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Effect of design and operating parameters on the gasification process of biomass in a downdraft fixed bed: An experimental study
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Effect of design and operating parameters on the gasification process of biomass in a downdraft fixed bed: An experimental study Feiqiang Guo a, Yuping Dong b,c,*, Lei Dong c, Chenlong Guo a a
School of Electric Power Engineering, China University of Mining and Technology, Xuzhou 221116, PR China School of Mechanical Engineering, Shandong University, Jinan 250061, PR China c Shandong Baichuan Tongchuang Energy Company Ltd., Jinan 250101, PR China b
article info
abstract
Article history:
The main objective of this paper is to study the effect of design and operating parameters,
Received 7 August 2013
mainly reactor geometry, equivalence ratio and biomass feeding rate, on the performance
Received in revised form
of the gasification process of biomass in a three air stage continuous fixed bed downdraft
22 December 2013
reactor. The gasification of corn straw was carried out in the gasifier under atmospheric
Accepted 21 January 2014
pressure, using air as gasifying agent. The results demonstrated that due to the three stage
Available online 1 March 2014
of air supply, a high and uniform temperature was achieved in the oxidation and reduction zones for better tar cracking. The designing of both the air supply system and rotating grate
Keywords:
avoided bridging and channeling. The gas composition and tar yield were affected by the
Biomass
parameters including equivalence ratio (ER) and biomass feeding rate. When biomass
Downdraft fixed bed
feeding rate was 7.5 kg/h and ER was 0.25e0.27, the product gas of the gasifier attained a
Gasification
good condition with lower heating value (LHV) about 5400 kJ/m3 and cold gas efficiency
Experiments
about 65%. An increase in equivalence ratio led to higher temperature which in turn resulted in lower tar yield which was only 0.52 g/Nm3 at ER ¼ 0.32. Increasing biomass feeding rate led to higher biomass consumption rate and process temperature. However, excessively high feeding rate was unbeneficial for biomass gasification cracking and reforming reactions, which led to a decrease in H2 and CO concentrations and an increase in tar yield. When ER was 0.27, with an increase of biomass feeding rate from 5.8 kg/h to 9.3 kg/h, the lower heating value decreased from 5455.5 kJ/Nm3 to 5253.2 kJ/Nm3 and tar yield increased from 0.82 g/Nm3 to 2.78 g/Nm3. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved .
1.
Introduction
With the fast economic growth, increasing demand for energy, particularly in developing countries like China, has
exacerbated the concerns over energy crisis caused by overexploitation of fossil fuels. Research has increasingly included efforts to partially replace fossil fuels with renewable energy sources [1]. Of all the renewable energy sources, biomass energy is the only renewable energy source of carbon
* Corresponding author. School of Mechanical Engineering, Shandong University, Jinan 250061, PR China. Tel./fax: þ86 531 88392199. E-mail addresses: [email protected] (F. Guo), [email protected] (Y. Dong). http://dx.doi.org/10.1016/j.ijhydene.2014.01.130 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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and is able to convert into convenient solid, liquid and gaseous fuels [2]. In China, Biomass is abundant and has been widely used for a long time. The thermochemical conversion is one of effective methods to convert biomass to gas, liquid or solid fuel [3,4]. This technology has significant environmental benefits from clean exploitation of biomass, and the fuels converted from these residues release a small net emission of CO2 and other greenhouse gases compared with fossil fuels. As a thermochemical conversion technology, gasification is an important method to convert biomass into combustible gaseous fuels by partial oxidation of the biomass at high temperature. Different gasifiers are employed in this process, mainly including fixed bed, entrained flow and fluidized bed [5,6]. The main difference among these reactors is concerned with how the biomass and oxidizer are moved in the reactor. Compared with the fluidized bed and entrained flow gasifiers, fixed bed gasifier is well suited for small scale applications. The fixed bed includes downdraft and updraft fixed bed gasifier, and the selection of gasifier is determined by their different features. Besides, the tar problem is still considered as one of the main bottlenecks for industrializing the technology of biomass gasification. The downdraft fixed bed gasifier, in comparison with updraft gasifier, has the advantage of low tar generation, which is caused by the effect of the gas passing through a high temperature zone, enabling a partial cracking of the tars formed during gasification [7e9]; therefore this investigation chooses downdraft fixed bed as the gasifier. Many researchers have paid special attention to characterize the biomass gasification process through experimental studies, not only in pilot plants, but also in installations at laboratory scale. Ma et al. [10] studied biomass gasification in a 190 kWe pilot-scale biomass fixed bed using a double air stage downdraft approach and found relations for the effect of the secondary air supply on the gas composition and heating value. Tinaut et al. [11] developed a study on the biomass gasification process in a fixed bed downdraft gasifier. The effect of air superficial velocity and particle size on the autothermal gasification process was studied taking into account the propagation velocity. Pe´rez et al. [12] studied effect of operating and design parameters on the gasification/combustion process of waste biomass in fixed bed downdraft reactors. They obtained the optimal gasification conditions by changing the air superficial velocity, biomass particle size and biomass moisture content. They found that the optimal gasification conditions of lower heating value of the producer gas ¼ 2965.6 kJ/Nm3, tar concentration ¼ 7.73 g/Nm3 were obtained with the following set of inlet conditions: air superficial velocity of 0.06 m/s, biomass particle size between 2 and 6 mm, and biomass moisture content of 10.62%. Other empirical works using similar experimental approaches have been developed. Kramreiter et al. [13] built a 125 kW twin-fire fixed bed gasification pilot plant. Basic parameters like the type of wood chips, power and air distribution were varied to investigate the effect on gas composition, tar content in the producer gas and carbon content in the ash. Sharma [14] studied the temperature, gas composition, heating value and trends for pressure drop across the porous gasifier bed, cooling-cleaning train and across the system as a whole in both firing as well as non-firing mode on a 75 kWth downdraft gasifier system. Experiments showed that the rise in the bed temperature due to chemical reactions strongly influences the
pressure drop through the porous gasifier bed. In order to enhance gas heat values, Thanapal et al. [15] studied dairy biomass gasification in a medium with enriched oxygen varying and the effect of enriched air mixture, equivalence ratio and steam fuel ratio on the performance of fixed bed gasifier was studied. They found that peak temperature and carbon dioxide production increases with corresponding decrease in carbon monoxide with increase in oxygen concentration. Yoon et al. investigated non-catalytic autothermal gasification of woody biomass with air and steam mixtures. They found that Hydrogen increased with both equivalence ratio and steam-tobiomass ratio with corresponding lower heating value of the dry gaseous product varied from 2000 to 3400 kJ/Nm3. The objective of this work was to present the design and experimental results of a fixed bed gasification system using a three air stage downdraft approach and rotating grate system with corn stalk as feedstock. The stable and continuous working of the gasifier with lower tar yield and higher gas quality was expected in the experiments. The results will provide a good reference for the design of a scale-up gasification system.
2.
Materials and methods
2.1.
Biomass material
The biomass used in this study is corn stalk with a relative homogeneity in size. The corn stalk used comes from Jinan, China. The production of corn stalk is about 220 million tons every year in China, and the material represents a kind of huge straw biomass resource. The moisture and heating value were measured by means of a drying oven DHG-9240A and calorimeter system HY-A9. Ultimate analysis and proximate analysis were determined by Elemental Analyzer (Vario ELCHNO) and Muffle furnace (XL-2006). Table 1 summarizes the properties of corn stalk.
2.2.
Three air stage downdraft gasifier
The current experiments were performed using a downdraft gasifier with three stage of air supply. The gasifier was
Table 1 e Properties of corn stalk. Fuel Proximate analysis (wt. %) Moisture Volatiles Fixed carbon Ash Ultimate analysis (wt.%-daf) Carbon Hydrogen Nitrogen Sulfur Oxygen Others LHV (kJ/kg) Bulk density (g/cm3) Particle size (cm)
Corn stalk 12.5 69.5 12.2 5.8 47.54 6.02 0.77 0.13 43.87 1.67 15,527 0.081 2e5
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Fig. 1 e Sketch of the experimental setup.
designed and built at Shandong Baichuan Tongchuang Energy Company Ltd. The gasifier has an internal diameter of 0.42 m and a total height of 1.05 m. The height from the reactor top to the grate center is 0.85 m and the height of feed material in the gasifier is kept about 0.63 m in the tests. The gasifier is built of carbon steel with an internal coating of refractory material which is surrounded by 15 cm of insulating blanket for safety and minimizing heat losses. The feedstock is fed continuously from the hopper by a screw conveyor into the gasifier at the top of the reactor. The biomass flow out of the hopper depends on the bed density and the rotation speed of the drive motor .The mass flow of biomass is in the range of 5e10 kg/h, and the amount of fuel can be varied by a frequency converter. An agitator is mounted above the feed material in the gasifier to agitate the feeding fuels, avoiding bed bridging [16]. The level of feed material in the gasifier can be measured with a manual dip stick. In order to make the uniform air distribution in the oxidation and reduction zones, air is fed by three stages as seen in Fig. 1. The distances from the grate center to the first, second and third stage air are 0.43, 0.7 and 0.36 m respectively. The first stage air is fed through a pipe along the vertical axis of the gasifier and is injected around by a nozzle. The second and third airs are injected by nine nozzles which are located on the crosssection of the gasifier, as shown in Fig. 2, along the circumferential uniform distribution. The air supply is controlled by blowers and can be read by flow meters respectively. A special rotating grate system is designed to dispose the ash continuously. The rotating grate system includes three rotating grates which are built by RQTAl22 heat resisting cast iron, and is driven by a motor through chains. There are many grooves and holes on the surfaces of the grates which can take ash out of the gasifier and let the product gas go through. The advantage of this construction is to avoid the formation of bridges and to promote the bed continuous movement. The ash falls down into the ash chamber and is transported out of the gasifier by a discharge device. Besides, the coke which may appear at high temperature during the gasification process can be crushed and disposed with ash by the rotating grate system, which also helps to ensure the continuous and stable working of the gasifier.
The downdraft gasifier has four distinct reaction zones from top to the grate: drying, pyrolysis, oxidation and reduction. The drying process occurs in drying zone in the upper part of the reactor, as well as lighter compounds devolatilization. Pyrolyzed gas and charcoal are generated in pyrolysis zone and flow downwards. Then, the pyrolysis gases pass through the combustion zone where oxidation reactions occur and release heat. The heat generated in the combustion zone is transferred to the pyrolysis, drying zones for biomass drying and devolatilization [17,18]. Tar generated in pyrolysis zone cracks into non-condensable gases due to the high temperature of the combustion zone. The gas stream flows to the reduction zone at the bottom of the reactor where the unconverted carbon and ash promotes its endothermic reduction reactions. The product gas, leaving the reduction zone at grate, is sampled for analysis.
2.3.
Operating procedures
The gasifier is operated at atmospheric pressure. First, a specified quantity of feedstock in the gasifier is ignited and induced fan is turned on to preheat the gasifier. Afterwards,
Fig. 2 e The air injection of the second and third air.
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biomass is fed into the gasifier at desired feeding rate by the screw feeder, and the air flow rate is adjusted properly to ensure the sufficient air atmosphere in the gasifier. The temperature of the gasifier is raised step-by-step. When the temperature at oxidation zone reaches about 900 C after about 1.5 h, the grate can be activated and the tests start up. Then, the first, second and third airs are adjusted to maintain the desired temperatures. As the fuel was gasified, the ash produced was gradually discharged by the rotating grate system. The bed was maintained at a constant height by adjusting the fuel feeding rate and ash discharging rate. When the gasifier has reached a steady-state, all parameters are kept constant for at least 60 min for gas sampling and analysis.
2.4.
Product gas sampling and measurement procedures
Temperatures along the gasifier are monitored every 60 s by type K thermocouples located at 0.06, 0.13, 0.2, 0.27, 0.34, 0.41, 0.48 and 0.55 m above of the grate center. In order to avoid possible problems with the flowing of the biomass as it is consumed, thermocouples are projected up to the internal reactor wall and the temperatures are read as the axial temperature distribution inside the gasifier [19]. The thermocouples can be adjusted at radial direction if necessary to detect the radial temperature distribution. The temperatures presented within this investigation were all given as the average. When the gasifier reaches a steady-state at each test, gas sampling is carried out. The condensable compounds which include tar, water and a few soot particles are absorbed in the acetone bottle in ice-water bath. MgSO4 is added into the tarcontained solution with 10e15 g per 100 ml to absorb water, and then the solution is filtered. The tar and acetone are separated on a rotary evaporator (RE-5299) at 40 C for about 2.5 h. Finally, the remaining tar is weighed to determine its quantity. The clean, cool and dry gas downstream the tar sampling system is sampled by a gas sampling bag at 10-min intervals. Then, the main gas composition is analyzed by a gas chromatography (GC, Agilent 3000 A) which can detect CO, H2, CO2, O2, CH4, CnHm (includes C2H4, C2H6 and C3H8) and N2. To assure the reliability of the test results, each test is repeated for five times, and the variability of measuring data is within 5%.
3.
Results and discussion
3.1.
Temperature distribution in the gasifier
After the biomass was fed into the gasifier, the biomass material underwent moisture evaporation, pyrolysis and char gasification primarily in the riser. When the biomass feeding rate was 7.5 kg/h, the height of the biomass in the gasifier was kept at 0.63 m, and the flow rates of the first, second and third stage air supply were 1.4 m3/h, 1.4 m3/h and 6.6 m3/ h(ER ¼ 0.29), the axial temperature distribution inside the gasifier was shown in Fig. 3 based on their average value from T1 to T8. The location above 0.48 m from the grate is the pyrolysis zone with temperature below 578 C (T2). The biomass particles are decomposed into volatile gases (CO, H2, CO2, CH4 and light hydrocarbons), charcoal and tar in the pyrolysis zone, as
Fig. 3 e Temperature axial profile inside the gasifier above the grate center.
shown in reaction (1). The pyrolysis gases, including volatile gases and tar, then flow downwards. The location below pyrolysis zone is oxidation zone where the charcoal and volatile gases burn fiercely (reactions (2)e(6)) with homogeneous air distribution, so that a great quantity of heat is released to meet the heat demand for the whole gasification process. The high temperature in the oxidation helps reduction reactions and thus tar cracking takes place to improve the calorific value and reduce the tar content in the gas, as suggested by Devi et al. [20] and Bhattacharya et al. [21]. Pyrolysis :
Cx Hy Oz /Charcoal þ Tar þ Volatile gases
Oxidation :
CðsÞ þ O2 ¼ CO2
Partial oxidation :
CO þ 0:5O2 ¼ CO2 H2 þ 0:5O2 ¼ H2 OðgÞ
Hydrogen oxidation : Methane oxidation :
CH4 þ 1:5O2 ¼ CO þ 2H2 O
Tar oxidation : Cp Hq ðTarÞ þ ðn=2 þ m=4ÞO2 ¼ nCO þ ðm=2ÞH2 O
(1) (2) (3) (4) (5)
(6)
Water gas shift :
CO þ H2 O4CO2 þ H2
(7)
Steam reforming :
CH4 þ H2 O ¼ CO þ 3H2
(8)
Boudouard :
CðsÞ þ CO2 ¼ 2CO
Water gas :
CðsÞ þ H2 O ¼ CO þ H2
Methanation reaction : Tar reforming : Tar cracking :
CðsÞ þ 2H2 ¼ CH4
Cp Hq ðTarÞ þ nH2 O ¼ nCO þ ðn þ m=2ÞH2 Cp Hq ðTarÞ/Cx Hy þ C þ H2 þ CO
(9) (10) (11)
(12) (13)
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Fig. 5 e Effect of the equivalence ratio on the temperatures in the gasifier.
Fig. 4 e Temperature radial profiles inside the gasifier.
The reduction zone is just next to the bottom of the oxidation zone. The oxygen is exhausted and only ash and unconverted carbon are left in this zone, so that the temperature decreases sharply as some endothermic reduction reactions occur (reactions (7)e(13)). It is believed that the ash of biomass was mainly composed by oxides, particularly CaO, MgO, K2O, Al2O3 and Fe2O3 which have obvious function on tar catalytic decomposition [22]. Therefore, a uniform and stable reduction zone is important in decreasing the tar content and improving gas quality. The use of three stage of air supply aims at realizing the uniform air distribution in oxidation and reduction zones, and consequently the uniform reaction can be gained in the same cross section of the gasifier. The radial temperature distribution in oxidation zone and reduction zone is shown in Fig. 4. The temperature at T4 (0.34 m above the grate) has a little fluctuations, while little difference is observed at the location below T5 which indicates that the air distribution is uniform in the same cross section and in turn a more consistent reaction intensity is achieved. The uniform reaction intensity has an important effect on avoiding the phenomenon of bridges and channeling resulting from uniform reaction. During the whole tests operation, the gasifier worked stable and no bridges or channeling phenomenon occurred. The stable working favored more stable gasification results, including gas composition and tar yield, and the variation of these parameters were relatively small in the following tests even though the equivalence ratio and biomass feeding rate were changed.
3.2.
Effect of equivalence ratio
The equivalence ratio, ER, defined as the ratio of the actual air supply to the stoichiometric air required for complete combustion on a daf (dry ash free) basis is calculated by Eq. (14) in this study. ER indicates the oxygen feed in the gasification and it is a crucial factor that affects the performance of the gasification process. ER was changed by adjusting the air feeding rate during the experiments. Seven experiments were performed by changing only air feeding rate with the biomass feeding rate kept constant, contributing to a detailed parametric study of the effect of ER. The experimental conditions are presented in Table 2. Fm;air =Fm;fuelðdafÞ Actual ER ¼ Fm;air =Fm;fuelðdafÞ stoich
(14)
where, 1:293 Cdaf Hdaf 1:866 þ 5:55 Fm;air =Fm;fuelðdafÞ stoich ¼ 0:21 100 100 Sdaf Odaf 0:7 þ 0:7 100 100
(15)
where, Fm,i(daf) is the mass flow rate of i on a daf basis [kg/h]; Idaf is the mass fraction of I on a daf basis. Fig. 5 shows the temperatures in the gasifier with the changing of ER. The increase of ER enhances the combustion reactions to release heat, which results in a higher operating temperature in the gasifier. The change trend of axial
Table 2 e Effect of the equivalence ratio on the gasification performance. No. Feeding rate, kg/h Air flow, Nm3/h First stage Second stage Third stage ER
Run1
Run2
Run3
Run4
Run5
Run6
Run7
7.5
7.5
7.5
7.5
7.5
7.5
7.5
0.9 0.9 4.1 0.18
1.1 1.1 5.0 0.22
1.2 1.2 5.7 0.25
1.3 1.3 6.2 0.27
1.4 1.4 6.6 0.29
1.6 1.6 7.3 0.32
1.8 1.8 8.5 0.37
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combustible gases. Thus, the lower heating value (LHV) of the product gas rapidly decreased. Variation of cold gas efficiency and carbon conversion efficiency are used to investigate the effect of ER on the energy and mass conversion [23]. The cold gas efficiency (h) in this study is calculated as: h¼
Fig. 6 e Effect of ER on gas composition.
temperature in the gasifier is similar at different ER. The temperature is relatively low in upper portion of the gasifier where is the pyrolysis zone, and the peak temperature occurs at oxidation zone (T4). The experimental results of gas composition by the variation of the ER are shown in Fig. 6. With the increase in ER, the volume fraction of H2 increased first and attained the highest value of 12.89% at ER ¼ 0.25. Keeping on increasing ER, the combustion reaction (reaction (4)) increased which resulted in the decrease in H2. The changing trend of CO was similar with H2 with the peak value of 19.41% at ER ¼ 0.27. The trends of CO and CO2 represent the main carbon conversion during the gasification process. Thus, it could be seen that the trend of CO2 was almost opposite to CO and approached the minimum value of 15.49% at ER ¼ 0.27. A higher value of ER represents that more CH4 burns with O2 (reaction (5)) and the formation of CH4 (exothermic reaction (11)) is inhibited at higher temperature. Therefore, the volume fraction of CH4 decreased with the increase of ER. CnHm represents light hydrocarbons (C2eC3) produced in the gasifier with total volume fraction about 1%. There was no evident change of CnHm with the varying ER. The dry gas lower heating value at the standard state of 101.3 kPa and 273 K can be estimated from the gas composition by Eq. (16): LHV ¼ 107:98H2 þ 126:36CO þ 358:18CH4 þ 629:09Cn Hm kJ=Nm3
½LHVgas Vg ½LHVfuel
100%
where, [LHV]gas is the lower heating value of the gas, kJ/Nm3; Vg is the specific gas yield at the standard state, Nm3/kg. Nitrogen tracer method was developed to determine the gas yield from the percentage of nitrogen in the gases produced during gasification. The corn stalk has very low percentage of N which is 0.77% (Table 2), which is very low compared to the amount of N2 which enters the reactor along with air. Thus N2 released from the fuel during gasification is negligible compared to N2 from air used as the gasification medium. From the concentration of nitrogen in the product gas and the total amount of nitrogen entering the reactor along with air in the gasification process, the total dry volumes of gas produced can be estimated using the following formula: Vg ¼
Qa 4N2 Nm3 =kg 40N2
(18)
where, Qa is the volume of air fed for 1 kg biomass, Nm3/kg; 4N2 represents the volume fraction of N2 in air at the standard state, 78.12%; 40N2 is the volume fraction of N2 in product gas at the standard stage, vol. %. The carbon conversion efficiency (hc) is defined as the ratio of carbon which is converted from the added fuel into gaseous carbon components to the carbon in the added fuel. As the tar was removed from the gas at ordinary temperatures, the carbon in tar was not taken into account in the experiments. Therefore, the carbon conversion efficiency can be calculated as: hc ¼
12ðCO2 þ CO þ CH4 þ nCn Hm Þ Vg 100% 22:4 C
(19)
where, C is the percentage of mass fraction of carbon in the fuel.
(16)
where, CO, H2, CH4 and CnHm are percentages of the volume fraction of carbon monoxide, hydrogen, methane and hydrocarbons in the product gas. Since the gas heating value was determined by the concentrations of gases, the LHV increased first and decreased afterwards with increase in ER and reached the peak value at ER ¼ 0.25e0.27. An increase in ER leads to higher temperature in the gasifier due to stronger combustion reactions (mainly reaction (2)), as shown in Fig. 5. More H2 and CO generate by biomass pyrolysis (reaction (1)) and tar decomposition (reaction (6), (12), (13)), which leads to the increase in LHV. However, too high ER results in the decrease in the volume fraction of combustible gases (H2, CO, CH4 and CnHm) as a result of their combustion reactions (reaction (3)e(5)). Besides, more N2 was supplied into the gasifier at higher ER which diluted the
(17)
Fig. 7 e Effect of ER on cold gas efficiency and carbon conversion efficiency.
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Fig. 7 gives the values of cold gas efficiency and carbon conversion efficiency at different ER. When ER 0.25, the cold gas efficiency increased sharply, and then remained at a high level about 65% when ER was between 0.25 and 0.27. The cold gas efficiency begun to decrease when ER 0.3, which indicated that the total energy conversion of biomass decreased. The carbon conversion efficiency represents the carbon conversion from biomass fuel to product gas, which is the main element conversion during biomass gasification process. With an increase ER from 0.18 to 0.27, the carbon conversion efficiency increased from 51.44% to 83.78%. As ER further increased, the carbon conversion efficiency did not show any significant change and remained at about 90%. That indicates that higher ER favors the reactions involving carbon and about 90% of the carbon in biomass fuel can be converted to gas by gasification when ER >0.27. A thick char bed was obtained by three stage air supply which was beneficial to tar cracking, and the cracking reaction was favored when the temperature increased with increasing ER. Fig. 8 shows the effect of change in ER on tar yield. The tar yield dropped to 1.07 g/Nm3 at ER ¼ 0.27. When ER >0.27, although the temperature further increased, excessively high ER resulted in a higher gas yield and a shorter gas residence, so that the tar dropping speed gradually slowed down. It was observed that the tar yield was only 0.52 g/Nm3 at ER ¼ 0.32, which meant that the tar yield could decrease to about 0.5 g/ Nm3 without catalyst in the downdraft gasifier with three airblown supply, while the lower heating value decreased as well only 3591.6 kJ/Nm3 (Fig. 6). The tar yield was close to the value obtained by Kramreiter et al. [13] using twin-fired downdraft fixed bed. Thus, it is possible to solve the tar problem by designing the structure of gasifier and changing the air distribution way inside the gasifier.
3.3.
Effect of feeding rate
For a specified gasifier, accelerating biomass feeding rate (FR) is beneficial for increasing production capacity. However, excessively high feeding rate will result in a higher gas yield and a shorter gas residence which leads to lower gas quality
Fig. 8 e Effect of ER on tar yield.
Table 3 e Effect of the feeding rate on the gasification performance. No. Air flow, Nm3/h First stage Second stage Third stage ER Feeding rate, kg/h
Run8
Run9
Run10
Run11
Run12
1.08 1.08 4.97 0.27 5.8
1.24 1.24 5.74 0.27 6.7
1.39 1.39 6.43 0.27 7.5
1.59 1.59 7.37 0.27 8.6
1.72 1.72 7.97 0.27 9.3
and higher tar yield. Therefore, a study is carried out to investigate its effect on gas and tar yield. The rotational frequency of grates changed with the biomass feeding rate to keep the height of feed material in the gasifier constant, at about 0.63 m. The experimental conditions are presented in Table 3. Fig. 9 shows the temperature in the gasifier as a function of the biomass feeding rate. It was observed that the temperature increased with feeding rate. A higher value of biomass feeding rate accelerates the rate of reactions in the gasifier, especially stronger oxidization reactions promoting the increase of temperature. As Fig. 10 illustrated, the concentrations of H2 and CO in the product gas decreased with the increase in biomass feeding rate. This proved that excessively high feeding rate is unbeneficial for biomass gasification cracking and reforming reactions, which leads to a reduction of H2 and CO content in gases. The concentration of CO2 increased gradually, which indicated that more CO2 was produced by the stronger oxidation reactions. Besides, the shorter gas residence in reduction zone led to less CO2 consumption via C. The pyrolysis reactions of biomass were reinforced by increasing feeding rate which resulted in a slight increase in CH4 and CnHm. Consequentially, increasing feeding rate trended to decrease the lower heating value. The lower heating value decreased from 5455.5 kJ/Nm3 to 5253.2 kJ/Nm3 when feeding rate changed from 5.8 kg/h to 9.3 kg/h.
Fig. 9 e Effect of biomass feeding rate on the temperatures in the gasifier.
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Fig. 10 e Effect of biomass feeding rate on gas composition.
Fig. 11 shows the effect of biomass feeding rate on tar yield. As discussed above, the higher gas yield resulted in a shorter gas residence time in high temperature zones, which was unfavorable to tar cracking and reforming reactions, so that the tar yield continuously increased with feeding rate. Lv et al. [24] conducted pine wood block gasification experiment in a self-heated downdraft gasifier using char as the catalyst and reported that the tar yield decreased with an increase in biomass feeding rate too. They also noticed that higher feeding rate resulted in the decrease in hydrogen content and lower heating value. In our previous research [4], the gasification of herb residues in a fluidized bed was investigated using air as gasifying agent and also found that tar yield increased with an increase in biomass feeding rate.
4.
Conclusions
This work presents the design and test results of a biomass fixed bed gasification using a three air stage continuous downdraft approach. Based on the tests, it was found that the
Fig. 11 e Effect of biomass feeding rate on tar yield.
three stage of air supply can yield a high and uniform temperature in the oxidation and reduction zones for better tar cracking. Use of both the air supply system and rotating grate system avoided bridging and channeling. By increasing ER, the combustion reactions were enhanced to release heat, which in turn led to higher temperature in the gasifier. For the range of ER investigated (0.18 < ER < 0.37), the volume fraction of CO and H2 underwent the process of a fall after a rise, whilst the volume fraction of CO2 rose after a fall. When biomass feeding rate was 7.5 kg/h and ER was 0.25e0.27, the product gas of the downdraft fixed bed attains a good condition with LHV about 5400 kJ/m3 and cold gas efficiency about 65%. As ER increased, the higher temperature also led to higher tar cracking rate, which resulted in lower tar yield. The tar yield as low as 0.52 g/Nm3 was achieved at ER ¼ 0.32. Increasing biomass feeding rate led to higher biomass consumption rates which in turn resulted in higher process temperatures. However, excessively high feeding rate is unbeneficial for biomass gasification cracking and reforming reactions, which leads to a reduction of H2 and CO content in gases and an increase in tar yield. When ER was 0.27, with an increase of biomass feeding rate from 5.8 kg/h to 9.3 kg/h, the low heating value decreased from 5455.5 kJ/Nm3 to 5253.2 kJ/ Nm3 and tar yield increased from 0.82 g/Nm3 to 2.78 g/Nm3.
Acknowledgements This project was supported by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology).
references
[1] Leung DYC, Yin XL, Wu CZ. A review on the development and commercialization of biomass gasification technologies in China. Renew Sustain Energy Rev 2004;8:565e80. [2] Demirbas A. Bioresource facilities and biomass conversion processing for fuels and chemicals. Energy Convers Manag 2001;42:1357e78. [3] Wang P, Zhan SH, Yu HB, Xue XF, Hong N. The effects of temperature and catalysts on the pyrolysis of industrial wastes (herb residue). Bioresour Technol 2010;101:3236e41. [4] Guo FQ, Dong YP, Dong L, Jing YZ. An innovative example of herb residues recycling by gasification in a fluidized bed. Waste Manag 2013;33:825e32. [5] Raman P, Ram NK. Design improvements and performance testing of a biomass gasifier based electric power generation system. Biomass Bioenergy 2013;56:555e71. [6] Calvo LF, Gil MV, Otero M, Mora´n A, Garcı´aa AI. Gasification of rice straw in a fluidized-bed gasifier for syngas application in close-coupled boiler-gasifier systems. Bioresour Technol 2012;109:206e14. [7] Van de steene L, Tagutchou JP, Mermoud F, Martin E, Salvador S. A new experimental continuous fixed bed reactor to characterise wood char gasification. Fuel 2010;89:3320e9. [8] Sheth PN, Babu BV. Production of hydrogen energy through biomass (waste wood) gasification. Int J Hydrogen Energy 2010;35:10803e10.
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[9] Yoon HC, Cooper T, Steinfeld A. Non-catalytic autothermal gasification of woody biomass. Int J Hydrogen Energy 2011;36:7852e60. [10] Ma ZQ, Zhang YM, Zhang QS, Qu YB, Zhou JB, Qin HF. Design and experimental investigation of a 190 kWe biomass fixed bed gasification and polygeneration pilot plant using a double air stage downdraft approach. Energy 2012;46:140e7. [11] Tinaut F, Melgar A, Pe´rez JF, Horrillo A. Effect of biomass particle size and air superficial velocity on the gasification process in a downdraft fixed bed gasifier: an experimental and modelling study. Fuel Process Technol 2008;89:1076e89. [12] Pe´rez JF, Melgar A, Benjumea PN. Effect of operating and design parameters on the gasification/combustion process of waste biomass in fixed bed downdraft reactors: an experimental study. Fuel 2012;96:478e96. [13] Kramreiter R, Url M, Kotik J, Hofbauer H. Experimental investigation of a 125 kW twin-fire fixed bed gasification pilot plant and comparison to the results of a 2 MW combined heat and power plant (CHP). Fuel Process Technol 2008;89:90e102. [14] Sharma AK. Experimental study on 75 kWth downdraft (biomass) gasifier system. Renew Energy 2009;34:1726e33. [15] Thanapal SS, Annamalai K, Sweeten JM, Gordillo G. Fixed bed gasification of dairy biomass with enriched air mixture. Appl Energy 2012;97:525e31. [16] Gai C, Dong YP. Experimental study on non-woody biomass gasification in a downdraft gasifier. Int J Hydrogen Energy 2012;37:4935e44.
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[17] Babu BV, Chaurasia AS. Parametric study of thermal and thermodynamic properties on pyrolysis of biomass in thermally thick regime. Energy Convers Manag 2004;45:53e72. [18] Shetha PN, Babu BV. Experimental studies on producer gas generation from wood waste in a downdraft biomass gasifier. Bioresour Technol 2009;100:3127e33. [19] Martı´neza JD, Loraa EES, Andradea RV, Jae´n RL. Experimental study on biomass gasification in a double air stage downdraft reactor. Biomass Bioenergy 2011;35(8):3465e80. [20] Devi L, Ptasinski KJ, Janssen F. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003;24:125e40. [21] Bhattacharya SC, Hla SS, Pham HL. A study on a multi-stage hybrid gasifier engine system. Biomass Bioenergy 2001;21:445e60. [22] El-Rub ZA, Bramer EA, Brem G. Experimental comparison of biomass chars with other catalysts for tar reduction. Fuel 2008;10e11:2243e52. [23] Li XT, Grace JR, Lim CJ, Watkinson AP, Chen HP, Kim JR. Biomass gasification in a circulating fluidized bed. Biomass Bioenergy 2004;26:171e93. [24] Lv PM, Yuan ZH, Ma LL, Wu CZ, Chen Y, Zhu JX. Hydrogenrich gas production from biomass air and oxygen/steam gasification in a downdraft gasifier. Renew Energy 2007;32:2173e85.
Available online at www.sciencedirect.com
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Effect of design and operating parameters on the gasification process of biomass in a downdraft fixed bed: An experimental study Feiqiang Guo a, Yuping Dong b,c,*, Lei Dong c, Chenlong Guo a a
School of Electric Power Engineering, China University of Mining and Technology, Xuzhou 221116, PR China School of Mechanical Engineering, Shandong University, Jinan 250061, PR China c Shandong Baichuan Tongchuang Energy Company Ltd., Jinan 250101, PR China b
article info
abstract
Article history:
The main objective of this paper is to study the effect of design and operating parameters,
Received 7 August 2013
mainly reactor geometry, equivalence ratio and biomass feeding rate, on the performance
Received in revised form
of the gasification process of biomass in a three air stage continuous fixed bed downdraft
22 December 2013
reactor. The gasification of corn straw was carried out in the gasifier under atmospheric
Accepted 21 January 2014
pressure, using air as gasifying agent. The results demonstrated that due to the three stage
Available online 1 March 2014
of air supply, a high and uniform temperature was achieved in the oxidation and reduction zones for better tar cracking. The designing of both the air supply system and rotating grate
Keywords:
avoided bridging and channeling. The gas composition and tar yield were affected by the
Biomass
parameters including equivalence ratio (ER) and biomass feeding rate. When biomass
Downdraft fixed bed
feeding rate was 7.5 kg/h and ER was 0.25e0.27, the product gas of the gasifier attained a
Gasification
good condition with lower heating value (LHV) about 5400 kJ/m3 and cold gas efficiency
Experiments
about 65%. An increase in equivalence ratio led to higher temperature which in turn resulted in lower tar yield which was only 0.52 g/Nm3 at ER ¼ 0.32. Increasing biomass feeding rate led to higher biomass consumption rate and process temperature. However, excessively high feeding rate was unbeneficial for biomass gasification cracking and reforming reactions, which led to a decrease in H2 and CO concentrations and an increase in tar yield. When ER was 0.27, with an increase of biomass feeding rate from 5.8 kg/h to 9.3 kg/h, the lower heating value decreased from 5455.5 kJ/Nm3 to 5253.2 kJ/Nm3 and tar yield increased from 0.82 g/Nm3 to 2.78 g/Nm3. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved .
1.
Introduction
With the fast economic growth, increasing demand for energy, particularly in developing countries like China, has
exacerbated the concerns over energy crisis caused by overexploitation of fossil fuels. Research has increasingly included efforts to partially replace fossil fuels with renewable energy sources [1]. Of all the renewable energy sources, biomass energy is the only renewable energy source of carbon
* Corresponding author. School of Mechanical Engineering, Shandong University, Jinan 250061, PR China. Tel./fax: þ86 531 88392199. E-mail addresses: [email protected] (F. Guo), [email protected] (Y. Dong). http://dx.doi.org/10.1016/j.ijhydene.2014.01.130 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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and is able to convert into convenient solid, liquid and gaseous fuels [2]. In China, Biomass is abundant and has been widely used for a long time. The thermochemical conversion is one of effective methods to convert biomass to gas, liquid or solid fuel [3,4]. This technology has significant environmental benefits from clean exploitation of biomass, and the fuels converted from these residues release a small net emission of CO2 and other greenhouse gases compared with fossil fuels. As a thermochemical conversion technology, gasification is an important method to convert biomass into combustible gaseous fuels by partial oxidation of the biomass at high temperature. Different gasifiers are employed in this process, mainly including fixed bed, entrained flow and fluidized bed [5,6]. The main difference among these reactors is concerned with how the biomass and oxidizer are moved in the reactor. Compared with the fluidized bed and entrained flow gasifiers, fixed bed gasifier is well suited for small scale applications. The fixed bed includes downdraft and updraft fixed bed gasifier, and the selection of gasifier is determined by their different features. Besides, the tar problem is still considered as one of the main bottlenecks for industrializing the technology of biomass gasification. The downdraft fixed bed gasifier, in comparison with updraft gasifier, has the advantage of low tar generation, which is caused by the effect of the gas passing through a high temperature zone, enabling a partial cracking of the tars formed during gasification [7e9]; therefore this investigation chooses downdraft fixed bed as the gasifier. Many researchers have paid special attention to characterize the biomass gasification process through experimental studies, not only in pilot plants, but also in installations at laboratory scale. Ma et al. [10] studied biomass gasification in a 190 kWe pilot-scale biomass fixed bed using a double air stage downdraft approach and found relations for the effect of the secondary air supply on the gas composition and heating value. Tinaut et al. [11] developed a study on the biomass gasification process in a fixed bed downdraft gasifier. The effect of air superficial velocity and particle size on the autothermal gasification process was studied taking into account the propagation velocity. Pe´rez et al. [12] studied effect of operating and design parameters on the gasification/combustion process of waste biomass in fixed bed downdraft reactors. They obtained the optimal gasification conditions by changing the air superficial velocity, biomass particle size and biomass moisture content. They found that the optimal gasification conditions of lower heating value of the producer gas ¼ 2965.6 kJ/Nm3, tar concentration ¼ 7.73 g/Nm3 were obtained with the following set of inlet conditions: air superficial velocity of 0.06 m/s, biomass particle size between 2 and 6 mm, and biomass moisture content of 10.62%. Other empirical works using similar experimental approaches have been developed. Kramreiter et al. [13] built a 125 kW twin-fire fixed bed gasification pilot plant. Basic parameters like the type of wood chips, power and air distribution were varied to investigate the effect on gas composition, tar content in the producer gas and carbon content in the ash. Sharma [14] studied the temperature, gas composition, heating value and trends for pressure drop across the porous gasifier bed, cooling-cleaning train and across the system as a whole in both firing as well as non-firing mode on a 75 kWth downdraft gasifier system. Experiments showed that the rise in the bed temperature due to chemical reactions strongly influences the
pressure drop through the porous gasifier bed. In order to enhance gas heat values, Thanapal et al. [15] studied dairy biomass gasification in a medium with enriched oxygen varying and the effect of enriched air mixture, equivalence ratio and steam fuel ratio on the performance of fixed bed gasifier was studied. They found that peak temperature and carbon dioxide production increases with corresponding decrease in carbon monoxide with increase in oxygen concentration. Yoon et al. investigated non-catalytic autothermal gasification of woody biomass with air and steam mixtures. They found that Hydrogen increased with both equivalence ratio and steam-tobiomass ratio with corresponding lower heating value of the dry gaseous product varied from 2000 to 3400 kJ/Nm3. The objective of this work was to present the design and experimental results of a fixed bed gasification system using a three air stage downdraft approach and rotating grate system with corn stalk as feedstock. The stable and continuous working of the gasifier with lower tar yield and higher gas quality was expected in the experiments. The results will provide a good reference for the design of a scale-up gasification system.
2.
Materials and methods
2.1.
Biomass material
The biomass used in this study is corn stalk with a relative homogeneity in size. The corn stalk used comes from Jinan, China. The production of corn stalk is about 220 million tons every year in China, and the material represents a kind of huge straw biomass resource. The moisture and heating value were measured by means of a drying oven DHG-9240A and calorimeter system HY-A9. Ultimate analysis and proximate analysis were determined by Elemental Analyzer (Vario ELCHNO) and Muffle furnace (XL-2006). Table 1 summarizes the properties of corn stalk.
2.2.
Three air stage downdraft gasifier
The current experiments were performed using a downdraft gasifier with three stage of air supply. The gasifier was
Table 1 e Properties of corn stalk. Fuel Proximate analysis (wt. %) Moisture Volatiles Fixed carbon Ash Ultimate analysis (wt.%-daf) Carbon Hydrogen Nitrogen Sulfur Oxygen Others LHV (kJ/kg) Bulk density (g/cm3) Particle size (cm)
Corn stalk 12.5 69.5 12.2 5.8 47.54 6.02 0.77 0.13 43.87 1.67 15,527 0.081 2e5
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Fig. 1 e Sketch of the experimental setup.
designed and built at Shandong Baichuan Tongchuang Energy Company Ltd. The gasifier has an internal diameter of 0.42 m and a total height of 1.05 m. The height from the reactor top to the grate center is 0.85 m and the height of feed material in the gasifier is kept about 0.63 m in the tests. The gasifier is built of carbon steel with an internal coating of refractory material which is surrounded by 15 cm of insulating blanket for safety and minimizing heat losses. The feedstock is fed continuously from the hopper by a screw conveyor into the gasifier at the top of the reactor. The biomass flow out of the hopper depends on the bed density and the rotation speed of the drive motor .The mass flow of biomass is in the range of 5e10 kg/h, and the amount of fuel can be varied by a frequency converter. An agitator is mounted above the feed material in the gasifier to agitate the feeding fuels, avoiding bed bridging [16]. The level of feed material in the gasifier can be measured with a manual dip stick. In order to make the uniform air distribution in the oxidation and reduction zones, air is fed by three stages as seen in Fig. 1. The distances from the grate center to the first, second and third stage air are 0.43, 0.7 and 0.36 m respectively. The first stage air is fed through a pipe along the vertical axis of the gasifier and is injected around by a nozzle. The second and third airs are injected by nine nozzles which are located on the crosssection of the gasifier, as shown in Fig. 2, along the circumferential uniform distribution. The air supply is controlled by blowers and can be read by flow meters respectively. A special rotating grate system is designed to dispose the ash continuously. The rotating grate system includes three rotating grates which are built by RQTAl22 heat resisting cast iron, and is driven by a motor through chains. There are many grooves and holes on the surfaces of the grates which can take ash out of the gasifier and let the product gas go through. The advantage of this construction is to avoid the formation of bridges and to promote the bed continuous movement. The ash falls down into the ash chamber and is transported out of the gasifier by a discharge device. Besides, the coke which may appear at high temperature during the gasification process can be crushed and disposed with ash by the rotating grate system, which also helps to ensure the continuous and stable working of the gasifier.
The downdraft gasifier has four distinct reaction zones from top to the grate: drying, pyrolysis, oxidation and reduction. The drying process occurs in drying zone in the upper part of the reactor, as well as lighter compounds devolatilization. Pyrolyzed gas and charcoal are generated in pyrolysis zone and flow downwards. Then, the pyrolysis gases pass through the combustion zone where oxidation reactions occur and release heat. The heat generated in the combustion zone is transferred to the pyrolysis, drying zones for biomass drying and devolatilization [17,18]. Tar generated in pyrolysis zone cracks into non-condensable gases due to the high temperature of the combustion zone. The gas stream flows to the reduction zone at the bottom of the reactor where the unconverted carbon and ash promotes its endothermic reduction reactions. The product gas, leaving the reduction zone at grate, is sampled for analysis.
2.3.
Operating procedures
The gasifier is operated at atmospheric pressure. First, a specified quantity of feedstock in the gasifier is ignited and induced fan is turned on to preheat the gasifier. Afterwards,
Fig. 2 e The air injection of the second and third air.
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biomass is fed into the gasifier at desired feeding rate by the screw feeder, and the air flow rate is adjusted properly to ensure the sufficient air atmosphere in the gasifier. The temperature of the gasifier is raised step-by-step. When the temperature at oxidation zone reaches about 900 C after about 1.5 h, the grate can be activated and the tests start up. Then, the first, second and third airs are adjusted to maintain the desired temperatures. As the fuel was gasified, the ash produced was gradually discharged by the rotating grate system. The bed was maintained at a constant height by adjusting the fuel feeding rate and ash discharging rate. When the gasifier has reached a steady-state, all parameters are kept constant for at least 60 min for gas sampling and analysis.
2.4.
Product gas sampling and measurement procedures
Temperatures along the gasifier are monitored every 60 s by type K thermocouples located at 0.06, 0.13, 0.2, 0.27, 0.34, 0.41, 0.48 and 0.55 m above of the grate center. In order to avoid possible problems with the flowing of the biomass as it is consumed, thermocouples are projected up to the internal reactor wall and the temperatures are read as the axial temperature distribution inside the gasifier [19]. The thermocouples can be adjusted at radial direction if necessary to detect the radial temperature distribution. The temperatures presented within this investigation were all given as the average. When the gasifier reaches a steady-state at each test, gas sampling is carried out. The condensable compounds which include tar, water and a few soot particles are absorbed in the acetone bottle in ice-water bath. MgSO4 is added into the tarcontained solution with 10e15 g per 100 ml to absorb water, and then the solution is filtered. The tar and acetone are separated on a rotary evaporator (RE-5299) at 40 C for about 2.5 h. Finally, the remaining tar is weighed to determine its quantity. The clean, cool and dry gas downstream the tar sampling system is sampled by a gas sampling bag at 10-min intervals. Then, the main gas composition is analyzed by a gas chromatography (GC, Agilent 3000 A) which can detect CO, H2, CO2, O2, CH4, CnHm (includes C2H4, C2H6 and C3H8) and N2. To assure the reliability of the test results, each test is repeated for five times, and the variability of measuring data is within 5%.
3.
Results and discussion
3.1.
Temperature distribution in the gasifier
After the biomass was fed into the gasifier, the biomass material underwent moisture evaporation, pyrolysis and char gasification primarily in the riser. When the biomass feeding rate was 7.5 kg/h, the height of the biomass in the gasifier was kept at 0.63 m, and the flow rates of the first, second and third stage air supply were 1.4 m3/h, 1.4 m3/h and 6.6 m3/ h(ER ¼ 0.29), the axial temperature distribution inside the gasifier was shown in Fig. 3 based on their average value from T1 to T8. The location above 0.48 m from the grate is the pyrolysis zone with temperature below 578 C (T2). The biomass particles are decomposed into volatile gases (CO, H2, CO2, CH4 and light hydrocarbons), charcoal and tar in the pyrolysis zone, as
Fig. 3 e Temperature axial profile inside the gasifier above the grate center.
shown in reaction (1). The pyrolysis gases, including volatile gases and tar, then flow downwards. The location below pyrolysis zone is oxidation zone where the charcoal and volatile gases burn fiercely (reactions (2)e(6)) with homogeneous air distribution, so that a great quantity of heat is released to meet the heat demand for the whole gasification process. The high temperature in the oxidation helps reduction reactions and thus tar cracking takes place to improve the calorific value and reduce the tar content in the gas, as suggested by Devi et al. [20] and Bhattacharya et al. [21]. Pyrolysis :
Cx Hy Oz /Charcoal þ Tar þ Volatile gases
Oxidation :
CðsÞ þ O2 ¼ CO2
Partial oxidation :
CO þ 0:5O2 ¼ CO2 H2 þ 0:5O2 ¼ H2 OðgÞ
Hydrogen oxidation : Methane oxidation :
CH4 þ 1:5O2 ¼ CO þ 2H2 O
Tar oxidation : Cp Hq ðTarÞ þ ðn=2 þ m=4ÞO2 ¼ nCO þ ðm=2ÞH2 O
(1) (2) (3) (4) (5)
(6)
Water gas shift :
CO þ H2 O4CO2 þ H2
(7)
Steam reforming :
CH4 þ H2 O ¼ CO þ 3H2
(8)
Boudouard :
CðsÞ þ CO2 ¼ 2CO
Water gas :
CðsÞ þ H2 O ¼ CO þ H2
Methanation reaction : Tar reforming : Tar cracking :
CðsÞ þ 2H2 ¼ CH4
Cp Hq ðTarÞ þ nH2 O ¼ nCO þ ðn þ m=2ÞH2 Cp Hq ðTarÞ/Cx Hy þ C þ H2 þ CO
(9) (10) (11)
(12) (13)
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Fig. 5 e Effect of the equivalence ratio on the temperatures in the gasifier.
Fig. 4 e Temperature radial profiles inside the gasifier.
The reduction zone is just next to the bottom of the oxidation zone. The oxygen is exhausted and only ash and unconverted carbon are left in this zone, so that the temperature decreases sharply as some endothermic reduction reactions occur (reactions (7)e(13)). It is believed that the ash of biomass was mainly composed by oxides, particularly CaO, MgO, K2O, Al2O3 and Fe2O3 which have obvious function on tar catalytic decomposition [22]. Therefore, a uniform and stable reduction zone is important in decreasing the tar content and improving gas quality. The use of three stage of air supply aims at realizing the uniform air distribution in oxidation and reduction zones, and consequently the uniform reaction can be gained in the same cross section of the gasifier. The radial temperature distribution in oxidation zone and reduction zone is shown in Fig. 4. The temperature at T4 (0.34 m above the grate) has a little fluctuations, while little difference is observed at the location below T5 which indicates that the air distribution is uniform in the same cross section and in turn a more consistent reaction intensity is achieved. The uniform reaction intensity has an important effect on avoiding the phenomenon of bridges and channeling resulting from uniform reaction. During the whole tests operation, the gasifier worked stable and no bridges or channeling phenomenon occurred. The stable working favored more stable gasification results, including gas composition and tar yield, and the variation of these parameters were relatively small in the following tests even though the equivalence ratio and biomass feeding rate were changed.
3.2.
Effect of equivalence ratio
The equivalence ratio, ER, defined as the ratio of the actual air supply to the stoichiometric air required for complete combustion on a daf (dry ash free) basis is calculated by Eq. (14) in this study. ER indicates the oxygen feed in the gasification and it is a crucial factor that affects the performance of the gasification process. ER was changed by adjusting the air feeding rate during the experiments. Seven experiments were performed by changing only air feeding rate with the biomass feeding rate kept constant, contributing to a detailed parametric study of the effect of ER. The experimental conditions are presented in Table 2. Fm;air =Fm;fuelðdafÞ Actual ER ¼ Fm;air =Fm;fuelðdafÞ stoich
(14)
where, 1:293 Cdaf Hdaf 1:866 þ 5:55 Fm;air =Fm;fuelðdafÞ stoich ¼ 0:21 100 100 Sdaf Odaf 0:7 þ 0:7 100 100
(15)
where, Fm,i(daf) is the mass flow rate of i on a daf basis [kg/h]; Idaf is the mass fraction of I on a daf basis. Fig. 5 shows the temperatures in the gasifier with the changing of ER. The increase of ER enhances the combustion reactions to release heat, which results in a higher operating temperature in the gasifier. The change trend of axial
Table 2 e Effect of the equivalence ratio on the gasification performance. No. Feeding rate, kg/h Air flow, Nm3/h First stage Second stage Third stage ER
Run1
Run2
Run3
Run4
Run5
Run6
Run7
7.5
7.5
7.5
7.5
7.5
7.5
7.5
0.9 0.9 4.1 0.18
1.1 1.1 5.0 0.22
1.2 1.2 5.7 0.25
1.3 1.3 6.2 0.27
1.4 1.4 6.6 0.29
1.6 1.6 7.3 0.32
1.8 1.8 8.5 0.37
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combustible gases. Thus, the lower heating value (LHV) of the product gas rapidly decreased. Variation of cold gas efficiency and carbon conversion efficiency are used to investigate the effect of ER on the energy and mass conversion [23]. The cold gas efficiency (h) in this study is calculated as: h¼
Fig. 6 e Effect of ER on gas composition.
temperature in the gasifier is similar at different ER. The temperature is relatively low in upper portion of the gasifier where is the pyrolysis zone, and the peak temperature occurs at oxidation zone (T4). The experimental results of gas composition by the variation of the ER are shown in Fig. 6. With the increase in ER, the volume fraction of H2 increased first and attained the highest value of 12.89% at ER ¼ 0.25. Keeping on increasing ER, the combustion reaction (reaction (4)) increased which resulted in the decrease in H2. The changing trend of CO was similar with H2 with the peak value of 19.41% at ER ¼ 0.27. The trends of CO and CO2 represent the main carbon conversion during the gasification process. Thus, it could be seen that the trend of CO2 was almost opposite to CO and approached the minimum value of 15.49% at ER ¼ 0.27. A higher value of ER represents that more CH4 burns with O2 (reaction (5)) and the formation of CH4 (exothermic reaction (11)) is inhibited at higher temperature. Therefore, the volume fraction of CH4 decreased with the increase of ER. CnHm represents light hydrocarbons (C2eC3) produced in the gasifier with total volume fraction about 1%. There was no evident change of CnHm with the varying ER. The dry gas lower heating value at the standard state of 101.3 kPa and 273 K can be estimated from the gas composition by Eq. (16): LHV ¼ 107:98H2 þ 126:36CO þ 358:18CH4 þ 629:09Cn Hm kJ=Nm3
½LHVgas Vg ½LHVfuel
100%
where, [LHV]gas is the lower heating value of the gas, kJ/Nm3; Vg is the specific gas yield at the standard state, Nm3/kg. Nitrogen tracer method was developed to determine the gas yield from the percentage of nitrogen in the gases produced during gasification. The corn stalk has very low percentage of N which is 0.77% (Table 2), which is very low compared to the amount of N2 which enters the reactor along with air. Thus N2 released from the fuel during gasification is negligible compared to N2 from air used as the gasification medium. From the concentration of nitrogen in the product gas and the total amount of nitrogen entering the reactor along with air in the gasification process, the total dry volumes of gas produced can be estimated using the following formula: Vg ¼
Qa 4N2 Nm3 =kg 40N2
(18)
where, Qa is the volume of air fed for 1 kg biomass, Nm3/kg; 4N2 represents the volume fraction of N2 in air at the standard state, 78.12%; 40N2 is the volume fraction of N2 in product gas at the standard stage, vol. %. The carbon conversion efficiency (hc) is defined as the ratio of carbon which is converted from the added fuel into gaseous carbon components to the carbon in the added fuel. As the tar was removed from the gas at ordinary temperatures, the carbon in tar was not taken into account in the experiments. Therefore, the carbon conversion efficiency can be calculated as: hc ¼
12ðCO2 þ CO þ CH4 þ nCn Hm Þ Vg 100% 22:4 C
(19)
where, C is the percentage of mass fraction of carbon in the fuel.
(16)
where, CO, H2, CH4 and CnHm are percentages of the volume fraction of carbon monoxide, hydrogen, methane and hydrocarbons in the product gas. Since the gas heating value was determined by the concentrations of gases, the LHV increased first and decreased afterwards with increase in ER and reached the peak value at ER ¼ 0.25e0.27. An increase in ER leads to higher temperature in the gasifier due to stronger combustion reactions (mainly reaction (2)), as shown in Fig. 5. More H2 and CO generate by biomass pyrolysis (reaction (1)) and tar decomposition (reaction (6), (12), (13)), which leads to the increase in LHV. However, too high ER results in the decrease in the volume fraction of combustible gases (H2, CO, CH4 and CnHm) as a result of their combustion reactions (reaction (3)e(5)). Besides, more N2 was supplied into the gasifier at higher ER which diluted the
(17)
Fig. 7 e Effect of ER on cold gas efficiency and carbon conversion efficiency.
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Fig. 7 gives the values of cold gas efficiency and carbon conversion efficiency at different ER. When ER 0.25, the cold gas efficiency increased sharply, and then remained at a high level about 65% when ER was between 0.25 and 0.27. The cold gas efficiency begun to decrease when ER 0.3, which indicated that the total energy conversion of biomass decreased. The carbon conversion efficiency represents the carbon conversion from biomass fuel to product gas, which is the main element conversion during biomass gasification process. With an increase ER from 0.18 to 0.27, the carbon conversion efficiency increased from 51.44% to 83.78%. As ER further increased, the carbon conversion efficiency did not show any significant change and remained at about 90%. That indicates that higher ER favors the reactions involving carbon and about 90% of the carbon in biomass fuel can be converted to gas by gasification when ER >0.27. A thick char bed was obtained by three stage air supply which was beneficial to tar cracking, and the cracking reaction was favored when the temperature increased with increasing ER. Fig. 8 shows the effect of change in ER on tar yield. The tar yield dropped to 1.07 g/Nm3 at ER ¼ 0.27. When ER >0.27, although the temperature further increased, excessively high ER resulted in a higher gas yield and a shorter gas residence, so that the tar dropping speed gradually slowed down. It was observed that the tar yield was only 0.52 g/Nm3 at ER ¼ 0.32, which meant that the tar yield could decrease to about 0.5 g/ Nm3 without catalyst in the downdraft gasifier with three airblown supply, while the lower heating value decreased as well only 3591.6 kJ/Nm3 (Fig. 6). The tar yield was close to the value obtained by Kramreiter et al. [13] using twin-fired downdraft fixed bed. Thus, it is possible to solve the tar problem by designing the structure of gasifier and changing the air distribution way inside the gasifier.
3.3.
Effect of feeding rate
For a specified gasifier, accelerating biomass feeding rate (FR) is beneficial for increasing production capacity. However, excessively high feeding rate will result in a higher gas yield and a shorter gas residence which leads to lower gas quality
Fig. 8 e Effect of ER on tar yield.
Table 3 e Effect of the feeding rate on the gasification performance. No. Air flow, Nm3/h First stage Second stage Third stage ER Feeding rate, kg/h
Run8
Run9
Run10
Run11
Run12
1.08 1.08 4.97 0.27 5.8
1.24 1.24 5.74 0.27 6.7
1.39 1.39 6.43 0.27 7.5
1.59 1.59 7.37 0.27 8.6
1.72 1.72 7.97 0.27 9.3
and higher tar yield. Therefore, a study is carried out to investigate its effect on gas and tar yield. The rotational frequency of grates changed with the biomass feeding rate to keep the height of feed material in the gasifier constant, at about 0.63 m. The experimental conditions are presented in Table 3. Fig. 9 shows the temperature in the gasifier as a function of the biomass feeding rate. It was observed that the temperature increased with feeding rate. A higher value of biomass feeding rate accelerates the rate of reactions in the gasifier, especially stronger oxidization reactions promoting the increase of temperature. As Fig. 10 illustrated, the concentrations of H2 and CO in the product gas decreased with the increase in biomass feeding rate. This proved that excessively high feeding rate is unbeneficial for biomass gasification cracking and reforming reactions, which leads to a reduction of H2 and CO content in gases. The concentration of CO2 increased gradually, which indicated that more CO2 was produced by the stronger oxidation reactions. Besides, the shorter gas residence in reduction zone led to less CO2 consumption via C. The pyrolysis reactions of biomass were reinforced by increasing feeding rate which resulted in a slight increase in CH4 and CnHm. Consequentially, increasing feeding rate trended to decrease the lower heating value. The lower heating value decreased from 5455.5 kJ/Nm3 to 5253.2 kJ/Nm3 when feeding rate changed from 5.8 kg/h to 9.3 kg/h.
Fig. 9 e Effect of biomass feeding rate on the temperatures in the gasifier.
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Fig. 10 e Effect of biomass feeding rate on gas composition.
Fig. 11 shows the effect of biomass feeding rate on tar yield. As discussed above, the higher gas yield resulted in a shorter gas residence time in high temperature zones, which was unfavorable to tar cracking and reforming reactions, so that the tar yield continuously increased with feeding rate. Lv et al. [24] conducted pine wood block gasification experiment in a self-heated downdraft gasifier using char as the catalyst and reported that the tar yield decreased with an increase in biomass feeding rate too. They also noticed that higher feeding rate resulted in the decrease in hydrogen content and lower heating value. In our previous research [4], the gasification of herb residues in a fluidized bed was investigated using air as gasifying agent and also found that tar yield increased with an increase in biomass feeding rate.
4.
Conclusions
This work presents the design and test results of a biomass fixed bed gasification using a three air stage continuous downdraft approach. Based on the tests, it was found that the
Fig. 11 e Effect of biomass feeding rate on tar yield.
three stage of air supply can yield a high and uniform temperature in the oxidation and reduction zones for better tar cracking. Use of both the air supply system and rotating grate system avoided bridging and channeling. By increasing ER, the combustion reactions were enhanced to release heat, which in turn led to higher temperature in the gasifier. For the range of ER investigated (0.18 < ER < 0.37), the volume fraction of CO and H2 underwent the process of a fall after a rise, whilst the volume fraction of CO2 rose after a fall. When biomass feeding rate was 7.5 kg/h and ER was 0.25e0.27, the product gas of the downdraft fixed bed attains a good condition with LHV about 5400 kJ/m3 and cold gas efficiency about 65%. As ER increased, the higher temperature also led to higher tar cracking rate, which resulted in lower tar yield. The tar yield as low as 0.52 g/Nm3 was achieved at ER ¼ 0.32. Increasing biomass feeding rate led to higher biomass consumption rates which in turn resulted in higher process temperatures. However, excessively high feeding rate is unbeneficial for biomass gasification cracking and reforming reactions, which leads to a reduction of H2 and CO content in gases and an increase in tar yield. When ER was 0.27, with an increase of biomass feeding rate from 5.8 kg/h to 9.3 kg/h, the low heating value decreased from 5455.5 kJ/Nm3 to 5253.2 kJ/ Nm3 and tar yield increased from 0.82 g/Nm3 to 2.78 g/Nm3.
Acknowledgements This project was supported by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology).
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