as a fuel are the increase of octane number in the blends (even small amounts cause disproportionately large increase in octane number) and the higher oxygen content which improves the efficiency of combustion [30,32]. Most important, the use of ethanol as fuel facilitates in the reduction of the emission of carbon monoxide, sulfur oxides, volatile organic compounds, fine particulate matter, benzene and hydrocarbons [33]. On the contrary, the disadvantages of using ethanol as fuel are the increase in the emissions of nitrogen oxides, acetaldehyde, formaldehyde and acrolein [32]. Moreover, ethanol has lower energy content compared to gasoline; equal to 66% of the energy content of gasoline [34].

MSW is a promising raw material for the production of ethanol. During ethanol fermentation, the carbohydrate fraction of MSW (e.g. glucose, fructose, starch, cellulose) can be converted to ethanol, whereas the proteins and minerals present in MSW are necessary for the growth of the fermenting microorganism.

There are many reports in the literature with different sources of MSW to be used. For example, Matsakas et al. [35] utilized household food waste (HFW) which were dried in situ. More specifically, a two-stage process was applied, where the dried HFW were initially fermented to ethanol after an enzymatic saccharification/liquefaction producing up to 43 g/L ethanol with a volumetric productivity of 2.85 g/Lh (Table 1). The remaining solids at the end of the fermentation were further utilized for the production of ethanol after the application of microwave-assisted hydrothermal pretreatment. During the second stage, an additional 16 g/L of ethanol were produced resulting in an ethanol yield of 107.6 g/kg dry material. FW from a university cafeteria in Seoul was also used Kim et al. [36]. The authors evaluated a variety of commercial enzyme solutions for the conversion of the food waste to glucose, with the most efficient combination to be glucoamylase (from Aspergillus niger) or carbohydrase (from Aspergillus aculeatus) supplemented with protease (from Bacillus licheniformis). In the fermentation stage, the SHF (separate hydrolysis and fermentation) process was more effective compared to the SSF (simultaneous saccharification and fermentation), resulting in an ethanol yield of 0.43 g/gTS (Total Solids) in comparison to 0.31 g/gTS of the SSF. SHF process was more beneficial probably due to the fact that enzymes can act under their optimal conditions prior to fermentation (normally they present higher optimal temperatures compared to the temperature that fermentation takes place). Same results of the advantage of SHF over SSF were also demonstrated in other works using various lignocellulosic materials such as sweet sorghum [37,38] and wheat straw [39].

FW from a university cafeteria was also used by Moon et al. [40], who also examined the effect of the sodium chloride concentration on ethanol fermentation. They used an enzyme mixture of amyloglucoside and carbohydrase which resulted in a glucose yield of 0.46 g/g FW in 3 h of treatment. This yield was higher compared to the yields obtained by using single enzymes, underpinning the synergistic effect of these two enzyme solutions. At the subsequent fermentation, 29.1 g/L of ethanol were produced with an ethanol yield of 0.23 g/g FW. It was also demonstrated that addition of NaCl up to 3% w/v did not had any negative impact on the fermentation, whereas for higher concentration than 4% w/v they observed an inhibition in cell growth and substrate uptake with the ethanol yield slightly decreased as well.

Starchy food waste from a university dinner center were also used by Walker et al. [41]; composed by corn, pasta and potatoes. Prior to fermentation, the FW were saccharified with the use of a-amylases and glucoamylases and ethanol was produced up to 8 g/L. On the hand, a combination of kitchen FW from both university cafeteria and households were used by Uncu and Cekmecelioglu [42]. The authors used a combination of enzymes to saccharify the FW 6 h prior to fermentation. Fermentation was optimized with experimental design and under optimal conditions 32.2 g/L of ethanol were obtained with a volumetric productivity of 0.55 g/Lh. FW with high starch content can use of starch-targeting hydrolytic enzymes were also used by other researchers resulting in ethanol production between 23.3 g/L and 81.5 g/L [43,44,45]. Other approaches to improve the ethanol production was the co-cultivation of different fermenting organisms (Saccharomyces coreanus and Pichia stipites) [46] and the application of continuous cultivation which resulted in productivity of up to 24 g/Lh [47].

High ethanol concentration could result in inhibition of the process. In order to overcome this problem, Huang et al. [48] proposed the application of vacuum for controlling ethanol under 100 g/L. The result of this process was the increase of ethanol yield from 0.327 g/g of waste to 0.358 g/g of waste. Other methods such as cell immobilization were also examined [49] in order to increase the ethanol production. More specifically they used cellulose treated corn stalk to immobilize S. cerevisiae cells, resulting in 87.9 g/L of ethanol which was 6.9% higher compared to the free cells during batch fermentation. When a continuous fermentation was applied with an HRT (hydraulic retention time) of 3.1 h, the ethanol production increased to 84.9 g/L, whereas an ethanol productivity as high as 43.5 g/Lh was achieved when the HRT was reduced to 1.55 h. Finally, Ugkun Kiran and Liu [50] used a process configuration where the enzymes necessary to hydrolyze the FW were produced by cultivating Aspergillus awamori on waste cake collected from a catering unit. The produced enzymes were a mixture of different enzymatic activities, with the main activity against starch to be the glucoamylase, and the subsequent fermentation resulted in 58 g/L of ethanol when the FW were saccharified for 24 h. Other sources of FW have also been used by different research groups, like food waste leachate from food waste resource recovery plant, where after process optimization the ethanol production reached 24.2 g/L [51]. Finally, Nwobi et al. [52] performed a compositional analysis of household wastes collected by a collection company over a period of 8 months. The main fraction of the waste was food wastes, accounting for the 74% of the total waste and followed by paper (11%). The authors prepared a model waste according to this composition which was mildly treated (85°C for 1 h) and pre-saccharified with a mixture of commercial enzymes for a variety of durations. In the following SSF process, ethanol productivity was as high as 2.14 g/Lh.

Table 1. Yields of ethanol from different sources of MSW.

Source of raw „ . . . . . . Pretreatment material Ethanol yield parameters

Enzymes _ ... , . Productivity Concentration (g/L) , ,, VM ' (g/L h) Yield (g/g solids) Reference

Household food wastes cellulases 42.8 2.85 0.10 [35]

Food waste from cafeteria - glucoamylase and carbohydrase n.a. n.a. 0.43 [36]

Food waste from cafeteria amyloglucoside and carbohydrase 29.1 1.94 0.23 [40]

Food waste from dinner -center a-amylases and glucoamylases 8 n.a. n.a. [41]

Food waste a-amylases,

from cafeteria and - amyloglucosidase, cellulase and p- 32.2 0.55 0.16 [42]

households glucosidase

Food waste from cafeteria - Glucoamylases and p-glucanase 48.6 2.03 n.a. [46]

Food waste from retail store - a-amylase, glucoamylase and protease n.a. n.a. 0.36 [48]

Food waste from dinner -center a-amylase and glucoamylase 87.9 1.83 n.a. [49]

Food waste from cafeteria - Enzyme solution produced in-situ by A. awamori 58 1.81 n.a. [50]

Leachate from

food waste resource - - 24.2 0.61* n.a. [51]

recovery plant

Model organic Mixture of cellulase,

fraction of municipal solid waste 85oC for 1 h amylase, protease, hemicellulose, lipase and pectate lyase 42.8 2.14 n.a. [52]

n.a., not available

The productivity was calculated by dividing the ethanol concentration with 40h which was mentioned in materials and methods as the period that the fermentation lasted.

3.2 Methane

Biogas (methane) is a gaseous biofuel which is produced through the digestion of organic materials in the absence of oxygen and consist mainly of methane and CO2. Other compounds that are produced during the digestion include sulfur compounds (hydrogen sulfide and alkythiols), ammonia, alcohols, carbonyl compounds (including aldehydes and ketones), carboxylic acids (such as formic acid, acetic acid, etc.), terpenes and aromatic compounds [53]. The process of anaerobic digestion is a complex procedure which involves different stages, each one catalyzed by different consortia of microorganisms. These steps involve the hydrolysis of complex molecules to monomers, which is followed by the steps of acidogenesis, acetogenesis and methanogenesis [54,55].

The applications of biogas involve its use as vehicle fuel (after enriching its composition in methane) and for the production of heat and electricity through burning in special equipment which is called CHP (Combined Heat and Power). The use of methane as vehicle fuel presents some benefits as it does not require extensive modification of the vehicle and it is a technology that has already proven in practice [56]. Compared to other fuels, such as ethanol, production of methane is more efficient, resulting in a higher ratio of output to input energy which reaches up to 28 [57,58]. For the production of combined heat and power different apparatus are used, such as internal combustion engines, microgas turbines and solid oxide fuel cells (SOFC) [59] with varied efficiencies. For example, the use of traditional internal combustion engines result in electrical efficiencies in the range of 30-35% (with the power ranging between 1 and 500 kW), whereas novel technologies such as solid oxide fuel cell systems can reach efficiency of 65-70% [60].

MSW has been widely used as raw material for anaerobic digestion as it presents a favorable composition as they are normally rich in carbohydrates, proteins and minerals. Apart from the source of MSW, other factors that affect the digestibility of MSW is the carbon to nitrogen (C:N) ratio, the particle size, the water content (wet vs dry digestion) and temperature (phychrophilic vs mesophilic vs thermophilic) of the digestion and the organic load that is fed in the digester. Presence of a pretreatment or conditioning step prior to digestion could have a positive impact on the digestion as it could result in easier decomposition of more recalcitrant compounds. Use of anaerobic digestion for MSW treatment instead of other processes, like incineration and gasification, requires significantly less investment costs and has a lower gate fee [61]; underpinning the importance of anaerobic digestion as MSW treatment method.

Generally, food waste result in higher methane yields compared to MSW that are consisting mainly from other organic materials (such as papers, cardboards, etc.).

For example, Zhang et al. [62] reported a methane yield of up to 435 mL CH4/gVS (volatile solids) on FW collected from different sources (restaurants, food markets, hotels and businesses) (Table 2). In order to increase the composition in FW and remove all the rest of organic materials (e.g. wood) and inorganic contaminants (e.g. plastic), the original waste went through a screening process in a waste management company. Davidsson et al. [63] evaluated different OFMSW (organic fraction of municipal solid waste) for their methane production potential in both at lab and pilot scale. The OFMSW differ from each other regarding their origin (single family houses or apartment blocks), the wrappings and sack type in refuse been (if any) and the pretreatment method subjected (screw press device, disc screen, shredder + magnetic separation, piston press device or food-waste-disposer-system). The methane yields during lab-scale varied between 300 and 570 mL CH4/gVS and the authors did not observe any systematic variations that were caused by the difference in origin or treatment. The same values for pilot-scale digestion varied between 275 and 450 mL CH4/gVS.

On the other hand, use of the organic fraction of MSW which is rich in paper waste can result in poor methane yields, such as the results reported by Macias-Corral et al. [64] where the methane yield was 37 mL CH4/gVS using OFMSW containing 62% of paper. A strategy proposed by the authors in order to improve this yield, was the co-digestion with cow manure, which resulted in improvement to 172 mL CH4/gVS. Co-digestion can generally result in improving the nutrient deficiencies of the waste digested. For example, nitrogen poor waste can be improved by the addition of nitrogen rich waste resulting in a better C:N ratio. This was verified by Rivard et al. [65], who supplemented the digestion of MSW with either yeast extract and minerals or with pre-anaerobically digested municipal sewage sludge with both acted as nutrient supplementation. The same positive impact of the co-digestion of OFMSW with primary sludge and thickened excess activated sludge was also demonstrated by Sosnowski et al. [66], when they operated the digestion in two-stage quasi-continuous mode. Under these conditions the biogas yield improved from 419 mL/gVS to 532 mL/gVS (with a methane content above 60% in all cases). The concept of co-digestion was also employed by Elango et al. [67] where MSW was co-digested with domestic sewage at different organic feeding rates. Under the optimal rate of 2.9 gVS/mL/day the methane yield reached a value of 360 mL CH4/gVS. Finally, Martín-González et al. [68] evaluated the co-digestion of source collected OFMSW with FOG (fat, oil and grease waste) from sewage treatment plants at different ratios between the two wastes; with a ratio of 7:1 (MSW:FOG) to be the optimal. During the continuous digestion it was demonstrated that with the co-digestion system the methane yield increased to 350 mL CH4/gVS compared to the continuous digestion of MSW which resulted to 240 mL CH4/gVS.

Concerning the effect of the temperature, the digestion normally takes place under either mesophilic (25-35°C) or thermophilic (45-60°C) conditions, with both of them to present different positive characteristics. Generally, thermophilic conditions are considered more favorable as they result in faster digestion, higher methane production, and in addition the higher temperatures result in hygienization of the sludge minimizing contamination problems [69,70]. Thermophilic conditions were found to more efficient for the digestion of MSW by Cecchi et al. [71] resulting in 2-3 times higher gas production rate compared to mesophilic digestion. In another study, these authors reported that after altering different operational conditions, like

increasing the organic loading rate, increase the hydraulic retention time, temperature and the total solids of the feeding, the digestion at thermophilic conditions resulted in stabilization of the system [72].

Forster-Carneiro et al. [73] evaluated the effect of the source of MSW on the performance of anaerobic digestion by comparing source separated MSW and mechanically selected MSW. Both sources resulted in similar accumulative methane production (25 L for source separated vs 29.9 L for mechanical selected) and high removal of organic matter with the mechanically sorted to score better results. The pattern of digestion was different between these two materials, with the source separated showing an initial fast digestion rate which then after following an acclimation stage was stabilized, whereas the mechanical selected showed different peak production rates throughout the digestion.

Another process configuration that has been evaluated is multi-stage digestion. This strategy has the benefit that it could result in improved digestion of more recalcitrant substrates, as Hartman and Ahring [74] proposed in their work. More specifically, they applied a post-digestion treatment at hyperthermophilic conditions (68°C) when using source-separated OFMSW in order to convert the most recalcitrant particles to volatile fatty acids (VFAs), which then were recycled back to the digestor and converted to biogas. At the same time, higher temperatures facilitated the stripping of ammonium which could be used as fertilizer. The use of the two-stage system resulted in 7% increase of VS reduction compared to the control (reaching 78-89%) with the biogas yield reaching 640-790 mL/gVS. The authors mention that there was no difference in the methane composition of biogas in between the two-stage and one-stage digestion.

Finally, another strategy to improve the biogas yield from MSW is the application of a pretreatment step prior to digestion. Pretreatment can result in better hydrolysis of complex molecules and especially more recalcitrant molecules such as cellulose, which in turns improves their digestibility. Different pretreatment techniques can be applied alone or in combination. For example, Del Borghi et al. [75] evaluated the effect of bacterial pre-hydrolysis on biogas yield, alone or in different combinations with heat treatment (121°C, 20 min) and chemical treatment (4 g/L NaOH). The optimal hydrolysis of the OFMSW was obtained with the combination of all these three together with the biogas yield to increase from 360 mL/gVS (180 mL CH4/gVS) for the untreated to 870 mL/gVS (400 mL CH4/gVS) for the treated MSW. Another pretreatment concept applied by Liu et al. [76] was to pretreat the un-digested solids that contain the more recalcitrant compounds with steam explosion (240°C, 5min) and further digest them in a secondary digestion. This system of the two digestions resulted in an increase by 43.3% of the methane yield from 247 mL CH4/gVS to 355 mL CH4/gVS, whereas application of another steam explosion pretreatment of the remaining solids from the second digestion resulted in a further increase of the methane yield to 381 mL CH4/gVS (54.2% higher than the first digestion). Ma et al. [77] examined the effect of a variety of pretreatment methods on biogas yield from industrial kitchen wastes. More specifically they evaluated the effect of acid pretreatment (HCl, pH 2, 18°C, 5 min), thermal (120°C, 30 min, with 30 min heating and cooling each), combined thermal and acid, pressure-depressure (10 bar with CO2 and depressurize) and freeze-thaw (from -80°C to 55°C). From all the tested methods, pressure-depressure resulted in the highest biogas yiels (520 mL/gCOD -

Chemical oxygen demand), with the freeze-thaw and thermal treatment to have a small improvement compared to the control and the combined acid-thermal and acid treatment to have a negative impact.

Table 2. Results of anaerobic digestion of different sources of MSW.

Source of raw material Treatment / other operational conditions Yield (mLCH4/gVS) Reference

Food waste from different sources Screening and grinding to remove impurities ^ 435 [62]

OFMSW from houses or apartments Different mechanical treatments 300-570 [63]

OFMSW rich in paper waste - f 37 [64]

OFMSW rich in paper waste Co-digestion with cow manure 172 [64]

MSW Supplemented with yeast extract and minerals 336 [65]

MSW Supplemented with pre-digested municipal solid sludge 307 [65]

OFMSW Operation in two-stage quasi-continuous mode 419 [66]

OFMSW Co-digestion with primary sludge and thickened excess activated sludge in two-stage quasi-continuous mode 532* [66]

MSW from yard Co-digestion with domestic sewage 360 [67]

OFMSW 4- 240 [68]

OFMSW Co-digestion with FOG waste at a ration 7:1 (MSW:FOG) 350 [68]

Source separated OFMSW Post-digestion treatment at hyperthermophilic conditions with recycling of the treated particles to the digestor 640-790* [74]

OFMSW from households 180 [75]

OFMSW from households Bacterial hydrolysis in combination with thermal and alkaline treatement 400 [75]

MSW - 247 [76]

MSW Two-stage digestion, with the remaining solids after first stage to get pretreated with steam explosion and digested 355 [76]

MSW As before with the remained solids of secondary r digestion to be pretreated with steam explosion and digested 381 [76]

Industrial kitchen waste Pretreatment with pressurize-depressurize 520 [77]

Results are presented as mL biogas per g VS. ** Results are presented as mL biogas per g COD.

OFMSW: Organic fraction of municipal solid waste; FOG: Fat, oil and grease waste

3.3 Hydrogen

Hydrogen (H2) is a form of gaseous fuel which is recognized as another promising alternative to the traditional fuels. Hydrogen is an environmentally friendly form of energy as it only produces water instead of GHGs during combustion and can also directly be used for the production of electricity through hydrogen fuel cells or other type of fuel cells such as SOFC [78,79]. It has a high energy yield (122 kJ/g; which is 2.75 times higher than that of hydrocarbons) and is mostly produced from fossil-derived materials (approximately 95% globally) [78,80,81]. In order to minimize the dependence of fossil sources, it is important to switch the production to microbial conversion of renewable resources. Biological production of H2 can be classified as fermentation (photofermentation or dark fermentation) or photosynthetic (direct bio-photolysis of water by algae and cyanobacteria or bioelectrohydrogenesis in i.e. microbial fuel cells) processes [81,82]. Due to the nature of the fermentative process

where an organic compound is required, it is considered more advantageous as it can utilize different wastes as raw materials. Moreover, production of H2 through dark-fermentation is estimated to have 340 timer lower process costs compared to photosynthetic processes [83]. Fermentation can take place by using either pure cultures (such us Clostridium sp.) or mixed cultures [78]. When mixed cultures are used, the produced H2 can potentially be utilized by hydrogenotrophic bacteria and for this reason heat treatment of the sludge is widely used in order to suppress this side path [7].

The use of MSW for the production of H2 is another alternative for the conversion of this material to a high value fuel. For example, Alzate-Gaviria et al. [84] digested OFMSW derived from a cafeteria (containing food waste, recycled paper and cardboards) in a packed bed reactor, resulting in 99 NmL H2/gVS (maximum hydrogen yield of 23%) at a mass retention time (MRT) of 50 d (Table 3). The nature and composition of the MSW used greatly affect the hydrogen yields, which was showed by Dong et al. [78] in studies based on different sources of MSW with varying composition. Materials rich in more simple forms of carbohydrates achieved higher H2 yields, whereas oil and lignocellulosic materials resulted in very poor yields (Table 3). When fat was used as raw material no H2 was detected. The authors also observed that with protein substrate (lean meat), only a few mL of gas was produced and no hydrogen was detected during incubation with carbon dioxide. A similar study evaluating different MSW materials [85] demonstrated similar trends, with the carbohydrate rich material resulting in higher yields, whereas the yields were significantly lower for protein-rich material and fats (Table 3). Another study evaluating the effect of the source of the material on hydrogen yield was performed by Kobayashi et al. [86], in which more complex waste was utilized. More specifically they used separated domestic waste which were classified as 20 different waste fractions, ranging from animal and vegetable kitchen waste, to newspaper waste and used paper. They demonstrated that the hydrogen yield varied significantly between the different waste fractions and after a statistical analysis of the relationship between the composition of the different fractions and the hydrogen yield by PCA, they concluded that carbohydrate rich feedstocks result in higher hydrogen yield ranges compared to protein rich, fat rich and cellulose rich feedstocks.

A process to improve the poor hydrogen yields from lipid-rich waste (more specifically lard) was later proposed by Kim et al. [80]. As they pointed out, presence of CO2 can result in the synthesis of succinate and formate from CO2, pyruvate and NADH, whereas removal of it will allow the NADH to be re-oxidized to NAD+ with simultaneous production of H2. For this reason, they evaluated the effect of CO2 removal on the hydrogen yields by stirring and/or CO2 scavenging with KOH. Stirring alone resulted in an increase of hydrogen yield from 4.5 mL H2/gVS to 40.3 mL H2/gVS. On the other hand, the use of a KOH trap for CO2-scavenging together with the stirring further improved the yield to 185.8 mL H2/gVS. This system had an important impact on the concentration of VFAs which were reduced from 522.2 mg/L to 278.8 mg/L with the main impact of this reduction to be observed on propionic acid, valeric acid and n-butyric acid.

The hydrolysis of the organic particles present in the MSW is often considered a crucial part and rate limiting factor of the process. One strategy to resolve this and improve the hydrolysis rates is the addition of surfactants like Tween 80 and

polyethylenglycol (e.g. PEG 6000), a strategy that was proposed by Elsamadony et al. [87]. More specifically the use of Tween 80 (2.8% v/v) yielded 109.9 mL H2/gcarbs and the use of PEG 6000 (16.7 g/L) resulted in 113.8 mL H2/gcarbs; both being higher compared to the yield of the control (87.4 mL H2/gcarbs). A combination of both (2.8% v/v Tween 80 and 1.7 g/L PEG 6000) further improved the production of hydrogen to 116.7 mL H2/gcarbs.

Generally, the MSW originating from kitchens (containing from food waste to papers) is considered a very good substrate for hydrogen production. Jayalakshmi et al. [88] used kitchen wastes from a hostel which were consisting mainly by food waste and vegetable waste (66% and 27% respectively), followed by packing material (1.4%), egg shells (1.1%) and tea waste (1%). After the initial stabilization of the hydrogen producing system which lasted 21 d, the hydrogen yield obtained was 72 mL H2/gVS added. In another study, the same group used a slightly different approach for the conversion of kitchen waste to hydrogen, where they isolated hydrogen producing organisms from the biogas plant sludge [89]. The most efficient hydrogen producing bacterium isolated was Clostridium acetobutylicum which produced 185.9 mL H2/kgTS/day, followed by Serratia marcescens with a production of 171.2 mL H2/kgTS/day. Other authors have also used food waste from different sources, such as dining halls, for the production of hydrogen, [90] achieving high hydrogen yields (91.5 mL H2/gVS) underpinning the important of the food waste fraction among the MSW for the biohydrogen process.

Finally, other research groups have evaluated the effect of ultrasonication pretreatment on the digestibility of food waste. Elbeshbishy et al. [91] examined the effect of ultrasonication time by keeping the TS constant at 6.5% and at the optimal conditions, hydrogen yield was 141 mL/gVS, with a productivity of 2.5 mL/h and a specific energy input of 23,000 kJ/kg TS. In a later work, Gadhe et al. [92] optimized both the ultrasonication time and the TS content that the treatment took place, and despite the fact that under optimal conditions (8% TS and 12 min ultrasonication) hydrogen yield was similar (149 mL/gVS), the hydrogen productivity was higher (5.2 mL/h) and the energy inputs was significantly reduced to 13,500 kJ/kg TS.

Table 3. Results of hydrogen production from different sources of MSW.

Source of raw material Treatment / other operational conditions Hydrogen yield (mL H2/gVS) Reference

OFMSW from cafeteria - 99 [84]

Rice - 134 [78]

Potato - 106 [78]

Lettuce - 50 [78]

Oil - 6.3 [78]

Banyan leaves - 1.8 [78]

Cabbage - 23.6-61.7 [85]

Carrot - 44.8-70.7 [85]

Rice - 19.3-96 [85]

Lean meat - 2.5-7.7 [85]

Chicken skin - 3.6-10.2 [85]

Egg - 2.6-7.1 [85]

Fats - 4.4-11.1 [85]

Lard - 4.5 [80]

Lard Fermentation was under stirring 40.3 [80]

Lard Fermentation was under CO2- 116.7 [80]

scavenging

Lard Fermentation was under stirring and CO2-scavenging 185.8 [80]

Restaurant wastes - 87.4 [87]

Restaurant wastes Addition of 2.8 v/v Tween 80 109.9 [87]

Restaurant wastes Addition of 16.7 g/L PEG 6000 113.8 [87]

Restaurant wastes Addition of 2.8% v/v Tween 80 and 1.7 g/L PEG 6000 116.7* [87]

Kitchen waste Inoculum was heat treated at 100oC for < 72 [88]

30 min prior to digestion \

Kitchen waste Digestion performed with isolated C. acetobutylicum 185.9** [89]

Kitchen waste Digestion performed with isolated S. marcescens 171.2** [89]

Food waste from dining room - 91.5 [90]

Pulp from facility processing municipal organic waste - 80 [91]

Pulp from facility processing municipal organic waste Pretreatment wth ultrasonication for 30 min with 2 sec sonication pulses 141 [91]

Food waste from cafeteria Pretreatment wth ultrasonication for 12 min with 2 sec sonication pulses 149 [92]

^Results are presented as mL H2/gcarbs ** Results are presented as mL H2/KgTS/day

3.4 Other biological products

In addition to the three main products described before, MSW has also been used for the production of other bio-products. For example the bacterium Clostridium beijerinckii P260 was used for the production of butanol through ABE (aceton, butanol, ethanol) fermentation [93]. The substrate used in this work was FW from a local retailer store and the amount of ABE produced reached 18.9 g/L with the butanol reaching 12.3 g/L. In two other studies, succinic acid was produced from waste bread and bakery waste [94,95]. In both studies, Actinobacillus succinogenes was used as the fermenting microorganism and prior to fermentation the waste was hydrolyzed by enzymes produced by A. awamori and Aspergillus oryzae in order to release glucose and free amino nitrogen. When bread waste was used, 47.3 g/L of succinic acid was produced [94], whereas succinic acid production from bakery waste reached was 24.8 g/L for cake hydrolysates and 31.7 g/L for pastry hydrolysate [95].

In another work, Zhang et al. [96] used food waste from a dining room in order to produce biopesticide from the bacterium Bacillus thuringiensis. During a semi-solid fermentation with a water content of 75%, the amount of 5-endotoxin produced in the media reached 862 pg/mL. After adjusting the pH and product inhibition, the authors observed a toxin production up to 2478 pg/mL after four fermentation loops. Food waste from a canteen has also been used for the cultivation of microalgae (Schizochytrium mangrovei and Chlorella pyrenoidosa) after hydrolysis of the waste with A. awanori and A. oryzae [97]. When S. mangrovei was used, the biomass concentration reached 14 g/L with an oil content of 16% (of which DHA reached 40mg/g of biomass). The biomass concentration of C. pyrenoidosa reached 20 g/L with an oil content of 20% (of which a-linolenic accounted for 30 mg/g biomass).

Finally, another category of microbial products that have been produced from MSW are different categories of enzymes. This involves the production of amylases from FW by A. niger [98] and from bread waste by using A. oryzae [99]. Other authors have observed production of different carbohydrase from food waste [4] or kitchen waste [100]. Pectinolytic enzymes have also been reported to be produced by a variety of different fruit wastes and proteases from a wide range of food wastes such as shrimp waste and waste bread [8,99,101]. Finally, another category of enzymes produced were lipases where waste such as waste cooking oil have been used to cultivate microorganisms such as Yarrowia lipolytica [102] or different Aspergillus and Penicillium strains [103].

4. Thermochemical processes

Thermochemical conversion technologies for sustainable integration of MSW mainly include gasification, pyrolysis and torrefaction. These processes are generally characterized by elevated temperatures and fast conversion rates in comparison with biochemical treatment processes. Gasification, pyrolysis, and torrefaction are all performed in the absence of oxygen or with significantly less oxygen present than what is required for complete combustion [104]. The operating conditions (e.g. temperature, heating rate and oxygen supply) and the yield of products (gas, oil/condensables and char) varies between these three processes. The fast heating rates and moderate temperatures of pyrolysis favor generation of liquid products, while the low temperatures and long residence times of torrefaction primarily yields chars, and the high temperatures and heating rates of gasification generate mainly gas products (condensable and non-condensable gases). Gasification, pyrolysis and torrefaction cannot be defined as completely separated processes in the sense that pyrolysis for example can be considered as an incomplete gasification process, and torrefaction an initial stage of gasification and pyrolysis. Combustion is naturally also a thermochemical conversion process, however, with heat and/or power as the main output, and is therefore not included in the scope of this review. In Figure 4, the major differences of combustion, gasification, pyrolysis and torrefaction are illustrated in terms of operating conditions and conversion products.

Combustion (--> heat & power)

02 (air)

Gasification (--> syngas): >700 °C

Pyrolysis (-> bio-oil): >400-650 °C

Torréfaction (--> bio-char): >200-350 DC

Non condensable gases /pj Water

Biomass

' Organic vapors Char

Heat (02 absent)

CO, co2

Organic vapors

Heat (02 absent)

Tar Char

Heat (under 02 and steam

CO 2 H20 Heat

Fig. 4. Comparison of combustion, gasification, pyrolysis and torrefaction, with major products illustrated (figure modified from Yin [105]).

The advantages of thermochemical conversion of MSW compared to traditional MSW incineration are mainly related to the increased energy efficiency and generation of value-added products, as well as improved pollution control [106]. The intermediate products from thermochemical conversion may be suitable for a wide range of applications, from high-quality fuels to fine chemicals. The low operating temperatures compared to MSW incineration can also potentially reduce the risk of alkali volatilization, fouling, slagging and bed agglomeration [107]. Furthermore, thermochemical conversion systems for gasification and pyrolysis are commonly equipped with product cooling and collection units which may enable improved control of emissions of organic and inorganic pollutants [108].

Despite the advantages in energy and material recovery, thermochemical conversion of MSW remains a subject of debate due to the potential negative environmental impact. Similar to combustion-based processes, trace amounts of inorganic and organic pollutants (e.g. heavy metals and dioxins) could be formed and emitted into the air, soil and water body. The characteristics and distribution of these pollutants are highly dependent on the operating conditions and technologies used, which will be discussed in each subsection.

Currently there are more than 100 commercial scale plants in continuous operation worldwide for MSW thermal treatment, with capacity ranging from 10 to 250 kton/year [109]. However, compared to conventional MSW incineration, thermal treatment at moderate temperatures (i.e., gasification and pyrolysis) involves more complex processes and is still at the stage of technical development. MSW is generally a complex mixture of various materials which largely differs in physical and chemical characteristics. An integrated waste management system involving different types of pre-treatment is often required, including mechanical treatments, mechanical biological treatments and mechanical heat treatments [110]. The aim of such preprocessing is to improve the combustibility of the waste as well as to recover as

many recyclable materials as possible. In addition to pretreatment strategies that involves mechanical sorting, torrefaction (conducted at moderate temperature and oxygen-deficient conditions) can also be applied to reduce the moisture content and to generate a MSW-based fuel for further thermochemical conversion [110].

Table 4 summarizes the typical processing parameters of different thermal conversion technologies and the influence on product yields and chemical compositions. The types of input MSW, operating conditions and the types of reactors are highlighted due to their relevance on the physical and chemical properties of the conversion products. The studies listed in Table 4 includes experiments ranging from lab-scale to up-scaled pilot and commercial plants.

4.1 Gasification

Gasification is a partial oxidation with lower oxygen supply than required for complete stoichiometric combustion [104]. The operating temperature is generally within the range of 800-1200°C, depending on the type of reactor and the feedstock composition. The process is largely exothermic but heat is sometimes required to initialize and sustain the gasification process. The partial oxidation can be carried out using air, oxygen, steam, carbon dioxide or a mixture of these as gasifier agents. The gas product (commonly referred to as syngas in literature) is a mixture of carbon monoxide, hydrogen, carbon dioxide, methane and other low molecular weight hydrocarbons. It also contains a number of undesired components such as particulate matter, tar, alkali metals, chlorine and sulphide [111].

Gasification of MSW is advantageous over MSW incineration, primarily due to the possibility to produce syngas that can be used as clean fuel gas in a conventional burner, or coupled to a boiler or a steam turbine. The possibility to combine different operating conditions with a specific reactor enables adjustment of the syngas composition for different applications. However, the MSW gasification technology is still in a development stage, and the number of plants commercially available worldwide is limited [107]. Flue gases from gasification contain particulate matters, acidic gases (e.g., nitrogen oxides and hydrogen chloride) and organic pollutants such as dioxins. In addition, the final process residues with potential leachability of heavy metals and organic pollutants represents a major environmental concern. The emissions of these pollutants are nowadays controlled by end-of-pipe technologies such as electrostatic precipitators, bag filters and addition of slaked lime [107]. The costs required for syngas conditioning and cleaning in gasification-based MSW treatment is high compared to MSW incineration.

Various gasification reactors have been used in practice, including entrained flow gasifiers, fluidized bed gasifiers, cyclone gasifiers and packed bed gasifiers [106]. The entrained flow gasifiers in commercial operation generally have large capacities with installed plant sizes of up to a few thousand tons per day [112]. Fluidized bed reactors have the advantage of sufficient mixing and high heat transfer, resulting in uniform and efficient reactions. For most types of reactors, the feeding materials must be properly granulated. Sample pre-processing is often needed, in particular for heterogeneous materials such as MSW, to obtain feeding materials with consistent physical and chemical characteristics. Such pre-processing methods include sorting/separation, shredding, grinding, blending, drying and pelletization.

Development and integration of various innovative technologies in thermochemical processes is thus desirable to reduce the need of pre-treatment of the MSW [113].

Table 4. Comparison of operating parameters and product yields in different thermochemical _conversion technologies._

Conversion Main findings (e.g., product yield, product _ .

techno!ogy Feedstock Key operating parameters Type of reactor properties) References

Gasification

Olive oil residues, meat and bone meal, dried sewage sludge

Temperature was in the range of 770-870°C. The air ratio varied from 0.23 to 0.43. Solid feeding flow rate ranged from 8 to 18 kg/h.

Pilot-scale bubbling fluidized bed

Gas yields ranged from 0.89 to 1.47 Nm3/kg (expressed as dry and nitrogen free gas per kg of dry and ash free feedstock). The gas yield increased with increasing stoichiometric ratio. Yield of CO decreased with increasing stoichiometric ratio. Yield of H2 decreased from 5% to 0% when stoichiometric ratio decreased from 0.25 to 0.45 for meat and bone meal. High potassium content in the olive oil residue promoted sintering of the bed. Meat and bone meal yielded gas with low heating value compared to other fuels.

Gasification

Polyethylene, bamboo

Processing temperature was between 600-800°C. Equivalence ratio varied from 0.2 to 0.5. Steam/feedstock ratio varied from 0.4 to 1.0.

The yield of H2 was between 7.0-17.7% depending on the gasification temperature. Bench-scale fixed Combustible gas components and heating value bed with air or steam °f syngas decreased as the equivalence ratio increased. Higher temperature favored H2 and CO production. The optimal temperature was about 700°C for obtaining syngas with high heating value. The yield of Co decreased as steam/feedstock ratio increased for polyethylene, but no significant effect on bamboo.

as gasification agents

Pyrolysis

MSW collected from a waste treatment plant

300-700°C with a residence time of 1-5 h at a heating rate of 5°C/min

Lab scale muffle furnace

Char yields decreased from 65.6% to 27.8% as temperature increased from 300 to 700°C. Increasing temperature increased aromatization and stability. Low temperatures resulted in higher cation exchange capacity and available nutrients. Residence time influenced hydrophobicity and thermal stability._

450-800°C with residence . , , . , , Highest char surface area was obtained at 550-I ah-scalp tubular

Pyrolysis MSW time of 15 min. CO2 and N2 g^ctncalfurrace 600°C. N2 or CO2 as inert gas gave comparable [117]

were used as sweeping gas. results. Moderate temperature favored the char

porosity development.

Pyrolysis/ torrefaction

Poultry litter

300-600°C with reaction time Lab-scale muffle varying from furnace

130-372 min

Char yields were 60%, 52% and 47% at pyrolysis temperatures of 300, 400 and 500°C, respectively. Organic carbon content and cation exchange capacity of char products decreased with increased temperature, while surface area and organic carbon stabilities of char increased with increased temperature

Torrefaction Synthetic MSW

250-450°C under N2 atmosphere for 30 min.

Quartz tube in muffle furnace with N2 purged.

Char yields increased from 58.4% to 86.8% when temperature decreased from 400°C to 250°C. Torrefaction at 300°C resulted in chars with calorific values comparable to those of standard coal. The chlorine content of the char (at 300°C) was 57% lower than in the feedstock.

Hydrothermal treatment

Food waste and paper

Reactors were heated at 250°C for 20 h.

Stainless steel tubular reactor (160 mL)

Hydrochar yields were 43.8% and 29.2% for food waste and paper, respectively. 49-75% of the initially present carbon was retained within the chars.

Hydrothermal treatment

Chicken manure

Reactors were heated to temperatures of 140-220°C and was maintained for a duration of 25-60 min under constant agitation. The_

Stainless steel reactor (200 mL) equipped with a cylindrical electric heater

Reduction of total solids in hydrochars ranged [121]

from 18 to 23% at a temperature of 220°C.

average heating rates were 11-14°C/min

A major challenge in MSW gasification is the formation of tar substances which can potentially lead to blocking, fouling and corrosion. Tar is a complex mixture of condensable high molecular weight hydrocarbons. The differences in tar compositions are mainly a function of processing conditions. Tar removal can be achieved by different methods including physical (e.g. filters, scrubbers, wet electrostatic precipitators) [122] and chemical processes (e.g. thermal and catalytic cracking) [114,123]. Steam gasification has recently emerged as a promising technology for generating syngas with reduced tar formation and high yield of hydrogen [124]. In this process, steam is the only gasifying agent used and almost no exothermic reaction is involved. Factors influencing the syngas yield are mainly MSW composition, particle size, temperature and steam-to-feedstock ratio. Addition of catalysts such as calcine dolomite has been shown to increase carbon conversion efficiency and tar content. Hu et al. [125] have proposed an in-situ MSW gasification technique with a downstream fixed bed reactor using CaO as catalyst and CO2 sorbent. They studied the effect of catalyst and reactor temperature on the yield and product composition and observed that complete tar decomposition can occur at elevated temperatures around 950°C when using calcined dolomite as catalyst.

Studies of MSW gasification reported in literature have also been focusing extensively on optimization of reaction conditions such as temperature, heating rate and catalysts, in order to improve the yield and quality of the syngas and meanwhile reduce the amount of undesired by-products [114,115,126]. Panepinto et al. [127] conducted a study on energy efficiency and environmental performance of MSW gasification and made a comparison with MSW incineration. They suggested that co-firing of syngas in large-scale power plants or the use of combined cycle gas turbines can improve energy conversion efficiency. A study by Consonni and Vigano [128] showed that gasification at pressures higher than atmospheric pressure can improve the energy conversion efficiency. It was further pointed out that the process of high pressure gasification is still challenging, currently for commercial stage applications.

An emerging technology involving thermal plasma gasification for MSW treatment has received increasing attention recently. The advantages are mainly related to improved energy recovery efficiency associated with the fast reaction times, low amount of oxidant and high heat flux densities. Studies have shown that the high temperature of the plasma arc can reduce the formation of tar and other undesirable products in the syngas [129]. The solid residues are produced in the form of a vitrified slag, which can be utilized in construction. One demonstration plant has been operating in Korea since 2010 [129]. An integrated furnace equipped with non-transferred thermal plasma torches was successfully applied for direct treatment of MSW with a capacity of gasification of MSW at 10 ton/d. A comprehensive overview on the types of plasma working gases and various types of reactors has been given by Tang et al. [117].

4.2 Pyrolysis

Pyrolysis is a thermochemical decomposition process conducted under oxygen-deficient conditions and typically at temperatures ranging between 300-650°C [130].

The major products of pyrolysis are char and condensable gases. Part of the condensable gas may be further decomposed into secondary products including CO, CO2, H2 and CH4. Pyrolysis for production of char as an energy carrier is not a new concept. It has been extensively utilized to produce charcoal and coke from biomass and coal [104,131]. For MSW pyrolysis, numerous studies have been carried out with main focus on technology development and system design [132]. The pyrolytic liquid as a result of condensation of volatiles can be utilized as a fuel product (referred to as bio-oil) after further upgrading and/or as an intermediate for synthesis of fine chemicals. The char may be used in a number of potential applications, including energy production, as soil amendment and for long-term carbon sequestration.

The yields and chemical composition of the pyrolysis products depends on e.g. feedstock properties, pyrolysis temperature and heating rate. Based on heating rate, pyrolysis can be classified as slow or fast pyrolysis, respectively. In fast pyrolysis, the residence time for vapors is a few seconds and the primary products are bio-oil and gas. In slow pyrolysis the residence time is longer (minutes or more) and the primary product is char [104]. In addition, the so-called flash pyrolysis with high heating rate has recently been successfully applied in demonstration scale for syngas production from solid recovered fuel derived from MSW [133]. There are also new conceptual designs by integration of pyrolysis with microwave heating, commonly at vacuum condition. The advantage of microwave-assisted pyrolysis includes shorter residence time, homogeneous heating and higher heating value of the volatile products [105]. The efficiency of the microwave-assisted process depends greatly on the nature of the processed material, in terms of its physical characteristics including structural arrangements, conductivity and dielectric properties [134].

The reactors used for conventional pyrolysis of MSW are mainly rotary kilns and tubular reactors in particular for up-scaled facilities, whereas fixed-bed and fluidized bed reactors have been frequently used for lab-scale studies [111]. N2 flush is commonly used to provide an inert atmosphere. Additionally, vacuum pyrolysis has been applied for recovery of animal-derived waste and printed circuit boards [135,136]. Pretreatment such as shredding and drying is often required especially for heterogeneous MSW fractions. The currently available pyrolysis plants for MSW treatment at demonstration and commercial scale are generally operated in combination with gasification or combustion systems. Fully integrated plants at demonstration and industrial scale have been in operation in European countries such as Germany, Italy and Sweden since the 1980s [137]. The MSW entering the gasifier is pyrolyzed directly and the recirculated gas and/or chars are combusted to maintain the required temperature in the gasifier [132,137]. Recovery of value-added products (such as ferrous and non-ferrous metals) from pyrolytic residues can be achieved by sorting or screening processes.

Operation of stand-alone pyrolysis is still under development for MSW treatment to produce liquid and char as end products. Application of pyrolysis using animal manure as feedstock has recently been demonstrated in a pilot-scale reactor [138]. Approximately 50% of the carbon in the feedstock was recovered in the char at a temperature of 620°C. The high P and K contents of the char suggest that it can be used as soil amendment to improve soil fertility. The heating value of the char was comparable with medium-rank coal. That study also demonstrated the possibility to co-pyrolyze animal waste with pelletized plastic waste to improve energy efficiency.

The influence of pyrolysis temperature, heating rate and the residence time on the yields and composition of pyrolysis products have been discussed in the literature. Williams and Williams [139] conducted pyrolysis of plastic waste (high- and low-density polyethylene, polypropylene and PVC) using a fixed-bed reactor at temperatures between 500 and 700°C. The study showed that increased temperature led to a dramatic increase of the yield of gas. It was also found that higher temperatures enhanced the yield of aromatic compounds in the liquid product. Chen et al. [140] described the influence of pyrolysis temperature on the adsorption capacity of organic molecules of the char by characterizing chars as being polarity-selective (produced at 200-400°C) and porosity-selective (500-600°C), respectively. This means that the polarity of the adsorbed compound is the most important factor for adsorption on low-temperature chars, while the size of the adsorbed compound is more crucial for adsorption on high-temperature chars [140]. Char produced at 700°C as well as conventional activated carbon were non-selective, due to less amount of functionalities and larger pore size, and these chars also demonstrated the highest adsorption capacities [140]. It has also been reported that high temperature pyrolysis will remove volatiles in the thermoplastic phase, which increase the pore size resulting in a non-selective type of char [141]. This type of char has similar properties to commercial activated carbon and it has good adsorption capacity due to the large surface area.

The residence time for feeding MSW remained in pyrolysis reactors can vary from a few seconds to 2 h [142]. It has been reported that increased residence time could lead to enhanced tar cracking and result in higher gas yields [142]. However, long residence times could reduce the processing efficiency in terms of the capacity of MSW treatment. Heating rates for MSW pyrolysis reported in literature varied from about 10°C/min in slow pyrolysis [143] to 600°C/s in flash pyrolysis [144]. It was commonly recognized that high heating rate could lead to a higher yield of volatile products and lower char yield [132]. In the case that volatiles are extracted immediately in the cooling trap, a higher yield of liquid products can be obtained. The effect of temperature on the stability, nutrient availability and hydrophobicity of chars derived from MSW have been studied recently [116]. It was found that increasing pyrolysis temperature increased aromatization and stability of the char, while lowering the temperature (300-400°C) could improve the cation exchange capacity and nutrient availability of the produced chars.

Di Blasi [145] has reported on the influence of particle size on MSW pyrolysis and concluded that reduced particle size in general resulted in increasing heating rate and consequently higher yield of the liquid product. Another study, however, showed that the influence of particle size on the yield of the liquid product was minor when the temperature was sufficiently high [146]. Additionally, operating pressure is also an influencing factor. High pressure pyrolysis increases the yield of char, although practically it may be difficult to operate [147].

The environmental impact, e.g. with regard to formation of inorganic (i.e. HCl and heavy metals) and organic (i.e. polychlorinated dibenzo-p-dioxins and dibenzofurans) contaminants in pyrolysis of solid waste containing PVC, discarded tires and demolition waste wood have been investigated [148,149,150,151]. Organic pollutants and heavy metals tended to be remained largely in the liquid and char fractions,

respectively. Studies also showed that formation of some undesired inorganic byproducts containing Cl, Br and S could be reduced by use of catalysts [152]. A comparison of the behavior of heavy metals in MSW pyrolysis with incineration has been conducted [153], showing that the release of heavy metals in rapid-heating combustion generally exceeded that observed in pyrolysis. The volatilities of some heavy metals, e.g. Zn and Pb, were reduced in pyrolysis compared to combustion. It was concluded from that study that pyrolysis is a better choice for MSW treatment in terms of controlling heavy metal contamination. To control the emission of organic and inorganic pollutants associated with MSW pyrolysis, flue gas scrubbing is to date the most efficient method [132].

4.3 Torréfaction

Torrefaction is a mild and slow pyrolysis, conducted at temperatures ranging between 200-350°C [154]. The process is generally operated at ambient pressure with an inert atmosphere to avoid oxidation and combustion of the feedstock [155]. The residence time can vary from a few minutes to several hours. The word torrefaction originates from the French word torréfaction, meaning roasting. Historically it was typically used for treatment of coffee beans, with the presence of oxygen though [104]. The torrefaction process is initiated by moisture evaporation, followed by partial devolatilization. The char, which is the major product, has a substantially higher energy density compared to the feedstock. Depending on the processing temperature torrefaction can be classified as light (below 240°C) and severe (above 270°C) torrefaction [156]. The advantage of torrefaction is basically associated with the increase in energy density but also the improved grindability, as well as reduced moisture content and susceptibility to microbial degradation. The char can be utilized as high-quality fuel in various applications including co-firing in power plants, entrained flow gasification and small scale combustion facilities [157]. The char can also be used as a water purification adsorbent and for in-situ soil remediation purposes [104].

Published studies on torrefaction technologies for MSW treatment is relatively sparse compared with literature on gasification and pyrolysis. Research have mainly been carried out to investigate the physical and chemical properties of torrefied MSW [110,119,153,158]. The types of materials used as feedstock include food waste, PVC plastic, discarded tires and wood residues. It has been commonly recognized that the torrefaction temperature is one of the crucial factors and that the yield of the char decreases with increased temperatures. Song and Guo [118] studied the properties of chars obtained from thermal treatment of animal litter at different temperatures. The results showed that chars obtained under torrefaction conditions at 300°C had improved adsorption capacity for nutrient substances due to the high surface functionalities, and was thus recommended to use for agricultural purposes. Similar observations were made in another study using animal manure and crop residues as feedstock, showing that relatively low treatment temperature led to increased nutrient availability and reduced alkalinity [116].

The behavior of organic and inorganic pollutants in the torrefaction process when waste materials are used as feedstock have been investigated [147,153,159]. Generally, heavy metals with high boiling points (e.g. Pb and Zn) tended to be retained in the chars, whereas those with low boiling points such as Hg tended to

enter the gas phase. Volatilization of heavy metals in torrefaction was suppressed at slow heating rates. With regard to organic pollutants such as dioxins and dioxin-like compounds, it has been reported that they were mainly retained in the chars with minor amounts found in the volatile fractions [159].

4.4 Hydrothermal carbonization

In both pyrolysis and torrefaction, the feedstock generally needs to be pre-dried in order to remove moisture, which is energy-demanding. For wet feedstocks it is therefore a more feasible alternative to subject the material to a wet thermochemical conversion technique, such as hydrothermal carbonization (HTC). Similar to pyrolysis and torrefaction, HTC is a thermal technique which is performed in absence of oxygen [160]. Unlike conventional pyrolysis, HTC is carried out in the presence of subcritical water. The process involves heating of the feedstock together with water at a temperature of 180-250°C in a pressure vessel for several hours. The water contained in the feedstock is used as solvent to pressurize the reaction medium and therefore there is no need for pre-treatment to remove water from the material. The transition from water to steam is avoided in HTC due to the high pressure. The energy consumed for heating the water is substantially lower than that required for water evaporation in pyrolytic treatment [160]. The formation of the HTC char (also known as hydrochar) is the result of a series of reactions including hydrolysis, condensation, decarboxylation and dehydration. The resulting liquid product is mechanically easy to separate from the hydrochar.

HTC was initially conducted in the early 1900s by Bergius as a means to convert cellulose into coal-like materials [161]. To date, HTC has mainly been applied on lignocellulosic biomass to explore the potential to produce chars for soil amendments and as energy-carriers. The feasibility to apply HTC to treatment of waste have been investigated using different types of feedstocks including poultry manure, PVC and food residues [120,121,162]. Compared to chars produced by conventional pyrolysis, hydrochars contain less aromatic carbon, while they have more surface functional groups containing hydrogen and oxygen, giving the char negative surface charge, which increases cation exchange capacity [163]. This makes the hydrochar suitable for use as adsorbent for organic pollutants in e.g. water purification applications. It has also been suggested that combining HTC with subsequent thermochemical treatment by pyrolysis or combustion is effective in practice for the application of waste remediation as well as energy recovery [162].

The effects of processing temperature and treatment time on the physical and chemical properties of hydrochars have been investigated in different scales, from stainless steel tubular reactors [163,164,165], to commercial plants with a processing capacity of up to 1 ton of MSW [166]. The energy contents of the produced hydrochars were positively correlated to the treatment times, and the heating values were comparable to low rank coals. It was also evident that at elevated temperature and pressure, water-insoluble organic chlorine were largely reduced in the hydrochars compared to feedstocks containing plastics [166]. This is probably due to the transformation of organic chlorine to water-soluble inorganic chlorine during the hydrothermal process. A comprehensive review was given by Zhao et al. [167] with focus on the recent development of HTC technology and the feasibility of HTC for upgrading of waste materials.

5. Conclusions

The increasing amount of MSW present a great challenge for their handling in order to minimize their environmental impact. Traditional methods of waste management such as landfill and burning are having a negative environmental impact and societies are trying to minimize their use. On the other hand, novel approaches that turn the waste to a valuable product or energy are gaining ground as methods for waste management. These methods involve both biological and thermochemical conversions and the intense research that has conducted the last decade has resulted in improved yields of product or energy formation together with decreased environmental impact. Biological methods include the production of fuels (e.g. ethanol, biogas, hydrogen and butanol), biopesticides, oils from microalgae and enzymes (such as amylases, carbohydrases, pectinases and lipases). Thermochemical methods of MSW utilization involve gasification, pyrolysis, torrefaction and hydrothermal carbonization. Lot of research has been conducted during the last years on the use of these methods for the conversion of MSW and this has resulted in very promising results as discussed in the current review article.

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