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Syngas production using straw pellet gasification in fluidized bed allothermal reactor under different temperature conditions
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Syngas production using straw pellet gasification in fluidized bed allothermal reactor under different temperature conditions Niels B.K. Rasmussena, Nabin Aryala,b, a b
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Danish Gas Technology Centre (DGC), Dr. Neergaards Vej 5B, DK-2970 Horsholm, Denmark Aarhus University, Biological and Chemical Engineering – Anaerobic Digestion Technologies, Hangovej 2, DK-8200 Aarhus N, Denmark
A R T I C LE I N FO
A B S T R A C T
Keywords: Straw Wood Gasification Syngas Allothermal Olivine Tar
Straw is one of the most available agriculture waste materials to be utilized as resource. Straw gasification for syngas production is one of the alternatives for resource recovery. Nevertheless, straw gasification might be problematic due to high alkali content, which may cause fouling of the gasifier reactor. In the present study, straw was gasified at different temperatures; in particular 750 °C, 800 °C, 850 °C, 900 °C, and 950 °C furnace temperature, was evaluated and then compared with wood feedstock using allothermal gasifier. The tested temperatures are not the limiting factor for the wood gasification using olivine (Mg2+,Fe2+)2SiO4 bed material and catalysis. Relatively, lower temperature is appropriate for straw gasification where agglomeration with bed material was observed at a higher temperature.
1. Introduction Syngas is a key energy carrier and intermediate product for the synthesis of fuels and chemicals. Based on the ratio of hydrogen and carbon monoxide (H2/CO), it has been used for the synthesis of liquid fuels, such as dimethyl ether (DME), methanol and synthetic natural gas [1,2]. Syngas composition in particular carbon dioxide (CO2, CO and H2) can be converted into biomethane coupling through anaerobic digestion [3–5] Currently, syngas is significantly produced from fossilbased fuels demanding the alternative approach due to environmental concern [6]. Alternatively, biomass is a renewable raw material for high-quality syngas production. Biomass is cleaner, more environmentally friendly and reliable than fossil fuels [7,8]. Biomass gasification and the technological development for syngas production were intensively researched to reduce the dependency on fossil fuels [9]. Over time, gasification process has been improved, and that offered the clean and efficient conversion process to convert biomass to a wide variety of applications, such as heat, electricity, chemicals and transport fuels [7]. Syngas production from biomass is often considered as CO2 neutral process due to the released CO2 being absorbed by photosynthesis of green plants. Recently, a different technology has been tested to convert biomass to syngas including biochemical, thermochemical, and mechanical extraction methods. Gasification is a thermochemical process that oxidizes biomass into a mixture of primarily H2, CO2, and CO with trace
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amounts of methane (CH4), nitrogen (N2), char, ash, tar, and oils in a temperature window of 973–1773 K [7,10]. Over the past decade, biomass gasification reactors and gasification technologies have been developed, and efficiency has been increased. Predominately applied gasifiers are fixed bed, fluidized bed and entrained flow gasifiers [11]. Briefly, fixed-bed gasifier are also designed either updraft where fuel enters from the top, gasifying agent from the bottom or downdraft where both enter from the top. Furthermore, fluidized bed gasifiers are designed as bubbling, circulating and dual fluidized bed gasifiers. In parallel, multistep gasification has also been tested to control and optimize the gasification process [12,13]. The composition of syngas and its constituents mainly depends on the type of gasifier technology, operating condition, for instance gasifying agents (O2, CO2, air or steam), catalysis and feedstock [11,14]. Primarily, gasification is endothermic reactions, where the necessary energy is generated by internal oxidation of biomass via an allothermal or an autothermal condition. During the allothermal process energy required for the biomass gasification is supplied externally, whereas in the autothermal process the gasifier is internally heated through the partial combustion of biomass [10]. The heat is transferred from either surface or transport bed material in an allothermal gasifier. The product gas produced in allothermal gasifier contains higher heating values compared to autothermal gasifier [7]. The composition of the product gas from gasifier significantly depends on the operating conditions, in particular temperature, feedstock, pressure, operational features,
Corresponding author at: Aarhus University, Biological and Chemical Engineering – Anaerobic Digestion Technologies, Hangovej 2, DK-8200 Aarhus N, Denmark. E-mail addresses: [email protected] (N.B.K. Rasmussen), [email protected] (N. Aryal).
https://doi.org/10.1016/j.fuel.2019.116706 Received 8 May 2019; Received in revised form 13 September 2019; Accepted 19 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Niels B.K. Rasmussen and Nabin Aryal, Fuel, https://doi.org/10.1016/j.fuel.2019.116706
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significant effort in order to characterize and identify issues arising from the feedstock, especially concerning ash sintering. One of the simplest ways to classify the feedstock is high and low-moisture-content plants. Woody biomass and herbaceous plants have low moisture contents; however, terrestrial biomass, marine biomass, and waste has high moisture contain. Interestingly, forest biomass, demolition wood, waste oil, and sawdust are ideal for gasification due to high cellulose and hemicelluloses [7]. In contrast, grasses, marine algae, energy crops, and cultivation are not ideal for gasification due to the high moisture contents thus requiring pretreatment. Alternative feedstock for gasification have been proposed such like municipal waste, sludge from wastewater treatment, agriculture crops, straw, and bio solids [9,11,17–21]. Recently, straw gasification has been reported as one of the best alternatives for the reuse of waste solids to produce syngas in the gasifier. In particular, straw is one of the most commonly available economical agriculture waste materials in Denmark and therefore suitable for syngas production. The Danish agriculture crop residues are dominated by wheat and barley straw representing around 6 Tg with cereal
Table 1 Standard proximate and ultimate value of wheat straw and wood pellet [23,24]. Feedstock
Straw Wood
Proximate (wt%)
Proximate (wt%)
C
H
O
N
S
Volatile matters
Fix carbon
Ash
43.2 48.2
5 5.8
39.41 45.0
0.6 0.2
0.1 0.02
71.31 82.8
19.79 16.4
8.9 0.8
including gasification agent, bed material, heat supply method, process temperature, pressure, the steam-to-biomass ratio (S/B), and equivalence ratio [7,11]. Biomass is a widely applied feedstock for gasification and syngas production. Different materials such as waste, agriculture residue, forest residues are applied as feedstock. There are several challenges concerning feedstock for gasification such as efficiency, competition between food and energy crops, cheap material, unavailable land, clearing of natural vegetation [2,11,15,16]. Researchers have made a
Fig. 1. Systematic diagram of the allothermal gasifier supplied from Highterm Research, Austria. 2
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Fig. 2. Temperature profiling in reactor A) Straw feed, B) Wood feed & Syngas composition (CO2, CO, H2, CH4), C) Straw feed, D) Wood feed.
contain MgO (42–45%), SiO2 (38%–40%) Fe2O3 (2%–9%), Al2O3(0.5%–1%), & CaO(0.2%–0.5%). The 800 g olivine catalysis (Mg2+, Fe2+)2SiO4 was used as bed material for supporting and enhancement of gasification.
straw accounting for more than 90%. A recent study reported that there are more than 1323 straw utilizing facilities in Denmark for heat production [22]. Unfortunately, operating heat recovery utilities is not the best alternative for resource recovery. However, recent studies reported that straw is a problematic feedstock for gasification in most of the existing combustion processes due to high alkali content which may cause slagging, fouling and agglomeration resulting in gasification and boiler operational problems [11]. Therefore, a current study tested straw pellets in allothermal gasifier as a feedstock for syngas production in different temperature profiles, and then compared this with the ideal feedstock, wood pellets. Moreover, the behavior of the straw pellet under allothermal laboratory scale fluidized-bed gasification in different temperatures was evaluated with optimum operational parameters for syngas production from straw.
2.2. Gasifier and operating conditions A laboratory scale allothermal gasifier 1.5 kW capacity was supplied from Highterm Research GesmbH, Austria as shown in Fig. 1. To fluidize the gasifier, around 350 g/h steam was supplied and 1 barg pressure was maintained during the gasification process. The reactor is heated by an electrically heated tube furnace surrounding the fluidized bed. The fluidized bed gasifier is operated with steam (allothermal gasification) supplied from steam generator at the entry into the reactor to 400 °C. The fuel is supplied from hopper located at the top of the reactor with a screw and conveyed into the reactor. The fuel feed is purged with CO2. During the gasification, complete monitoring and recording of temperatures, pressures, differential pressures, and mass flows are continuously recorded using acron software. At least duplicate experiments were performed for 1 hr in selected temperatures. The gasification was performed selecting 750 °C, 800 °C, 850 °C, 900 °C and 950 °C allothermal temperature of the furnace where the actual bed temperature was 50–160 °C lower, which is associated with the heat consumption by the allothermal gasification and heat loss across the wall.
2. Material and method 2.1. Materials To achieve the proposed objective of study, commercially available straw with 8 ± 0.3 mm diameter supplied from the Xstraw ApS, Denmark was purchased and then compared with 6 ± 0.4 mm diameter wood supplied from the Stora Enso, Finland therefore 2 mm difference in diameter was consider for the test. The wood pellet as recommended one of the best fuel for gasification has been selected as control to compare with straw pellet. Wheat straw was used for the pellet production that contains 8–10% moisture < 0.15% sulfur, 1 g of pellet can absorb 3.4 gm of water. Wood pellets contains 0.3% ash and 8% moisture. Furthermore, ash content in wheat straw is approximately 11 fold higher then wood as shown standard proximate and ultimate analysis value in table 1[23,24]. The feedstock was continuously fed at the rate of 330 g/h through hopper. Commercially available 0.2–0.34 mm size olivine powder was used as bed material usually
2.3. Gas cleaning and measurement A gas cleaning system was used to remove the undesirable impurities from the product gas and then to continuously measure the quality of the gas. The gas treatment filter system used was supplied by Hubei Cubi-Ruiyl Instrument Co Ltd. China. Briefly, the hot product gas was cooling at room temperature and then passed to water container 3
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Fig. 3. Straw pellet gasification and composition of syngas with different allothermal operating temperature conditions A) 750 °C, B) 800 °C, C) 850 °C, & D) 900 °C.
and filter containing cotton fibers to remove the tar and dust. The gas was then passed to the settlement container, which contains water to sediment possible further dust and impurities. Furthermore, activated carbon filter, aerosol filter and silica gel were used to remove impurities and moisture. The gas was then continuously measured by using Online Infrared Syngas Analyzer—Gasboard 3100 & 3100 PRO supplied by Hubei. The product gas composition which contains CO2, H2, CO, CH4, C2H4, & C3H8 was analyzed at 0.9 L/min flow rate at room temperature. Online response was recorded using Gas Analyzer Data Load software version 2.0.5.0 provided by Hubei and then analyzed using Microsoft XL program. The Propane (C3H8) and Ethane (C2H6) are both recorded as C3H8 but statistically 80% is C2H6. Furthermore, Ethylene (C2H4) and Propylene (C3H6) are both recorded as C2H4 but statistically 95% is C2H4 and the tar mass balance was performed based on stoichiometric molar flow of fuel at inlet compared to outlet. The measured concentrations of these components were corrected according to these statistics. However, the concentrations were small and hence their influence on mass balance is small. The inflow of CO2 flushing was measured using flow meter.
temperature of reactor, which is associated with the no combustion of fuel and the heat loss across the wall. The temperature distribution is homogeneous through each experiment. Moderately, the lower bed temperature of straw is slightly higher than that of wood pellet, which perhaps is associated with rapid thermal degradation of straw. Meanwhile, the freeboard and upper cap temperatures differs only about 50 °C. Nevertheless, the lower bed temperatures of straw and wood pellets are almost comparable when allothermal temperature is maintained at 950 °C, which may be associated with agglomeration of bed material which might cause the slow thermal degradation of straw. The lower heating value (LHV) of the gas was slightly increased with the temperature. The highest 8.4 (MJ/m3) LHV was observed at 900 °C for wood, whereas a slightly lower value of LHV was recorded with straw. Principally, during autothermal gasification process air is usually supplied for partial oxidation of fuel that cause the dilution with nitrogen of the product gas. Thereby LHV below 6 MJ/Nm3 [25] is observed. In the allothermal process gas is produced with medium heating value due to absence of nitrogen[10]. Furthermore, the excess steam ratio was around 4 in that the mass flows of fuel and steam were about equal. The stoichiometric steam ratio is about 1:4, hence the actual excess ratio around 4. Additionally, the tar elimination from biomass gasification through catalytic cracking is one of the most critical bottlenecks of the downstream applications of syngas. In-situ tar cracking applying the catalytic bed material is one of the promising approaches compared to other physical separation methods, such as aqueous scrubbing, filtration and membrane treatment. Currently, naturally occurring cheap minerals have been widely researched, and olivine (Mg, Fe)2SiO4 is reported as one of the best catalytic bed materials due to its higher catalytic activity
3. Result and discussion 3.1. Performance of gasification The temperature profile is measured online to evaluate the stability of the reactor. The lower bed temperature of the reactor is about 50–160 °C lower than the allothermal temperature as shown in Fig. 2. The highest temperature was observed at the bed of the gasifier. The temperature profiling is decreasing from bed, freeboard and upper cap 4
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Fig. 4. Wood pellet gasification and composition of syngas with different allothermal operating temperature conditions A) 750 °C, B) 800 °C, C) 850 °C, D) 900 °C, E) 950 °C & F) 950 °C for straw gasification.
Interestingly, higher hydrocarbon, such as, C3H8 and C2H4 have not been changed drastically as the temperature increased from 750 to 900 °C, but production is low.
and mechanical strength [26-28]. The performance of (MgFe)2SiO4 as bed material and catalytic activity was satisfactory observed for wood pellet gasification; however at higher temperatures agglomeration of bed material was observed when gasifying straw.
3.3. Wood pellet for syngas gasification 3.2. Straw pellets for syngas production Wood pellets is one of the ideal feedstocks for syngas production. Allothermal wood gasification was performed at different temperature conditions such as 750 °C, 800 °C, 850 °C, 900 °C and 950 °C as shown in Fig. 4. The composition of product gas been substantially changed. The CO2 content was relatively higher, ie 63% at lower temperature, whereas at 900 °C the CO & H2 concentration is approximately 50% combining both. In the syngas compositions, particularly CO and H2 were relatively higher at higher temperatures compared to lower, while methane is comparable in straw and wood gasification at all temperatures. The cellulose and hemicellulose content in wheat straw is 40% and 28%, which is almost similar to the soft wood biomass [11]. It has been reported that gasification of straw at higher temperatures might be
Allothermal straw gasification was performed at different temperature conditions such as 750 °C, 800 °C, 850 °C, 900 °C and 950 °C furnace temperature, as shown in Fig. 3A-D. The composition of product gas has been substantially changed, in particular CO2, CO, and H2. The CO2 content was relatively higher, ie 70% at lower temperature, whereas for wood it was below 70% at 750 °C. Nevertheless, the CO2 content has decreased when furnace temperature was increased. The syngas composition of CO, H2, and CH4 produced from straw feedstock was compared to wood. The CO & H2 concentration was below about 20%, whereas product gas produced on wood pellet yields up to 25% of CO & H2 concentration. Furthermore, the CH4 composition has been increased from 750 to 900 °C, however, below 10% was observed. 5
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production to synthesis optimal composition of syngas for biomethanation [21]. 3.4. Hydrocarbon production The hydrocarbons such as Ethylene (C2H4)/Propylene (C3H6) and Ethane (C2H6)/Propane (C3H8) were continuously measured. The Ethylene (C2H4) was comparatively higher at low allothermal temperature. The hydrocarbon concentration is inversely related to the allothermal temperature, which indicates the rapid degradation of the hydrocarbon thus producing the syngas composition. The higher composition of syngas, in particular, CO and H2 at higher allothermal temperature might be associated with the hydrocarbon degradation and thermal CO2 conversion to CO. In comparison, hydrocarbon concentration is relatively higher for straw fed syngas than wood fed syngas. 3.5. Mass balance The mass balance was performed based on the fuel supplied to the gasifier and on measurements. Mass flow in particular fuel, steam and CO2 was continuously measured. 328 g/h wood or straw pellet were supplied with CO2 flow of 2.1 L/min at 20 °C at 1 bar-g pressure and flow of water. N2 and O2 is always zero during the measurement, which is a test of correct measurements as no air is supplied. The mass balance was performed based on the input and output sources of gasification process. The gasification process input stream includes dry fuel material (wood or straw pellets) and water/steam for fluidization (including moisture present in fuel) and CO2 flushing to maintain the pressure in fuel tank and prevent syngas backflow into fuel tank. The output stream involves syngas, tar and water. The elemental composition of input and output of gasification was balanced as shown below. The empirical stoichiometric mass balance was done based on the stoichiometric molar flow of fuel at inlet compared to outlet, as shown in Eq. (1) and (2) [32-34].
Fig. 5. A) Straw pellet, B) Agglomeration of bed materials when using straw at 950 °C, C) Wood pellet & D) Bed materials when using wood at 950 °C.
problematic due to the presence of high alkali and cause the agglomeration thereby blocking the process [11]. The agglomeration of ash and bed material was observed with straw gasification at 950 °C thus experiment was stopped, whereas no agglomeration was observed with wood pellet gasification as shown in Fig. 5B & 5D. A study reported that operating parameters in particular bed temperature, fluidization velocity, and feedstock have an effect on the bed agglomeration [29]. Nuutinen et al. reported that the alkali content from biomass ash could be transferred to the surface of bed material particles that resulted in the formation of coating layer [30]. Subsequently, superimposed layers of bed materials have mainly alkali silicates at the innermost whereas the outermost layer is rich in calcium and magnesium resulting in the agglomeration. In the present study, the agglomeration is observed when the high temperature is applied to the furnaces and molten ash particles was absorbed in olivine. Hence, straw gasification in an allothermal gasifier is possible only while maintaining lower reactor temperatures. The produced syngas could be utilized as chemical production platform. Recently, the coupling of gasification and anaerobic digestion has been proposed for biomethanation [31]. The lignocellulose biomass in particular fibrous materials, straw, undigested biomass from biogas plants are consider as waste materials due to difficulties of microbial mediated degradation therefore such material could be recycled as feedstock to produce the syngas [3,31]. The ideal stoichiometric condition for microbial methanation of gas is H2: CO2 60:40 percentage (v/ v), therefore, the steam gasification perhaps enhances for hydrogen
Fuel + flushing CO2 + converted H2O = measured gas components out + tar (1) Eq. (1) is developed as shown below: x(C13H19O8 + bH2O + aCO2) + yH2O = %CO+%CO2+%CH4+ %C3H8+%H2+%C2H4 + zC10H8 (2) Eq. (2) is explained as follows:where, C13H19O8 represent wood that gives the correct relation of masses of C, H and O. The known amount of moisture in the fuel is represented by bH2O and b is calculated variable. The relation between flushing CO2 and fuel is known, which determines variable a. The fluid bed of the gasifier is fluidized by steam and yH2O represents the amount of steam converted to other gases during the gasification process. Only a part of the H2O is converted. “x” is the stoichiometric amount of fuel in inlet. On the right side of the equation all, the molar concentrations of the
Table 2 Product gas composition produced from straw and wood pellet. Feedstock
Furnace Temperature (°C)
Reactor Temperature (°C)
CO (%)
CO2 (%)
CH4 (%)
H2 (%)
C3H8 (%)
C2H4 (%)
LHV (MJ/m3)
Straw
750 800 850 900 950 750 800 850 900 950
685 750 767 802 824 661 690 720 751 790
9.38 12.93 16.18 21.83 24.67 12.55 15.44 19.15 23.66 26.52
69.96 61.51 57.21 50.50 49.46 63.28 55.96 49.82 46.24 44.62
4.34 4.30 4.01 3.95 4.66 4.72 5.29 4.87 4.88 5.01
13.42 18.77 20.22 21.75 19.52 16.87 20.61 23.84 23.18 22.18
0.34 0.26 0.20 0.09 0.02 0.31 0.30 0.21 0.09 0.04
2.56 2.23 2.18 1.88 1.67 2.38 2.37 2.11 1.95 1.6
6.5 6.7 7.1 7.7 7.9 6.8 7.2 8.2 8.5 8.0
Wood
6
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Fig. 6. Hydrocarbon composition when different allothermal temperatures were maintained A) Straw & B) Wood pellets.
Fig. 7. Tar production and H2O conversion in gasification process (mass/mass) A) Wood gasification and B) Straw gasification respectively.
contained in fuel for the lowest temperatures.
output gases are assumed to be the arbitrary chosen stoichiometric amounts. Tar is represented by naphthalene C10H8. According to literature, Naphthalene contributes to a high amount of all the compounds in tar from gasification and furthermore taking the average C/H ratio in tars it usually ends at 10/8 [33,34]. In this work we assume Naphthalene to represent all tars. “z” is then the stoichiometric amount of C10H8 in the outlet. With these assumptions it is possible to calculate the three unknowns, x, y and z from the three elemental balance equations for C, O and using Problem Solver in Microsoft Excel. [21,35]. The Elemental material mass balance analysis were performed from volume basis to molar basis using the gas composition measured as shown in Table 2. Then the molar composition of dry gas and fuel was calculated with respect to its individual elements to form empirical stoichiometric equation for the gasification of wood and straw in different temperature profiling. The inflow of CO2 flushing was measured using flow meter. During the wood pellet gasification, 23% tar is produced at low temperatures, falling to around 10% at higher temperature. In parallel, water is not converted at lower temperature, which might be due to lack of catalytic effect of olivine. Furthermore, at higher temperature water was consumed and tar might dissociate thereby produces syngas components that was also observed in Fig. 6. Moreover, at lower temperatures, tar is produced at 28% with straw input falling to around 16% at higher temperatures. Water is more or less not converted at any temperature of straw gasification. This might be due to lack of catalytic effect of the olivine as the bed material agglomerated for straw gasification at higher temperature. The negative water consumption sign in Fig. 7 illustrated that there is production of water from moisture
4. Conclusion Straw is one of the economical and most available agriculture waste materials in Denmark. Straw might be a problematic feedstock for gasification due to high alkali content, which may cause fouling of gasifier. In this study, straw was gasified at 750 °C, 800 °C, 850 °C, 900 °C and 950 °C furnace temperature and then compared with wood. A higher concentration of CO, H2, and CH4 was observed at higher allothermal temperature when straw gasification was performed and it is comparable to wood pellet. Hence, straw might be a good alternative to produce the syngas, however, agglomeration of bed materials is a risk which might create problems for the operation of the gasifier at higher temperatures. Furthermore, a method was developed of calculating the tar produced in gasification. This method is based on mass balance of input/output conditions for the gasifier and on measurements of other components than the tar.
Acknowledgement NA acknowledges the financial support for FutureGas (ID 516000006A) project from Innovation Fund, Denmark – Innovationsfonden and NBR acknowledges the financial support from the LIGNOSYS and SYNFERON projects.
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simultaneous syngas and biochar production in a top-lit updraft biomass gasifier. Renew Energy 2018;117:116–24. https://doi.org/10.1016/j.renene.2017.10.034. Saw W, McKinnon H, Gilmour I, Pang S. Production of hydrogen-rich syngas from steam gasification of blend of biosolids and wood using a dual fluidised bed gasifier. Fuel 2012;93:473–8. https://doi.org/10.1016/j.fuel.2011.08.047. Atnaw SM, Sulaiman SA, Yusup S. Syngas production from downdraft gasification of oil palm fronds. Energy 2013;61:491–501. https://doi.org/10.1016/j.energy. 2013.09.039. Al-Rahbi AS, Williams PT. Hydrogen-rich syngas production and tar removal from biomass gasification using sacrificial tyre pyrolysis char. Appl Energy 2017;190:501–9. https://doi.org/10.1016/j.apenergy.2016.12.099. Bentsen NS, Nilsson D, Larsen S. Agricultural residues for energy - A case study on the influence of resource availability, economy and policy on the use of straw for energy in Denmark and Sweden. Biomass Bioenergy 2018;108:278–88. https://doi. org/10.1016/j.biombioe.2017.11.015. Li H, Mahmood N, Ma Z, Zhu M, Wang J, Zheng J, et al. Preparation and characterization of bio-polyol and bio-based flexible polyurethane foams from fast pyrolysis of wheat straw. Ind Crops Prod 2017;103:64–72. https://doi.org/10. 1016/j.indcrop.2017.03.042. Telmo C, Lousada J, Moreira N. Proximate analysis, backwards stepwise regression between gross calorific value, ultimate and chemical analysis of wood. Bioresour Technol 2010;101:3808–15. https://doi.org/10.1016/j.biortech.2010.01.021. Zhou, Qiu Z, Long Ma L, Li Yin X, Zhi Wu C, Cheng Huang L, Wang C. Study on biomass circulation and gasification performance in a clapboard-type internal circulating fluidized bed gasifier. Biotechnol Adv 2009;27:612–5. https://doi.org/10. 1016/j.biotechadv.2009.04.016. Tursun Y, Xu S, Abulikemu A, Dilinuer T. Biomass gasification for hydrogen rich gas in a decoupled triple bed gasifier with olivine and NiO/olivine. Bioresour Technol 2019;272:241–8. https://doi.org/10.1016/j.biortech.2018.10.008. Ma X, Zhao X, Gu J, Shi J. Co-gasification of coal and biomass blends using dolomite and olivine as catalysts. Renew Energy 2019;132:509–14. https://doi.org/10.1016/ j.renene.2018.07.077. Li ZJ, Guo JL, Dong Z, Chen JW. Insight into interactions of olivine-scCO2-water system at 140 °C and 15 MPa during CO2mineral sequestration. Geosci Front 2018;9.. https://doi.org/10.1016/j.gsf.2017.12.008. Liu H, Feng Y, Wu S, Liu D. Bioresource Technology The role of ash particles in the bed agglomeration during the fluidized bed combustion of rice straw. Bioresour Technol 2009;100:6505–13. https://doi.org/10.1016/j.biortech.2009.06.098. Nuutinen LH, Tiainen MS, Virtanen ME, Enestam SH, Laitinen RS. Coating layers on bed particles during biomass fuel combustion in fluidized-bed boilers. Energy Fuels 2004;2004:127–39. https://doi.org/10.1021/ef0300850. Pecchi M, Baratieri M. Coupling anaerobic digestion with gasification, pyrolysis or hydrothermal carbonization: A review. Renew Sustain Energy Rev 2019;105:462–75. https://doi.org/10.1016/j.rser.2019.02.003. Rao MS, Singh SP, Sodha MS, Dubey AK, Shyam M. Stoichiometric, mass, energy and exergy balance analysis of countercurrent fixed-bed gasification of post-consumer residues. Biomass Bioenergy 2004;27:155–71. https://doi.org/10.1016/j. biombioe.2003.11.003. Rapagna A, Jand N, Kiennemann A, Foscolo PU. Steam-gasification of biomass in a fluidised bed of olivine particles. Biomass Abd Bioenergy 2000;19. https://doi.org/ 10.1016/S0961-9534(00)00031-3. Devi L, Ptasinski KJ, Janssen FJJG. Pretreated olivine as tar removal catalyst for biomass gasifiers: Investigation using naphthalene as model biomass tar. Fuel Process Technol 2005;86:707–30. https://doi.org/10.1016/j.fuproc.2004.07.001. Method developed from SYNFERON Project, cited 02-04-2019 http://www.kt.dtu. dk/Om-os/Nyheder/Nyhed?id=1e5e8281-2db1-46a7-b34e-188b578c28ad)).
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Short communication
Syngas production using straw pellet gasification in fluidized bed allothermal reactor under different temperature conditions Niels B.K. Rasmussena, Nabin Aryala,b, a b
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Danish Gas Technology Centre (DGC), Dr. Neergaards Vej 5B, DK-2970 Horsholm, Denmark Aarhus University, Biological and Chemical Engineering – Anaerobic Digestion Technologies, Hangovej 2, DK-8200 Aarhus N, Denmark
A R T I C LE I N FO
A B S T R A C T
Keywords: Straw Wood Gasification Syngas Allothermal Olivine Tar
Straw is one of the most available agriculture waste materials to be utilized as resource. Straw gasification for syngas production is one of the alternatives for resource recovery. Nevertheless, straw gasification might be problematic due to high alkali content, which may cause fouling of the gasifier reactor. In the present study, straw was gasified at different temperatures; in particular 750 °C, 800 °C, 850 °C, 900 °C, and 950 °C furnace temperature, was evaluated and then compared with wood feedstock using allothermal gasifier. The tested temperatures are not the limiting factor for the wood gasification using olivine (Mg2+,Fe2+)2SiO4 bed material and catalysis. Relatively, lower temperature is appropriate for straw gasification where agglomeration with bed material was observed at a higher temperature.
1. Introduction Syngas is a key energy carrier and intermediate product for the synthesis of fuels and chemicals. Based on the ratio of hydrogen and carbon monoxide (H2/CO), it has been used for the synthesis of liquid fuels, such as dimethyl ether (DME), methanol and synthetic natural gas [1,2]. Syngas composition in particular carbon dioxide (CO2, CO and H2) can be converted into biomethane coupling through anaerobic digestion [3–5] Currently, syngas is significantly produced from fossilbased fuels demanding the alternative approach due to environmental concern [6]. Alternatively, biomass is a renewable raw material for high-quality syngas production. Biomass is cleaner, more environmentally friendly and reliable than fossil fuels [7,8]. Biomass gasification and the technological development for syngas production were intensively researched to reduce the dependency on fossil fuels [9]. Over time, gasification process has been improved, and that offered the clean and efficient conversion process to convert biomass to a wide variety of applications, such as heat, electricity, chemicals and transport fuels [7]. Syngas production from biomass is often considered as CO2 neutral process due to the released CO2 being absorbed by photosynthesis of green plants. Recently, a different technology has been tested to convert biomass to syngas including biochemical, thermochemical, and mechanical extraction methods. Gasification is a thermochemical process that oxidizes biomass into a mixture of primarily H2, CO2, and CO with trace
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amounts of methane (CH4), nitrogen (N2), char, ash, tar, and oils in a temperature window of 973–1773 K [7,10]. Over the past decade, biomass gasification reactors and gasification technologies have been developed, and efficiency has been increased. Predominately applied gasifiers are fixed bed, fluidized bed and entrained flow gasifiers [11]. Briefly, fixed-bed gasifier are also designed either updraft where fuel enters from the top, gasifying agent from the bottom or downdraft where both enter from the top. Furthermore, fluidized bed gasifiers are designed as bubbling, circulating and dual fluidized bed gasifiers. In parallel, multistep gasification has also been tested to control and optimize the gasification process [12,13]. The composition of syngas and its constituents mainly depends on the type of gasifier technology, operating condition, for instance gasifying agents (O2, CO2, air or steam), catalysis and feedstock [11,14]. Primarily, gasification is endothermic reactions, where the necessary energy is generated by internal oxidation of biomass via an allothermal or an autothermal condition. During the allothermal process energy required for the biomass gasification is supplied externally, whereas in the autothermal process the gasifier is internally heated through the partial combustion of biomass [10]. The heat is transferred from either surface or transport bed material in an allothermal gasifier. The product gas produced in allothermal gasifier contains higher heating values compared to autothermal gasifier [7]. The composition of the product gas from gasifier significantly depends on the operating conditions, in particular temperature, feedstock, pressure, operational features,
Corresponding author at: Aarhus University, Biological and Chemical Engineering – Anaerobic Digestion Technologies, Hangovej 2, DK-8200 Aarhus N, Denmark. E-mail addresses: [email protected] (N.B.K. Rasmussen), [email protected] (N. Aryal).
https://doi.org/10.1016/j.fuel.2019.116706 Received 8 May 2019; Received in revised form 13 September 2019; Accepted 19 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Niels B.K. Rasmussen and Nabin Aryal, Fuel, https://doi.org/10.1016/j.fuel.2019.116706
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significant effort in order to characterize and identify issues arising from the feedstock, especially concerning ash sintering. One of the simplest ways to classify the feedstock is high and low-moisture-content plants. Woody biomass and herbaceous plants have low moisture contents; however, terrestrial biomass, marine biomass, and waste has high moisture contain. Interestingly, forest biomass, demolition wood, waste oil, and sawdust are ideal for gasification due to high cellulose and hemicelluloses [7]. In contrast, grasses, marine algae, energy crops, and cultivation are not ideal for gasification due to the high moisture contents thus requiring pretreatment. Alternative feedstock for gasification have been proposed such like municipal waste, sludge from wastewater treatment, agriculture crops, straw, and bio solids [9,11,17–21]. Recently, straw gasification has been reported as one of the best alternatives for the reuse of waste solids to produce syngas in the gasifier. In particular, straw is one of the most commonly available economical agriculture waste materials in Denmark and therefore suitable for syngas production. The Danish agriculture crop residues are dominated by wheat and barley straw representing around 6 Tg with cereal
Table 1 Standard proximate and ultimate value of wheat straw and wood pellet [23,24]. Feedstock
Straw Wood
Proximate (wt%)
Proximate (wt%)
C
H
O
N
S
Volatile matters
Fix carbon
Ash
43.2 48.2
5 5.8
39.41 45.0
0.6 0.2
0.1 0.02
71.31 82.8
19.79 16.4
8.9 0.8
including gasification agent, bed material, heat supply method, process temperature, pressure, the steam-to-biomass ratio (S/B), and equivalence ratio [7,11]. Biomass is a widely applied feedstock for gasification and syngas production. Different materials such as waste, agriculture residue, forest residues are applied as feedstock. There are several challenges concerning feedstock for gasification such as efficiency, competition between food and energy crops, cheap material, unavailable land, clearing of natural vegetation [2,11,15,16]. Researchers have made a
Fig. 1. Systematic diagram of the allothermal gasifier supplied from Highterm Research, Austria. 2
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Fig. 2. Temperature profiling in reactor A) Straw feed, B) Wood feed & Syngas composition (CO2, CO, H2, CH4), C) Straw feed, D) Wood feed.
contain MgO (42–45%), SiO2 (38%–40%) Fe2O3 (2%–9%), Al2O3(0.5%–1%), & CaO(0.2%–0.5%). The 800 g olivine catalysis (Mg2+, Fe2+)2SiO4 was used as bed material for supporting and enhancement of gasification.
straw accounting for more than 90%. A recent study reported that there are more than 1323 straw utilizing facilities in Denmark for heat production [22]. Unfortunately, operating heat recovery utilities is not the best alternative for resource recovery. However, recent studies reported that straw is a problematic feedstock for gasification in most of the existing combustion processes due to high alkali content which may cause slagging, fouling and agglomeration resulting in gasification and boiler operational problems [11]. Therefore, a current study tested straw pellets in allothermal gasifier as a feedstock for syngas production in different temperature profiles, and then compared this with the ideal feedstock, wood pellets. Moreover, the behavior of the straw pellet under allothermal laboratory scale fluidized-bed gasification in different temperatures was evaluated with optimum operational parameters for syngas production from straw.
2.2. Gasifier and operating conditions A laboratory scale allothermal gasifier 1.5 kW capacity was supplied from Highterm Research GesmbH, Austria as shown in Fig. 1. To fluidize the gasifier, around 350 g/h steam was supplied and 1 barg pressure was maintained during the gasification process. The reactor is heated by an electrically heated tube furnace surrounding the fluidized bed. The fluidized bed gasifier is operated with steam (allothermal gasification) supplied from steam generator at the entry into the reactor to 400 °C. The fuel is supplied from hopper located at the top of the reactor with a screw and conveyed into the reactor. The fuel feed is purged with CO2. During the gasification, complete monitoring and recording of temperatures, pressures, differential pressures, and mass flows are continuously recorded using acron software. At least duplicate experiments were performed for 1 hr in selected temperatures. The gasification was performed selecting 750 °C, 800 °C, 850 °C, 900 °C and 950 °C allothermal temperature of the furnace where the actual bed temperature was 50–160 °C lower, which is associated with the heat consumption by the allothermal gasification and heat loss across the wall.
2. Material and method 2.1. Materials To achieve the proposed objective of study, commercially available straw with 8 ± 0.3 mm diameter supplied from the Xstraw ApS, Denmark was purchased and then compared with 6 ± 0.4 mm diameter wood supplied from the Stora Enso, Finland therefore 2 mm difference in diameter was consider for the test. The wood pellet as recommended one of the best fuel for gasification has been selected as control to compare with straw pellet. Wheat straw was used for the pellet production that contains 8–10% moisture < 0.15% sulfur, 1 g of pellet can absorb 3.4 gm of water. Wood pellets contains 0.3% ash and 8% moisture. Furthermore, ash content in wheat straw is approximately 11 fold higher then wood as shown standard proximate and ultimate analysis value in table 1[23,24]. The feedstock was continuously fed at the rate of 330 g/h through hopper. Commercially available 0.2–0.34 mm size olivine powder was used as bed material usually
2.3. Gas cleaning and measurement A gas cleaning system was used to remove the undesirable impurities from the product gas and then to continuously measure the quality of the gas. The gas treatment filter system used was supplied by Hubei Cubi-Ruiyl Instrument Co Ltd. China. Briefly, the hot product gas was cooling at room temperature and then passed to water container 3
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Fig. 3. Straw pellet gasification and composition of syngas with different allothermal operating temperature conditions A) 750 °C, B) 800 °C, C) 850 °C, & D) 900 °C.
and filter containing cotton fibers to remove the tar and dust. The gas was then passed to the settlement container, which contains water to sediment possible further dust and impurities. Furthermore, activated carbon filter, aerosol filter and silica gel were used to remove impurities and moisture. The gas was then continuously measured by using Online Infrared Syngas Analyzer—Gasboard 3100 & 3100 PRO supplied by Hubei. The product gas composition which contains CO2, H2, CO, CH4, C2H4, & C3H8 was analyzed at 0.9 L/min flow rate at room temperature. Online response was recorded using Gas Analyzer Data Load software version 2.0.5.0 provided by Hubei and then analyzed using Microsoft XL program. The Propane (C3H8) and Ethane (C2H6) are both recorded as C3H8 but statistically 80% is C2H6. Furthermore, Ethylene (C2H4) and Propylene (C3H6) are both recorded as C2H4 but statistically 95% is C2H4 and the tar mass balance was performed based on stoichiometric molar flow of fuel at inlet compared to outlet. The measured concentrations of these components were corrected according to these statistics. However, the concentrations were small and hence their influence on mass balance is small. The inflow of CO2 flushing was measured using flow meter.
temperature of reactor, which is associated with the no combustion of fuel and the heat loss across the wall. The temperature distribution is homogeneous through each experiment. Moderately, the lower bed temperature of straw is slightly higher than that of wood pellet, which perhaps is associated with rapid thermal degradation of straw. Meanwhile, the freeboard and upper cap temperatures differs only about 50 °C. Nevertheless, the lower bed temperatures of straw and wood pellets are almost comparable when allothermal temperature is maintained at 950 °C, which may be associated with agglomeration of bed material which might cause the slow thermal degradation of straw. The lower heating value (LHV) of the gas was slightly increased with the temperature. The highest 8.4 (MJ/m3) LHV was observed at 900 °C for wood, whereas a slightly lower value of LHV was recorded with straw. Principally, during autothermal gasification process air is usually supplied for partial oxidation of fuel that cause the dilution with nitrogen of the product gas. Thereby LHV below 6 MJ/Nm3 [25] is observed. In the allothermal process gas is produced with medium heating value due to absence of nitrogen[10]. Furthermore, the excess steam ratio was around 4 in that the mass flows of fuel and steam were about equal. The stoichiometric steam ratio is about 1:4, hence the actual excess ratio around 4. Additionally, the tar elimination from biomass gasification through catalytic cracking is one of the most critical bottlenecks of the downstream applications of syngas. In-situ tar cracking applying the catalytic bed material is one of the promising approaches compared to other physical separation methods, such as aqueous scrubbing, filtration and membrane treatment. Currently, naturally occurring cheap minerals have been widely researched, and olivine (Mg, Fe)2SiO4 is reported as one of the best catalytic bed materials due to its higher catalytic activity
3. Result and discussion 3.1. Performance of gasification The temperature profile is measured online to evaluate the stability of the reactor. The lower bed temperature of the reactor is about 50–160 °C lower than the allothermal temperature as shown in Fig. 2. The highest temperature was observed at the bed of the gasifier. The temperature profiling is decreasing from bed, freeboard and upper cap 4
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Fig. 4. Wood pellet gasification and composition of syngas with different allothermal operating temperature conditions A) 750 °C, B) 800 °C, C) 850 °C, D) 900 °C, E) 950 °C & F) 950 °C for straw gasification.
Interestingly, higher hydrocarbon, such as, C3H8 and C2H4 have not been changed drastically as the temperature increased from 750 to 900 °C, but production is low.
and mechanical strength [26-28]. The performance of (MgFe)2SiO4 as bed material and catalytic activity was satisfactory observed for wood pellet gasification; however at higher temperatures agglomeration of bed material was observed when gasifying straw.
3.3. Wood pellet for syngas gasification 3.2. Straw pellets for syngas production Wood pellets is one of the ideal feedstocks for syngas production. Allothermal wood gasification was performed at different temperature conditions such as 750 °C, 800 °C, 850 °C, 900 °C and 950 °C as shown in Fig. 4. The composition of product gas been substantially changed. The CO2 content was relatively higher, ie 63% at lower temperature, whereas at 900 °C the CO & H2 concentration is approximately 50% combining both. In the syngas compositions, particularly CO and H2 were relatively higher at higher temperatures compared to lower, while methane is comparable in straw and wood gasification at all temperatures. The cellulose and hemicellulose content in wheat straw is 40% and 28%, which is almost similar to the soft wood biomass [11]. It has been reported that gasification of straw at higher temperatures might be
Allothermal straw gasification was performed at different temperature conditions such as 750 °C, 800 °C, 850 °C, 900 °C and 950 °C furnace temperature, as shown in Fig. 3A-D. The composition of product gas has been substantially changed, in particular CO2, CO, and H2. The CO2 content was relatively higher, ie 70% at lower temperature, whereas for wood it was below 70% at 750 °C. Nevertheless, the CO2 content has decreased when furnace temperature was increased. The syngas composition of CO, H2, and CH4 produced from straw feedstock was compared to wood. The CO & H2 concentration was below about 20%, whereas product gas produced on wood pellet yields up to 25% of CO & H2 concentration. Furthermore, the CH4 composition has been increased from 750 to 900 °C, however, below 10% was observed. 5
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production to synthesis optimal composition of syngas for biomethanation [21]. 3.4. Hydrocarbon production The hydrocarbons such as Ethylene (C2H4)/Propylene (C3H6) and Ethane (C2H6)/Propane (C3H8) were continuously measured. The Ethylene (C2H4) was comparatively higher at low allothermal temperature. The hydrocarbon concentration is inversely related to the allothermal temperature, which indicates the rapid degradation of the hydrocarbon thus producing the syngas composition. The higher composition of syngas, in particular, CO and H2 at higher allothermal temperature might be associated with the hydrocarbon degradation and thermal CO2 conversion to CO. In comparison, hydrocarbon concentration is relatively higher for straw fed syngas than wood fed syngas. 3.5. Mass balance The mass balance was performed based on the fuel supplied to the gasifier and on measurements. Mass flow in particular fuel, steam and CO2 was continuously measured. 328 g/h wood or straw pellet were supplied with CO2 flow of 2.1 L/min at 20 °C at 1 bar-g pressure and flow of water. N2 and O2 is always zero during the measurement, which is a test of correct measurements as no air is supplied. The mass balance was performed based on the input and output sources of gasification process. The gasification process input stream includes dry fuel material (wood or straw pellets) and water/steam for fluidization (including moisture present in fuel) and CO2 flushing to maintain the pressure in fuel tank and prevent syngas backflow into fuel tank. The output stream involves syngas, tar and water. The elemental composition of input and output of gasification was balanced as shown below. The empirical stoichiometric mass balance was done based on the stoichiometric molar flow of fuel at inlet compared to outlet, as shown in Eq. (1) and (2) [32-34].
Fig. 5. A) Straw pellet, B) Agglomeration of bed materials when using straw at 950 °C, C) Wood pellet & D) Bed materials when using wood at 950 °C.
problematic due to the presence of high alkali and cause the agglomeration thereby blocking the process [11]. The agglomeration of ash and bed material was observed with straw gasification at 950 °C thus experiment was stopped, whereas no agglomeration was observed with wood pellet gasification as shown in Fig. 5B & 5D. A study reported that operating parameters in particular bed temperature, fluidization velocity, and feedstock have an effect on the bed agglomeration [29]. Nuutinen et al. reported that the alkali content from biomass ash could be transferred to the surface of bed material particles that resulted in the formation of coating layer [30]. Subsequently, superimposed layers of bed materials have mainly alkali silicates at the innermost whereas the outermost layer is rich in calcium and magnesium resulting in the agglomeration. In the present study, the agglomeration is observed when the high temperature is applied to the furnaces and molten ash particles was absorbed in olivine. Hence, straw gasification in an allothermal gasifier is possible only while maintaining lower reactor temperatures. The produced syngas could be utilized as chemical production platform. Recently, the coupling of gasification and anaerobic digestion has been proposed for biomethanation [31]. The lignocellulose biomass in particular fibrous materials, straw, undigested biomass from biogas plants are consider as waste materials due to difficulties of microbial mediated degradation therefore such material could be recycled as feedstock to produce the syngas [3,31]. The ideal stoichiometric condition for microbial methanation of gas is H2: CO2 60:40 percentage (v/ v), therefore, the steam gasification perhaps enhances for hydrogen
Fuel + flushing CO2 + converted H2O = measured gas components out + tar (1) Eq. (1) is developed as shown below: x(C13H19O8 + bH2O + aCO2) + yH2O = %CO+%CO2+%CH4+ %C3H8+%H2+%C2H4 + zC10H8 (2) Eq. (2) is explained as follows:where, C13H19O8 represent wood that gives the correct relation of masses of C, H and O. The known amount of moisture in the fuel is represented by bH2O and b is calculated variable. The relation between flushing CO2 and fuel is known, which determines variable a. The fluid bed of the gasifier is fluidized by steam and yH2O represents the amount of steam converted to other gases during the gasification process. Only a part of the H2O is converted. “x” is the stoichiometric amount of fuel in inlet. On the right side of the equation all, the molar concentrations of the
Table 2 Product gas composition produced from straw and wood pellet. Feedstock
Furnace Temperature (°C)
Reactor Temperature (°C)
CO (%)
CO2 (%)
CH4 (%)
H2 (%)
C3H8 (%)
C2H4 (%)
LHV (MJ/m3)
Straw
750 800 850 900 950 750 800 850 900 950
685 750 767 802 824 661 690 720 751 790
9.38 12.93 16.18 21.83 24.67 12.55 15.44 19.15 23.66 26.52
69.96 61.51 57.21 50.50 49.46 63.28 55.96 49.82 46.24 44.62
4.34 4.30 4.01 3.95 4.66 4.72 5.29 4.87 4.88 5.01
13.42 18.77 20.22 21.75 19.52 16.87 20.61 23.84 23.18 22.18
0.34 0.26 0.20 0.09 0.02 0.31 0.30 0.21 0.09 0.04
2.56 2.23 2.18 1.88 1.67 2.38 2.37 2.11 1.95 1.6
6.5 6.7 7.1 7.7 7.9 6.8 7.2 8.2 8.5 8.0
Wood
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Fig. 6. Hydrocarbon composition when different allothermal temperatures were maintained A) Straw & B) Wood pellets.
Fig. 7. Tar production and H2O conversion in gasification process (mass/mass) A) Wood gasification and B) Straw gasification respectively.
contained in fuel for the lowest temperatures.
output gases are assumed to be the arbitrary chosen stoichiometric amounts. Tar is represented by naphthalene C10H8. According to literature, Naphthalene contributes to a high amount of all the compounds in tar from gasification and furthermore taking the average C/H ratio in tars it usually ends at 10/8 [33,34]. In this work we assume Naphthalene to represent all tars. “z” is then the stoichiometric amount of C10H8 in the outlet. With these assumptions it is possible to calculate the three unknowns, x, y and z from the three elemental balance equations for C, O and using Problem Solver in Microsoft Excel. [21,35]. The Elemental material mass balance analysis were performed from volume basis to molar basis using the gas composition measured as shown in Table 2. Then the molar composition of dry gas and fuel was calculated with respect to its individual elements to form empirical stoichiometric equation for the gasification of wood and straw in different temperature profiling. The inflow of CO2 flushing was measured using flow meter. During the wood pellet gasification, 23% tar is produced at low temperatures, falling to around 10% at higher temperature. In parallel, water is not converted at lower temperature, which might be due to lack of catalytic effect of olivine. Furthermore, at higher temperature water was consumed and tar might dissociate thereby produces syngas components that was also observed in Fig. 6. Moreover, at lower temperatures, tar is produced at 28% with straw input falling to around 16% at higher temperatures. Water is more or less not converted at any temperature of straw gasification. This might be due to lack of catalytic effect of the olivine as the bed material agglomerated for straw gasification at higher temperature. The negative water consumption sign in Fig. 7 illustrated that there is production of water from moisture
4. Conclusion Straw is one of the economical and most available agriculture waste materials in Denmark. Straw might be a problematic feedstock for gasification due to high alkali content, which may cause fouling of gasifier. In this study, straw was gasified at 750 °C, 800 °C, 850 °C, 900 °C and 950 °C furnace temperature and then compared with wood. A higher concentration of CO, H2, and CH4 was observed at higher allothermal temperature when straw gasification was performed and it is comparable to wood pellet. Hence, straw might be a good alternative to produce the syngas, however, agglomeration of bed materials is a risk which might create problems for the operation of the gasifier at higher temperatures. Furthermore, a method was developed of calculating the tar produced in gasification. This method is based on mass balance of input/output conditions for the gasifier and on measurements of other components than the tar.
Acknowledgement NA acknowledges the financial support for FutureGas (ID 516000006A) project from Innovation Fund, Denmark – Innovationsfonden and NBR acknowledges the financial support from the LIGNOSYS and SYNFERON projects.
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N.B.K. Rasmussen and N. Aryal
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