Scholarly article on topic 'Assessment of Operational Results of a Downdraft Biomass Gasifier Coupled with a Gas Engine'

Assessment of Operational Results of a Downdraft Biomass Gasifier Coupled with a Gas Engine Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Nikolaos. K. Margaritis, Panagiotis Grammelis, David Vera, Francisco Jurado

Abstract Biomass gasification is one of the several technologies with a very high potential for rural power generation applications. The proposed system is greenhouse gas neutral because the CO2 emissions from biomass are considered “carbon neutral”. In this paper a prototype gasification system (70 kWe- 245 kWth) combined with a gas engine is presented while its performance is tested using olive industry wastes (olive kernel) as fuel. The above CHP system represents a decentralized conversion system that functions economically even for small scale, fulfilling the operation conditions for an olive oil cooperative. The open top, downdraft gasifier consists of reactor, cooling and cleaning system. The producer gas exits the reactor at about 500°C, and includes contaminants in form of particulate matter and tar. The hot gas is further purified in the gas cooling and cleaning system in order to reach a state that is acceptable for engine operations. Initial tests in the prototype are carried out in order to achieve a profitable exploitation. Critical parameters such as producer gas composition and pressure drop of gasifier, are examined and assessed in order to enable the complete adaptation of the system to the operational specifications of an olive cooperative. The main objective is the optimization of the system in terms of gasification efficiency, gas composition, gas quality and electrical efficiency. The presented results are repeatable and give information about the steady state of gasifier when working at full load, producing a good quality gas with an electric efficiency of 16.1%.

Academic research paper on topic "Assessment of Operational Results of a Downdraft Biomass Gasifier Coupled with a Gas Engine"

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Procedía - Social and Behavioral Sciences 48 (2012) 857 - 867

Transport Research Arena- Europe 2012

Assessment of operational results of a downdraft biomass gasifier coupled with a gas engine

Nikolaos. K. Margaritisa'*9 Panagiotis Grammelisa, David Verab9 Francisco

Juradob

aInstitute for Solid Fuels Technology & Applications, Ptolemaida, Greece bUniversity of Jaén, Spain

Abstract

Biomass gasification is one of the several technologies with a very high potential for rural power generation applications. The proposed system is greenhouse gas neutral because the C02 emissions from biomass are considered "carbon neutral". In this paper a prototype gasification system (70 kWe- 245 kWth) combined with a gas engine is presented while its performance is tested using olive industry wastes (olive kernel) as fuel. The above CHP system represents a decentralized conversion system that functions economically even for small scale, fulfilling the operation conditions for an olive oil cooperative. The open top, downdraft gasifier consists of reactor, cooling and cleaning system. The producer gas exits the reactor at about 500°C, and includes contaminants in form of particulate matter and tar. The hot gas is further purified in the gas cooling and cleaning system in order to reach a state that is acceptable for engine operations. Initial tests in the prototype are carried out in order to achieve a profitable exploitation. Critical parameters such as producer gas composition and pressure drop of gasifier, are examined and assessed in order to enable the complete adaptation of the system to the operational specifications of an olive cooperative. The main objective is the optimization of the system in terms of gasification efficiency, gas composition, gas quality and electrical efficiency. The presented results are repeatable and give information about the steady state of gasifier when working at full load, producing a good quality gas with an electric efficiency of 16.1%.

© 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of the Programme Committee of the T ransport Research Arena 2012

Keywords: downdraft gasifier, gas composition, olive kernel, gas chromatograph

* Corresponding author. Tel.: +30 24630 55300; fax: +30 24630 55301 E-mail address', margaritis@lignite.gr

1877-0428 © 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of the Programme Committee of the Transport Research Arena 2012

doi:10.1016/j.sbspro.2012.06.1063

1. Introduction

Data concerning olive kernel gasification are scarce in the scientific literature and almost all studies for olive kernel thermo chemical conversion are reported by researchers from the Mediterranean. In Mediterranean countries olive oil production is an agricultural activity of great importance, producing annually large amounts of olive kernel as a valuable byproduct. Spain, Greece, Italy and Tunisia represent 65% of the olive tree cultivation area, 76% of the olive trees in production and 74% of the olive production worldwide On a global scale, the olive oil production reaches 1,600,000 tons annually. Main solid by-products from olive oil production are olive kernel, as well as, olive tree pruning and harvest residues. Conventional combustion of residues from olive and olive oil production is already applicable in many regions due to the extended local agricultural activity and additional energy consumption demands. The idea of replacing this technology with a new one such as gasification, under more 'environmental friendly' terms, seems very attractive. Moreover, difficulties that many rural areas face in order to achieve connection with a central power supply system as well as unstable fossil fuel prices act as the driving force for biomass exploitation in small decentralized gasification units integrated in small energy production systems. Although conversion of biomass to gas results in loss of energy of 10 to 25%, use of gas for production of heat and power can be highly efficient.

Several experiments regarding biomass gasification were conducted by different authors having as a common target the optimization of producer gas quality. Martinez et al [2] made a review of published papers on the effects of the particle size and the moisture content of biomass feedstock and the air/fuel ratio used in the gasification process with regard to the quality of the producer gas. The low heating value and the process cold efficiency for a downdraft type reactor were around 4-6 MJ/Nm3 and 50-70% respectively. In another paper [3] the same author presented an assessment of wood gasification in a 50 kWth downdraft double stage reactor. The gasifier enabled the achievement of a combustible gas with levels of CO, CH4 and H2 of 19.04, 0.89 and 16.78%(v.db) at 20 NmV of total air flow and an AR equal to 0.8. At these conditions, the lower calorific value of the producer gas was around 4.54 MJ/Nm3.

Jaojaruek et al [4] conducted experiments on three different downdraft gasification approaches: single stage, conventional two-stage and an innovative two stage air and premixed air/gas supply approach. Through the last approach, HHV and tar improved up to 6.5 MJ/Nm3 and down to 43.2 mg/Nm3, respectively. Therefore, this method can lower tar content sufficiently to feed the gas directly to the internal combustion engine. A small scale fixed bed downdraft gasifier to be fed with agricultural and forestry residues has been designed and constructed by Olgun et al [5]. No operational problems such as tar blockage, agglomeration, process breakdown were observed with wood chips and hazelnut shells.

Sharma et al. [6] conducted an experimental study on a downdraft biomass gasifier system including cooling-cleaning unit to obtain fluid flow characteristics in isothermal ambient airflow. In the reactive bed, the initial increase in gas flow rate resulted in raised reaction temperatures leading to higher calorific value of the product gas and hence better performance. An experimental study on a 75 kWth, downdraft gasifier system has been carried out by the same author [7] to obtain temperature profile, gas composition, calorific value and trends for pressure drop across the porous gasifier bed and cooling-cleaning system. The higher temperature in bed tends to better conversion from non-combustibles components (like C02, H20) into combustible component (like CO, H2) in the produced gas and, thus, improves its calorific value. The study of Skoulou et al [8] presented a laboratory fixed-bed gasification of olive kernels and olive tree cuttings. Experimental results showed that gasification with air at high temperatures (950°C) favoured gas yields. It was found that gas from olive tree cuttings (at 950°C and with an air equivalence ratio of 0.42) had a higher LHV (9.41 MJ/Nm3) in comparison to olive kernels (8.60 MJ/Nm3) at the same gasification conditions.

The purpose of this paper is to present in detail a prototype gasification system coupled with a gas engine and to make a first assessment of its operation in terms of gas composition and pressure drop. The fuel used during the experiments presented in this study was olive kernel whereas the testing of olive leaves and pruning is currently in progress.

2. Gasifier system description

A schematic of the Ankur [9] Biomass Gasifier System WBG-120 is given in Fig. 1 where the critical components have been identified.

Biomass inlet 1

'Yi'ii lu I' Scrubber / (cools & removes I particulates from gas)

Safety filter

(prevents particulate matter entering engine in case of failures')

S lev e f o r ch a r-water separation

Fine filter 1 + fine filter 2 (to further remove tar & particulates)

Gas blower Flare (for (delivers gas to testing gas gen set) quality)

Engine genset (operating in 100% producer gas)

Fig. 1. Gasifier with gas engine (Ankur)

The biomass is fed through the skip charger into feed shell and is stored in the hopper. Limited and controlled amount of air for partial combustion enters through the air nozzles. The reactor holds charcoal for reduction of partial combustion products while allowing the ash to escape. The dry ash that falls out of reactor gets collected in the slanted table of reactor and from there it is taken out with the help of a screw conveyor. Parameters of the gasifier, gas engine and generator are depicted in Table 1.

Table 1. Gasifier, engine and generator parameters

Model ANKUR WBG-120

Gasifier type Downdraft

Rated gas flow 300 Nm3/hr

Average gas calorific value 4.61 MJ/Nm!

Gasific ation temperature 1.050 - 1.100 °C

Biomass fuel type Olive kernel

Permissible inoistum: contentin biomass 5 to 20%

Rated hourly con sumption 100-110tg

Ash Discharge 6 to S %

ENGINE UNIT VALUE

Length x Height mm 3.500 x2.300

Engine make and model Cummins G-S55-G

Generator make and model Stamford UC 27E- (125KVA)

Revalutionsperminute KPM 1.500

Voltage/frequency V/Hz 400/50

Cylinder displacement volume L 2.33

Number of cylinders 6

Total displacement L 14

Generating set power kW 70

The gas outlet tube is connected with reactors outlet which drives the producer gas to a cyclone. Then, a Venturi scrubber and a heat exchanger with chiller and a mist eliminator are following. In addition, a parallel set of fine filters and a pleated-safety filter contribute much to the cleaning process of the

producer gas. Gas blower after the filters is necessary for delivering the gas to the engine. After the filters a header box with flare assembly and FCV (Fully Closed Valve) valves for the engine is placed, in order to facilitate running of the system in ultra clean gas mode.

2.1. Feeding system and reactor

The feed shell stores biomass in air tight compartment before discharging it into the hopper of the gasifier. The feed shell has two pneumatically operated doors, one on top and the other at the bottom. This is to prevent ingress of excess air into the gasifier. There's a vibrator motor fitted on the feed shell which operates only when the bottom feed door is in open condition and assists the discharge of biomass into the hopper. The hopper performs three functions steps of gasification, namely, drying, pyrolysis and combustion. It has pneumatic/manual air nozzles for injection of air into combustion zone. Two wood level sensors are provided to give signals for controlling biomass feed, as well as safe shut-down of the gasifier system. There's a vibrator motor for uniform flow of biomass. A pressure point is provided for reading of pressure drop across nozzles (APN) to determine air flow. A combustion cone is installed between the hopper and the reactor.

The reactor performs the reduction step and thus production of producer gas. In case of our system, the reactor is filled with recommended size charcoal. The reactor table with proprietary rotor holds the charcoal bed and the comb rotor helps in maintaining a consistent charcoal bed by removing ash from it. A pressure point is provided on the reactor (Fig. 1), for reading the pressure drop (APG) up to the exit of the charcoal bed, which determines the charcoal bed condition. A water column manometer/ pressure transmitter is connected to measure the gasifier pressure drop, known as APG. The ash removal system from the reactor is dry discharge. In case of dry ash discharge system, there is a screw conveyor connected at the bottom of the reactor. Water is circulated in thejacket of the screw conveyor. This brings down the temperature of ash to almost ambient temperature.

2.2. Gas cleaning and cooling system

The hot gas cleaning and cooling system consists of cyclone, Venturi scrubber, wet blower, heat exchanger with chiller, mist eliminator, fine filters and pleated cartridge filter.

1. Cyclone: The carbon particulates from the hot gas are trapped when passing through the cyclone and are collected in the ash collection box. There are two pneumatic/manual valves and an ash collection box at the bottom of the cyclone.

2. Hot gas line: The hot gas line has a temperature of 400 °C to 500 °C. This line is from the gasifier outlet to the Venturi scrubber inlet.

3. Venturi scrubber: Performs cooling and cleaning of hot gas by spraying pressurized waterjet on it, and reducing the gas temperature to 40 - 45 0 C. Water drains down and the cold gas passes through drain box gas outlet to the wet blower. There is a temperature sensor fitted on the drain box to measure the cold gas temperature.

4. Wet blower: The wet blower helps in sucking of controlled amount of air into the gasifier for partial combustion and suction and delivery of producer gas to the point of end use. The wet blower performs the gas flow control and an additional cleaning of gas through water injection.

5. Separation box: Producer gas and water get carried forward to the separation box, where water and producer gas get separated.

6. Heat exchanger: Cold water from chiller is circulated in the heat exchanger and producer gas passes through the tubes in the heat exchanger. The temperature of chilled water should be maintained around 10-12 °C. The chilled water enters from the bottom of the heat exchanger, circulates across the tubes and comes out from the top of the heat exchanger. The water vapour and traces of light tar condense, go down the bottom of the heat exchanger and drain out to the combo tank.

7. Chiller: The chiller is of 10 TR (Tonnage Refrigeration) capacity having water flow rate of 5.34 m3/hr. The water inlet temperature is 12°C and water outlet temperature is 7°C. The chiller is used to circulate chilled water to the externaljacket of heat exchanger. The chilled water from chiller outlet is fed to heat exchanger inlet. The water from heat exchanger is then returned back to the chiller for cooling. The chilled water in heat exchanger is used to condense the vapour content in the producer gas.

8. Mist eliminator: It assists in removal of mist present in the producer gas after the wet blower or the heat exchanger, but before the active fine filter. The producer gas in the mist eliminator enters from the bottom inlet pipe and comes out from the top outlet pipe after passing through the filter media. The mist eliminator is filled with wood chips (size: 25 mm X 25 mm X 10 mm) or wood pieces (size: 25 mm diameter X 25 mm long) as the filtering media up to the level indicated.

9. Fine filters: These filters having the role of further cleaning the gas, are operating on a temperature slightly above the ambient (25-30°C). Fine carbon particles from the producer gas get trapped in the filter media. These fine filters are filled with graded sawdust as filtering media. Producer gas enters the fine filters through the inlet from top side and after filtering comes out from the outlet at the bottom side.

10. Pleated-safety filter: It performs the function of final cleaning of the producer gas and also acts as a check/ safety filter to block the gas flow in case of dirty gas reaching up to this point. Finally, the main flare is used at the start-up of the gasifier. When the producer gas has a good quality, the system starts, the main flare is by-passed and the gas goes to the engine. The test flare is provided to check whether the producer gas gets ignited or not.

3. Instrumentation and measurement arrangements

Varian CP4900 Gas Chromatograph was used to measure the composition of producer gas. The GC was calibrated using the following calibration gas mixture of CO:19%, H2:18%, CH4:3%, C02:8% and N2:52%. The calibration mixture was injected to the Molsieve column of GC (Channel 1) with Argon as carrier gas. Channel 2 equipped with a Cp-PoraPlot U column was used for detecting C02 in the gas sample. The sampling point was located after the pleated-safety filter as shown in Fig. 2.

Fig. 2. Gas Sampling point (left) and gas Varian CP-4900 with PC (right), (G: Gasifier, C: Cyclone, S: Scrubber, WB: Wet Blower, SB: Separation Box, HE: Heat Exchanger, FF1: Fine Filter 1, FF2: Fine Filter 2, SF: Safety Filter)

By comparing the areas of peaks for calibration gas and gas sample, the composition of gas sample was predictedby the computer software (Star Workstation v6.41).

3.1. Working mode ofgasifier

During the measurement the gasifier was working at full load with a rated gas flow of 300 Nm3/hr. According to the technical specifications of the gasifier the average gas calorific value is 4.61 MJ/Nm3. After an hour of operation, the steady state of gasifier was accomplished. In this state, gas measurement started and lasted for about 1 hour. Every 5 minutes a chromatogram (peaks corresponding to different gases) was monitored in the PC while pressure drop of gasifier (APG) and pressure drop of nozzles (APN) were recorded manually. The same measurement was repeated after 24 hours (2nd day) in order to check the repeatability of the gas composition results.

3.2. Feedstock analysis and methodologyfollowed

Fig. 3 shows a picture of the olive kernel used in this gasification system. These residues are obtained in the mills during the olive oil extraction process. The ultimate and proximate analyses conducted in the laboratory of CERTH/ISFTA are depicted in Table 2. Each analysis was conducted following the European Technical Specifications for solid biofuels. Since for most of the analyses a particle size smaller than 0.5 mm was required, it was necessary to reduce the granulometry of the sample. Therefore a Fritsch universal cutting mill with a sieve size of 0.25 mm was used.

Table 2. Chemical analysis results of olive kernel

Proximate Analvsis Result Unit Methndnlnpv

Moisture 10 9.0 %ar CF.N/TS 14774-3

Volatiles 77 91 %dh CF.N/TS 15148

Ash ?.06 %dh CF.N/TS 14775

Fixed carbon 20.03 %dh FC=100-Vo1-Ash

Ultimate analvsis

Carbon 50.08 %dh CF.N/TS 15104

Hvdropen 5 90 %dh CF.N/TS 15104

Nitropen 0 64 %dh CF.N/TS 15104

Sulfur 0 00 %dh CF.N/TS 1 5289

Chlorine 0.02 %dh CF.N/TS 15289

Oxwen 41.31 %dh 0=100-C-H-N-S-Ash

Gross Heatinp Value 4953 Kcal/Kp CF.N/TS 14918

Moisture content: A simplified method of drying in a furnace is used to determine the moisture content. The sample to be analyzed is extended on a plate and introduced into a furnace where it remains at a temperature of 105 °C during a period not longer than 24 h. The moisture content is obtained by weighing the material.

Proximate analysis: In order to determine the volatile matter content, the sample material is covered and then heated in a muffle furnace at a temperature of 900 °C for 7 min. When the temperature has cooled down, the sample is introduced into a desiccator so that the loss of volatile matter can be calculated by weighing the differences. As far as the ash content, the samples are introduced into the muffle furnace whose temperature is increased up to 250 °C (increase of 5 °C/ min). The temperature is kept for 60 min

and is then increased again until it reaches 550 °C, keeping that temperature for 120 min. Once more, we obtain the ash content by weighing the difference.

Ultimate analysis: The equipment used in this test is an elemental analyzer (type 2400 CHNS series II from PerkinElmer) and the option of analyzing the C, H, N and S elements, simultaneously. Chlorine content was determined through combustion in an oxygen bomb and absorption of the chloride gas components in a solution.

Heating value: A bomb calorimeter of the type Leco AC-350 is used because it is able to withstand with security the high pressure generated during the combustion process.

Fig.3. Olive kernel used in the gasifier.

4. Experimental results

Fig. 4 shows the chromatogram of the CP-Molsieve channel. Separation of nitrogen (N2), oxygen (02) hydrogen (H2) and methane (CH4) was achieved. The first peak corresponds to nitrogen whereas oxygen, hydrogen and methane are following.

II —irVr-rY-r—frV --1-T'--

i1 V-Hr \ 1 vj_!_jJ___

1 I1 , , ill ^ 1M V 1 »

i a. ¡1J1J1 i ¿ A ¿ A A A

S S S!

Fig. 4. Gas chromatograms of first seven sample injections.

Calibration for 02 was made with air. When calibrating for oxygen the amount of argon has to be taken into account. Usually the concentration of Ar in air is 0.93 %. So we assume that the second peak is a representative amount of 02 and argon. C02 was detected in channel 2 equipped with CP-PoraPlot U column, with He as the carrier gas whereas CO is extracted from equation 1. As far as gas impurities, the content of tars is below 10 mg/m3, the content of particulate matters is below 10 mg/m3, whereas the moisture content (H20) is approximately 2% and maximum 02 content is 0.5% (limit values reported by the manufacturer of the gasifier). In addition, the concentration of heavier hydrocarbons (CxHy) was assumed as zero.

C0=100 - (N2 + 02 + Ar + H2 + CH4 + C02 + H20) (1)

The total time period of each analysis is less than 2 minutes. The average results of ten gas samplings are shown in Table 3. The average composition of gas (1st day) is N2:53.1%, 02-Ar:1.33%, H2:24.13%, CH4:4.18%, C02:4.6% and C0:10.66%. The standard deviation has been calculated for each component indicating the accuracy of gas analysis. The results are presented through a better way in Fig.5 where all components of gas are depicted. During the measurement the pressure drop of gasifier and nozzles were recorded manually and are presented in Fig.6. APG is in the range of 120-160 mm of water column whereas pressure drop in nozzles APN, an indication of air flow, is in the range of 10-25 mm of water column. The same measurement is repeated in the 2nd day with almost the same results as shown in Fig. 7 and Fig. 8. Gas sampling starts again, after lh of gasifier working at full load, in order to assure that the steady state of gasifier is accomplished.

Table 3. Average results of ten measurements during 1st and 2ntl day

Nitrogen Oxygen-Argon- Hydrogen Methane Carbon Dioxide Carbon Monoxide

N2[%] 02- Ar [%] H2[%] CH4 [%] co2 [%] CO [%]

1st day

Average 53.103 1.332 24.129 4.181 4.6 10.655

RSD % 1.369 0.097 1.616 0.558 0.19 2.630

2nd day

Average 55.039 1.397 23.66 3.798 4.6 9.506

RSD % 3.25 0.053 0.966 0.234 0.19 3.275

Time [min]

Fig. 7. Values of gas components (N2, H2, CH4, CO, C02) during operation of gasifier at full load. (2ntl day).

Fig. 8. Pressure drop across the gasifier (APG) and nozzles (APn) during operation of gasifier at full load (2ntl day).

4.1. Gas quality

The HHV of producer gas is dependent on the percentage quantities of CO, CH4, and H2 in producer gas and it can be calculated from the equation (2)

HHVgas=Yco *HHVco + Ych4*HHVCH4 + Yh2*HHVH2 (2)

where Y is the mole fraction of each gas species obtained from the Chromatograph whereas the heating value of each gas species is presented in the Table 4 [10] . In the same way the LHVgas can be calculated. The average values for gas species mole fractions (Yco, YCH4, YH2) were extracted from Table 3.

Table 4. Higher and lower heating values of each gas species and of produced gas.

Gas species HHV [MJ/Nm3l LHV [MJ/Nm3l

CO 13.1 13.1

CH, 41.2 37.1

H2 13.2 11.2

Gas 1st day 6.30 5.65

Gas 2nd day 5.93 5.30

4.2. System efficiencies

Table 5 shows the CHP parameters of the system when gasifier is fed with olive kernels. The electrical, overall energy and gasification efficiencies of CHP plant (downdraft gasifier-gas engine) are calculated according to [\ n].

Table 5. CHP operation parameters

Parameter/biomass Olive kernel

Biomass consumption (kg/h) 90-95

Product gas yield (m3/h) 240-260

Calorific value, LHV (MJ/kg) 5.65

Biomass energy input (kW) 435

Cold-gas efficiency (%) 75

Gas-to-shaft efficiency (%) 26

Electric efficiency (%) 16.1

Overall efficiency (%) 55.2

5. Conclusions

In this paper a prototype gasification system (70 kWe) combined with a gas engine is presented while its performance is tested using olive industry wastes (olive kernel). The system consists of feeding system, hopper, reactor and a gas cleaning and cooling system. At the end the ultra clean gas is ready for powering the gas engine and providing electric power to the network. Measurement of gas composition by means of a gas chromatograph after the safety filter is conducted while pressure drop of gasifier and air nozzles are manually recorded. The average gas composition produced during 1st day was N2:53.1%, 02-Ar:1.33%, H2:24.13%, CH4:4.18%, C02:4.6% and C0:10.66% resulting in a high heating value of 6.30 MJ/Nm3. During the 2nd day the average gas composition was N2:55.04%, 02-Ar:1.4%, H2:23.66%, CH4:3.8%, C02:4.6% and CO:9.51% resulting in a high heating value of 5.93 MJ/Nm3. In both two days of measurements, gasifier was working at full load producing a good quality gas with an electric efficiency of 16.1%. The results are repeatable and give information about the steady state of gasifier. The initial tests carried out, showed that continuous operation of gasifier with this type of fuel can be achieved fulfilling the operation conditions for an olive oil cooperative. Moreover, the experimental results contribute to the better assessment of the prototype in order to identify potential operational problems and fix them.

Acknowledgements

This work was supported in part by Project entitled "Adaptation of renewable energy solutions for the olive oil industry" (RESOLIVE), Grant Agreement Number: 218453. This Project is funded by the European Commission within the Seventh Framework Programme (2007-2013).

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