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Oct 25, 2013 - Texas AgriLife Research, Vernon, Texas 76384, United States. ABSTRACT: Gasification of woody biomass is an environmentally promising ...
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Updraft Gasification of Mesquite Fuel Using Air/Steam and CO2/O2 Mixtures Wei Chen,† Siva Sankar Thanapal,† Kalyan Annamalai,*,† Robert J. Ansley,‡ and Mustafa Mirik‡ †

Department of Mechanical Engineering, MS 3123, Texas A&M University, College Station, Texas 77843, United States Texas AgriLife Research, Vernon, Texas 76384, United States

‡

ABSTRACT: Gasification of woody biomass is an environmentally promising technology that provides an alternative to the use of fossil fuel. Typically, partial air oxidation is used to gasify biomass fuels, resulting in low-quality product gases due to dilution by nitrogen (N2) in air, which limits the application of the product gases for thermal power generation. The objective of this study was to identify optimum conditions for producing gases from woody biomass with enhanced heating values. Air, air/steam, and a carbon dioxide/oxygen (CO2/O2) mixture were used as gasification media for the gasification of pure mesquite (lowquality fuel) and a blend of mesquite and coal (high-quality fuel) using a 10 kW adiabatic fixed-bed reactor. Thermogravimetric analysis (TGA) along with limited emission studies using Fourier transform infrared (FTIR) spectrometry were performed to obtain basic kinetic data on raw mesquite pyrolysis. The effects of the steam-to-fuel ratio (S/F) and equivalence ratio (ER) on the peak temperature (Tpeak) in the gasifier, product gas composition, and higher heating value (HHV) of the product gases were investigated. It was found that Tpeak decreased from 1050 to 850 °C as S/F was increased from 0 to 0.45. However, the carbon dioxide (CO2) and hydrogen (H2) concentrations increased and the carbon monoxide (CO) percentage decreased with the introduction of larger amounts of steam. For air/steam gasification, the HHV of the producer gas was estimated to be in the range of 2800−3800 kJ/Nm3. The highest HHV was obtained at S/F = 0.3 and ER = 2.7 (optimum). The HHV of the gas from air/steam gasification was found to be almost the same as that produced from pure-air gasification (2800−4000 kJ), except for an increased H2 yield for air/steam. However, the HHVs of the gases obtained from CO2/O2 (79:21 on a volume basis) gasification (4000−6000 kJ/Nm3) were generally higher. Separation of CO2 from the product gases obtained using CO2/O2 gasification can further enhance the heating value of the product gas such that the HHV of the resulting gas mixture is 40% of the HHV of natural gas. Hence, the use of a CO2/O2 mixture as the gasification medium and of coal blended with raw biomass as the fuel is the best method for upgrading the quality of product gases from the gasification of woody biomass.

1. INTRODUCTION Biomass is a renewable energy source that can be utilized to help meet energy demands. Apart from being converted into a vast array of chemical products, fuels, and fertilizers, it can also be utilized to produce steam, power, and electricity.1 Efforts are underway to extract the energy from abundant supplies of biomass in developing countries to provide power to remote villages.2 However, there are still some limitations that would affect the wide application of biomass. These limitations are due to the high cost of feedstock transportation and lower biomass density in such areas. It therefore would be beneficial to develop small-scale, less expensive, and localized conversion facilities that require less feedstock and save on feedstock transportation costs.3 Woody species (brush) growing on semiarid uncultivated lands (rangelands) in the upper Midwest of the United States, such as mesquite (Prosopis glandulosa), might have the potential to be used as bioenergy feedstocks. It is estimated that mesquite covers 21 million hectares (Mha) of land in Texas alone, about 20% (or 4.2 Mha) could be harvested for bioenergy needs.4,5 At an average of 18 dry Mg/ha, this could amount to a total available mass of over 75 Tg.6 This species can be used as a feedstock to produce syngas and bio-oil in small-scale gasification units. Gasification is a thermochemical process that converts biomass into combustible gases and other chemical products. © 2013 American Chemical Society

Gasifiers can generally be classified into two different types: fixed-bed and fluidized-bed. For fixed-bed gasifiers, the flow velocity is low, and ash is disposed through a grate at the bottom of the gasifier. For fluidized-bed gasifiers, the flow velocity is high, and there is no grate. Fixed-bed gasifiers can, in turn, be classified as updraft, downdraft, and crossdraft. Updraft fixed-bed gasifiers are also called counterflow reactors, as the fuel is fed into the top of the reactor and the gasification medium (air or steam) is supplied at the bottom. In a downdraft fixed-bed gasifier, both fuel and gases flow in the same direction. Fixed-bed gasifiers are well-suited for smallscale applications (power < 10 MW).7 Fluidized-bed gasifiers usually are large-scale size and are used for industrial applications. Considering the different gasification configurations, an updraft fixed-bed gasifier was used for the current study because such gasifiers are easy to construct and operate and have high thermal efficiencies. The temperature of gas exiting the updraft gasifier was below 200 °C, resulting in a higher energy efficiency when compared to a downdraft gasifier. The results from the current study can also be applied to agricultural residues available in large amounts all over the world. Determination of the optimum gasification conditions Received: August 8, 2013 Revised: October 25, 2013 Published: October 25, 2013 7460

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a steam/biomass ratio of 2.05 was determined to be the optimum value in all steam gasification runs. Thanapal et al.13 studied the effects of using enriched-air and carbon dioxide/oxygen mixtures on the gasification of dairy biomass. They observed that the nitrogen in air acted as a diluent in reducing the heating value of the resulting gas mixture. The use of carbon dioxide/oxygen mixtures for gasification resulted in the production of gases with much higher HHVs than those obtained using air, air/steam, and enriched-air mixtures. Co-gasification of biomass with bituminous coal was experimentally studied by Gordillo and coworkers13 in an updraft fixed-bed gasifier. The effects of the equivalence ratio and steam/fuel ratio were studied using air and air/steam as gasification media. It was observed that the gas quality was much higher for the coal/biomass blend than for the raw dairy biomass. Gasification of biomass with coal in an oxy-co-combustion gasifier was modeled by Valero and Usón.14 Their model predicted the possibility of oxy-co-gasification of different biomass materials along with coal with negligible variations in gas quality despite varying gasification parameters. Mathieu and Dubuisson15 analyzed the effects of different parameters affecting the gasification efficiency using the ASPEN PLUS process simulator. They noted that oxygen enrichment in the incoming gasification medium and preheating of the gasification medium improved the gasification efficiency from 77% to 80% for increases in the oxygen concentration from 21% to 40% and from 76.5% to 80% when the temperature of the gasification medium was increased from 298 to 1098 K. However, the effects of using different gasification media on the gasification of woody biomass, such as mesquite, in a smallscale gasification facility have not been analyzed. Even though mesquite covers 21 Mha in Texas, its location is scattered in this area. Hence, a portable gasification unit that can be moved from one spot to another will be an ideal choice for thermochemical energy conversion applications. Because the costs of processing and transporting biomass are high, identifying optimum gasification conditions for small-scale gasification units will help to increase the efficiency of energy extraction from the gasification process. The main objective of this study was to investigate the effects of the gasification media, equivalence ratios, and steam-to-fuel ratio on the gasification temperature (i.e., Tpeak), gas yield, and quality (i.e., HHV) of the resulting gas mixture from mesquite gasification. This will enable the determination of the optimum gasification conditions for producing a gas mixture with a high HHV in a portable gasification unit that can be used in gas turbines, internal combustion engines, and other heating applications.

will result in increased gasification conversion efficiency and production of gases with good heating values that can be used directly for different applications. Air, pure oxygen, air/steam, supercritical water, and carbon dioxide have all been used as media for biomass gasification. Because air contains a high percentage of nitrogen, the heating value of the gas obtained from air gasification is very low. The heating value of the producer gas from the air gasification of biomass is in the range of 3.5−7.8 MJ/Nm3.7 Oxygen-blown gasification produces a syngas with a medium heating value, and steam-blown gasification leads to the production of a syngas with an acceptable higher heating value (HHV) of around 10− 16 MJ/Nm3.8 In addition to air and oxygen, steam can also be mixed with air to promote the steam-reforming reaction to produce H2-rich gas mixtures and lower the gasification temperature to avoid ash melting, which causes agglomeration and clogging in the bed.

2. LITERATURE REVIEW AND OBJECTIVE Extensive studies have been carried out on biomass gasification using different gasification media. Gordillo and Annamalai9 used air/steam mixtures as gasification media for dairy biomass gasification in an updraft gasifier. They found that the peak temperature (Tpeak) and CO concentration decreased, whereas the H2 and CO2 concentrations increased with increasing equivalence ratio (ER). Also, increased values of the steam-tofuel ratio (S/F) produced H2- and CO2-rich mixtures with low CO concentrations. The effects of preheated air and steam as the gasifying agent were studied by Lucas et al.,10 who found that, in a high-temperature air/steam updraft fixed-bed gasifier, the preheated air and steam maximized the gas yield due to the high heating rates. Increasing the feed-gas temperature reduced the production of tars, soot, and char residue and increased the heating value of the producer gas. Overall, it was seen that the yield of gas and the heating value of the dry fuel gas increased with increasing gasification temperature. Lv et al.11 studied the characteristics of biomass air/steam gasification in a fluidized bed. Their results showed that the introduction of steam improved the gas quality when compared with that obtained by biomass air gasification. For air/steam gasification, the lower heating value (LHV) of the gas decreased with ER. The gas yield varied between 1.43 and 2.57 N m3/(kg of biomass), and the LHV of the fuel gas was between 6741 and 9143 kJ/Nm3. It was concluded that higher gasification temperatures contributed to more hydrogen production; however, excessively high temperatures lowered the gas heating value. In addition, excessive steam would lower the gasification temperature and, hence, degrade the fuel gas quality. Gasification of mesquite and juniper using air as the medium was carried out by Chen et al.3 in an updraft fixed-bed gasifier. They reported that the H2 and CO mole percentages decreased with increasing ER, whereas the CO2 and N2 mole percentages increased. Gao et al.12 investigated the hydrogen-rich gas produced from biomass in an updraft gasifier with a continuous biomass feeder. The results showed that hydrogen-rich syngas with a high calorific value, in the range of 8.10−13.40 MJ/Nm3, was produced and that the hydrogen yield was in the range of 45.05−135.40 g of H2/(kg of biomass). Higher temperatures were found to favor the formation of hydrogen. When the gasifier temperature was increased from 800 to 950 °C, the hydrogen yield increased from 74.84 to 135.4 g of H2/(kg of biomass). The LHV of the gas increased first and then decreased as the ER was increased from 0 to 0.3. Furthermore,

3. EXPERIMENTS 3.1. Thermogravimetric Analysis (TGA) and Fourier Transform Infrared (FTIR) Spectrometry Tests. To obtain the fuel pyrolysis characteristics, TGA and FTIR studies were performed. Pyrolysis or thermal decomposition of biomass using inert (e.g., Ar, N2) and noninert (e.g., air) gases is the first step toward understanding fuel properties and energy conversion processes such as gasification, liquefaction, carbonization, and combustion.16 In this study, an SDT Q600 TGA/ DSC apparatus from TA Instruments was used to determine the effects of temperature on the weight loss characteristics of mesquite fuel under different environments. The different environments included air (N2/O2), a carbon dioxide/oxygen (CO2/O2) mixture with the same volume percentages as air (79% CO2 and 21% O2), pure CO2, and pure N2. In these 7461

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Figure 1. Gasification facility (adapted from ref 9).

experiments, around 10 mg of mesquite fuel was heated at a heating rate of 20 °C/min from room temperature to 900 °C using the above gases. Using the TGA−pyrolysis data from a single point where the volatile release rate is a maximum [called the single-reaction-model−maximum-volatile-release (SRM− MVR) method], the kinetics were extracted in addition to the global Arrhenius fit and parallel reaction models. SRM− MVR data are particularly useful in determining the upper limit to the torrefaction temperature of biomass fuels.17 Limited studies were performed on the composition of the gases liberated as a result of mesquite fuel pyrolysis (N2 as the purge gas) using an FTIR spectrometer (MultiGas 2030). To avoid condensation, a heated gas-transfer line was maintained at 200 °C by an external heater.18 The methodology is described by Eseltine et al.19 The liberated gases such as CO, CO2, H2O, and CH4 were measured for their compositions every 20 s by FTIR spectrometry. A chemical formula for volatiles was determined and is presented in section 4.3. 3.2. Gasification Facility and Procedure. The small-scale (10 kW) batch-type fixed-bed gasifier employed for this research is shown in Figure 1. The well-insulated gasifier had an outer diameter of 24.5 cm and a height of 72 cm. More details are provided elsewhere.9 The gasifier was also equipped with an ash disposal system. A conical gyratory cast iron grate drilled with large number of 6.4-mm-diameter holes was coupled to a pneumatic vibrator of variable frequency that vibrated the grate to remove the ash continuously from the bed. The rate of ash removal was controlled by changing the vibration frequency of the vibrator. The condensers were made of stainless steel and installed right after the reactor to ensure that the tar was collected by the condenser without sticking to the piping. At the beginning of each experiment, the empty bed was preheated to 600 °C using a propane torch. After the temperature had reached 600 °C, the torch was turned off, and biomass samples were gradually added to the gasifier. The addition continued until the bed height of the gasifier reached

22 cm (8.5 in). Afterward, the fuel port was closed, and the gasification medium, for example, air and steam, was sent into this system at the desired rate. The same procedure was used for different gasification media (air, CO2/O2 mixture) and fuel blends (coal and biomass blend). For CO2/O2 gasification, a mixture of CO2 and O2 gas in a ratio of 79:21 on a volume basis (same as N2/O2 ratio in the air) was used. Carbon dioxide and oxygen cylinders were used, and the gases were well mixed before being sent into the gasifer. The equivalence ratio (ER) used for the present study was determined as17 stoichiometric number of moles of air actual number of moles of air (A/F)stoichiometric = (A/F)actual

ER =

Hence, an increase in equivalence ratio indicates less air being sent into the gasifier for the gasification process. The steam-tofuel ratio (S/F) was determined as17 S/F =

mass of steam mass of biomass (as‐received)

3.3. Steam Generation. The steam generator was made of a 10-cm-diameter cylindrical vessel surrounded by a variablepower (0.1−1.2 kW) tape-type heating element.7 The steam generator was first calibrated to ensure that the desired flow was supplied to the gasifier. The desired rate of steam leaving the steam generator was equal to the rate of water entering the vessel. The steam generation rate was adjusted by controlling the power supplied to the heater. The steady state of the steam generator was verified before each experiment. 3.4. Temperature Measurements. Eight K-type thermocouples were located 2, 4, 7, 10, 13, 20, 24, and 28 cm from the bottom along the gasifier axis to measure the temperature in the gasification chamber. The temperature inside the gasifier was recorded every 60 s into a flash card. 7462

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Table 1. Properties of Mesquitea,b

3.5. Gas Composition Measurements. A mass spectrometer was used to measure the composition of the product gases. The gas was passed first through a condenser to remove tar and condensable vapors and then through a series of filters to capture associated particles. A small amount of filtered gas was then supplied to the gas analyzer. 3.6. Uncertainty of Measurements. Variable-area flow meters with an accuracy of ±3% were used to measure the flow rate of the gasification medium into the gasification facility. The K-type thermocouples used for temperature measurements had an accuracy of ±1.5 °C. The mass spectrometer was precalibrated using a standard gas mixture (N2, CO, CO2, H2, C2H6, and CH4) of known composition and inert gas (helium) every 3 days to ensure accurate measurements. In limited cases, particularly for experiments with air17 and CO2/O2 (ER = 2.7 and 4.2), the experiments were repeated to check for the repeatability of the results.

property

value As-Received Mesquite

content (%) moisture ash VM FC carbon oxygen hydrogen nitrogen sulfur HHV (kJ/kg)

15.5 1.67 66.1 16.7 43.6 33.6 4.98 0.62 0.03 16700 Dry, Ash-Free Mesquite

content (%) moisture ash VM FC carbon oxygen hydrogen nitrogen sulfur HHV (kJ/kg) volatile HHV (kJ/kg)c chemical formula RQd of mesquite21 RQprocessinge of mesquite21

4. RESULTS AND DISCUSSION 4.1. Fuel Properties. A solid fuel generally consists of combustibles, ash, and moisture, and the combustibles contain fixed carbon (FC) and volatile matter (VM). Ultimate analysis was performed to obtain the percentages of constituent elements in the biomass. Table 1 provides the results of proximate and ultimate analyses of the mesquite fuel. On a dry, ash-free (DAF) basis, the HHV of mesquite was found to be 20128 kJ/kg (Table 1), which is a typical value for most biomass fuels on a DAF basis but is still less than the typical DAF heating values of coals, 30000 kJ/kg . The empirical formula for the mesquite sample was found to be CH1.3582O0.5779N0.0122S0.0003. Mesquite biomass has a higher content of volatile matter (70−80%) than coal. Bituminous coal has approximately 30−40% VM, and lignite has approximately 40−50% VM.20 It is also noted that the N content of mesquite on a heat basis is almost same as that of coal, unlike for most biomass fuels. 4.2. TGA Tracing of Mesquite Fuel. Figure 2a shows TGA traces of the mesquite fuel. It can be seen that all of the TGA trace curves exhibit the same trend. With an increase in temperature, the moisture is first liberated from the fuel, which contributes a 10% weight loss. Volatile matter starts to be released from the mesquite at temperatures around 240 °C. When the temperature reached the ignition temperature (the point where the trace obtained in air deviated from the N2 trace), the slope of the air tracing curve became sharper than that of the CO2/O2 curve. Thus, the fuel was oxidized more rapidly when air was used as the purge gas than when CO2/O2 was used, especially when the temperature was above 350 °C. This might be because of comparatively higher specific heat of CO2 compared to N2, which results in some heat being carried away by the larger CO2 molecules. When CO2 was used as the purge gas, the mass loss rate was higher than that obtained using N2 as the purge gas, especially at high temperatures. This is because of the Boudouard reaction (e.g., CO2 + C → 2CO, ) which occurs at high temperatures. It can be seen in Figure 2a that, when the temperature reached 600 °C, more than 80% of the weight of the mesquite fuel (including moisture and volatile matter) was liberated from the fuel on using nitrogen as the medium, which means that, under pure pyrolysis condition, biomass will release more gas than coal. Figure 2b shows the weight loss rate as a function of temperature under different environments. The weight loss curve for CO2/O2 was found to be similar to that for air. In Figure 2b, there are two peaks with

0 0 79.8 20.2 52.7 40.5 6.01 0.75 0.04 20100 16900 CH1.3582O0.5779 N0.0122S0.0003 0.95 0.05

a Adapted from ref 17. bSee TAMU Fuel data bank Web site24 for properties of many coal and biomass fuel including sludge. cEstimated using the relation HHVDAF = FCDAF × HVFC + VMDAF × HHVDAF.22 d RQ (respiratory quotient) is defined as the ratio of the number of moles of CO2 emitted to the number of moles of O2 consumed for the oxidation reaction of a fuel. RQ > 1 indicates higher CO2 emissions potential from a fuel. eRQprocessing is an equivalent RQ value which indicates the amount of CO2 released from the consumption of fossil fuels used for processing biomass. Assuming that mesquite is carbonneutral and that energy from fossil fuels is consumed only during the harvesting stage.23

oxidation: rapid mass loss due to hemicellulose and cellulose oxidation, followed by rapid mass loss due to lignin oxidation in air. Similar mass loss behavior was also reported by Ghetti et al.25 and Ergüdenler and Ghaly26 for different lignocellulosic biomass samples. The occurrence of these two steps depends on the percentages of hemicellulose, cellulose, and lignin contained in the biomass samples. However, with CO2, the rate of weight loss was lower because of the higher heat capacity of CO2 and the reaction of carbon dioxide with carbon in the biomass. As a result, the sample was relatively cooler, so that mass loss occurred over a wider temperature range during the second combustion step at T < 600 °C. 4.3. FTIR Analysis of Pyrolysis Gas Composition. Because of the complex structure of tar, it was not identified by FTIR spectrometry. Panels a−e of Figure 3 present the concentrations of CO2, CO, H2O, CH4, and formaldehyde, respectively, from mesquite pyrolysis. For CO2 and CO, peak emission was observed in the temperature range of 340−370 °C as a result of the decomposition of cellulose and hemicellulose. Giuntoli et al.27 found that the release peaks of CO, CO2, and H2O occurred at temperatures between 300 and 7463

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volatile matter when considered on a dry, ash-free basis. However, char is not pure carbon, and if it contains H and O, then for the same “fixed carbon” obtained in the proximate analysis, less carbon would actually remain in char containing H and O. The FTIR data were used to check the mass balance of the total carbon in the raw biomass. From the data, it was found that around 74% of the total carbon was released along with the volatile matter and only 26% of the carbon was left in the char. This is important in countercurrent gasifiers, where most moisture leaves from the top of the bed. However, char still contains H and O, and the water−gas shift reaction might also occur. 4.4. Temperature Profile for Air/Steam Gasification. Figure 4 shows the temperature profiles for mesquite gasification at different steam/fuel ratios (S/F). The results obtained from the TGA study (Figure 2a) on the pyrolysis of mesquite samples can be used to determine the pyrolysis starting point in the gasifier (Figure 4). The temperature profiles for air/steam gasification exhibited the same trend as that for air gasification (S/F = 0): The temperature first increased and reached a peak value (Tpeak) and then decreased along the axis of the gasifier. However, Tpeak was lower for air/ steam gasification than for air gasification. It was found that, for air/steam gasification, Tpeak < 1000 °C in most cases. For air gasification, Tpeak reached 1050 °C for ER = 2.7, whereas it was only 1010 °C for air/steam gasification at S/F = 0.15 and ER = 2.7. The difference is due to both chemical effects, in that most steam gasification reactions are endothermic processes, which results in a lowering of the gasification temperature, and physical effects, in that the specific heat at constant pressure (Cp) of the steam and air mixture is higher than that of air.13 Typically, the peak temperature inside the gasifier exhibited the following trend for air gasification: TER=2.7 > TER=3.2 > TER=3.7 > TER=4.2. From Figure 4, it can be seen that the peak temperatures of the gasification temperature profiles follow the order TS/F=0 > TS/F=0.15 > TS/F=0.30 > TS/F=0.45. A higher S/F ratio means that more steam was sent into the gasifier, so that the endothermic reactions and increased Cp lowered the temperature inside the gasifier. The variation of peak temperature within the bed for different steam/fuel ratios is shown in Figure 5. However, the peak temperature was high for the gasification of the coal/ mesquite blend (Figure 6), because of the higher amount of fixed carbon available in the coal, which resulted in the release of a large amount of heat from the char oxidation process. 4.5. Temperature Profile for CO2/O2 Gasification. When the carbon dioxide/oxygen mixture was used for gasification, the peak temperature was much lower (870 °C for ER = 2.7 with S/F = 0); in particular, for ER = 4.2, Tpeak dropped below 750 °C. The lower peak temperature can be attributed to the higher specific heat of carbon dioxide at higher temperatures compared to that of nitrogen and the endothermic Boudouard reaction. Figure 6 shows the variation of temperature profile for different gasification conditions. 4.6. Gas Composition for Air/Steam Gasification. Figure 7 presents the gas compositions (CO, CO2, H2, and CH4) as a function of ER at S/F = 0.45. Increasing the ER at constant S/F lowered the CO concentration while increasing the CO2 percentage. Under a high-steam environment, a lower gasification temperature favors the formation of CO2 and H2 while decreasing CO. These trends occur because higher temperature shifts the equilibria of the endothermic reactions (e.g., CO2 + H2 ↔ CO+ H2O) toward the products and those

Figure 2. TGA traces of (a) mesquite using air, CO2, N2, and CO2/O2 (sampler size, 10 mg; heating rate, 20 °C/min) and (b) rate of mesquite weight loss using air, CO2, N2, and CO2/O2.

320 °C for the pyrolysis decomposition of untreated wheat straw. In contrast to the wheat straw fuel, the main water release peak from the mesquite fuel occurred when the temperature was below 200 °C, and between 250 and 420 °C, its emission decreased almost to zero. This might be because most of the cellulose and hemicellulose decomposed into CO, CO2, and a small amount of formaldehyde .When the temperature increased to 420 °C, H2O was released from fuel as a result of the decomposition of lignin. In Figure 3c, it can be seen that, when the temperature reached 350 °C, CH4 started being released from the fuel as a result of the decomposition of methoxyl groups in the lignin part of the biomass. 28 Formaldehyde (CH2O) is a colorless gas with a characteristic pungent odor.27 In Figure 3e, it can be seen that formaldehyde was released from the fuel in the temperature range of 380−750 °C, with a peak at ∼600 °C. Neglecting nitrogen and sulfur in the biomass, the carbonnormalized chemical formula for DAF mesquite fuel is CH1.3582O0.5779. During pyrolysis, the mesquite fuel released the volatile matter, and the fixed carbon remained unconsumed in the char. This process can be represented as CH1.3582O0.5779 → αCHhvOov + (1 − α)C(s), where the two terms on the righthand side represent the volatile matter (hv and ov represent the number of hydrogen and oxygen atoms respectively in the released volatile matter) and fixed carbon, respectively. From the proximate analysis results in Table 1 (approximately 80% VM and 20% fixed carbon on a DAF basis), one can express the fixed-carbon content of the carbon-normalized fuel as (1 − α) × (12.01/M) = 0.2, where M is the molecular weight of the fuel. From this expression, the value of α was determined to be 0.62. Then, using an oxygen-atom balance, we obtained hv and ov. Around 38% of the total carbon in the biomass was estimated to remain in the char, whereas 62% was released as 7464

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Figure 3. Variations in the concentrations of (a) CO2, (b) CO, (c) H2O, (d) CH4, and (e) formaldehyde with temperature during mesquite pyrolysis in N2.

Figure 5. Peak air/steam gasification temperature at different S/F ratios.

Figure 4. Temperature profile of mesquite fuel at ER = 2.7 for several S/F ratios.

CH4 + H 2O ↔ CO + 3H 2

ΔH = 206 000 kJ/kmol (2)

of the exothermic reactions (e.g., CO + H2O ↔ CO2+ H2) toward the reactants. As a result, a series of endothermic and exothermic biomass gasification reactions can be used to explain the gasification mechanism:29 C(s) + H 2O ↔ CO + H 2

C(s) + CO2 ↔ 2CO

C(s) + O2 ↔ CO2

ΔH = 172 320 kJ/kmol

(3)

ΔH = −393 180 kJ/kmol

(4)

CH4 + 2H 2O ↔ CO2 + 4H 2

ΔH = 131 160 kJ/kmol (1)

ΔH = 165 000 kJ/kmol (5)

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Figure 6. Variation of Tpeak with gasification medium. Figure 9. CO mole percentage vs ER for different S/F ratios.

concentration and a decrease in the CO percentage. As discussed earlier, a lower gasification temperature and higher steam concentration would shift the equilibrium of the water− gas shift reaction toward the formation of CO2 and H2. Figure 10 presents the effects of ER and S/F on the H2 concentration. It was found that increasing S/F increased the

Figure 7. Gas composition for a typical experiment at S/F = 0.45 for several ER values.

C(s) + 0.5O2 ↔ CO

ΔH = −110 000 kJ/kmol

CO2 + H 2 ↔ CO + H 2O

(6)

ΔH = − 16 180 kJ/kmol (7)

Increasing the gasification temperature would result in the equilibria of reactions 2, 3, and 6 moving forward, favoring the formation of H2. Also, endothermic reactions 2−4 would prevail over reaction 7, thus increasing the CO concentration.17,29 Additionally, reaction 5 would be favored at low temperature, and thus, more CO2 would be generated. The CH4 percentage did not vary much within the range of ER values investigated, and its concentration remained almost constant at 1−2%. Figures 8 and 9 present the CO2 and CO concentrations, respectively, for different S/F ratios. At a constant ER, increasing the S/F ratio led to an increase in the CO2

Figure 10. H2 mole percentage vs ER for different S/F ratios.

H2 concentration in the product gas. At constant S/F, increasing ER also resulted in an increase in the steam/air ratio. The heterogeneous reaction of char with steam could occur in a steam-rich environment, producing more H2.9 In addition, the equilibrium of the water−gas shift reaction shifts in the direction of the formation of H2 in a H2O-rich environment.9 Moreover, an increase in S/F leads to more steam being available to react with char to produce CO and H2. For instance, the H2 concentration was less than 4% under pure-air gasification (i.e., S/F = 0), increased to 4−7% for S/F = 0.15, and reached the range of 7−10.5% for S/F = 0.3. The highest H2 percentage was around 11%, which occurred at S/F = 0.45 and ER = 4.2. 4.7. Gas Composition for CO2/O2 Gasification. To obtain high-quality gas, CO2 was considered as an alternate gasification medium to air and an air/steam mixture. The composition of the producer gas obtained using a CO2/O2 mixture as the gasification medium is shown in Figure 11. Because CO2 was used as the medium, the percentage of CO2 in the product stream was higher. It can be seen that CO reached as high as 32% for CO2/O2 gasification, whereas the maximum was 25% for air gasification, because the C + CO2 reaction occurs and the water−gas shift reaction favors the direction CO2 + H2 → CO + H2O at high CO2 concentration,

Figure 8. CO2 mole percentage vs ER for different S/F ratios. 7466

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Figure 13. Heating values of the gas mixtures obtained using different gasification media and fuels.

Figure 11. Gas composition from the CO2 gasification process.

as evidenced by the lower H2 concentration for CO2/O2 gasification. 4.8. HHV of the Gas Mixture. Figure 12 shows the HHVs of the gas mixtures produced during the gasification of

Because the concentration of carbon dioxide was high when the CO2/O2 mixture was used, the CO2 reacted with the fixed carbon in the biomass to yield more CO, which increased the gas HHV. However, at higher ER, the HHV of the gas mixture obtained using the CO2/O2 mixture was lower because of the lower Tpeak within the bed. Blending mesquite with subbituminous Powder River Basin (PRB) coal in the ratio of 80:20 on a mass basis and gasifying the blend with air resulted in a gas mixture with a much higher heating value because of the presence of a higher amount of fixed carbon in the resulting fuel blend.17 Gasifying the coal/biomass blend with the CO2/ O2 mixture further enhanced the HHV of the resulting gas mixture. Produced gas with a high HHV can be used as a fuel in gas turbines. The effect of enhancing the heating value of the produced gases in gas turbine applications has been reported elsewhere.17 It was estimated that increasing the heating value reduced the volumetric flow of the syngas into the gas turbine. A 30% reduction in the volumetric flow of gases was achieved when gases with a higher HHV were used. The gasification conversion efficiency varies between 20% and 70% depending on the equivalence ratio (ER) employed.7,17 The main gasification products include gas, ash, and tar. A higher ER value results in lower gas production because of reduced amount of oxygen available for gasification. The lower the amount of oxygen supplied to the gasifier, the lower the gasification temperature, and thus, the less tar cracked into gas and more unburned char remaining in the ash. Hence, the heatbased gasification conversion efficiency30 decreases with increasing ER. Because of the higher percentage of N2 or CO2 in the producer gas, the gas HHV is very low. Producer gas quality can be further improved if CO2 and N2 are removed from the producer gas. Also, the sequestered CO2 can be recycled for gasification. It can be seen in Figure 14 that the HHV of the producer gas can reach 36% of the natural-gas HHV for CO2/ O2 gasification, whereas it can reach only 26% of the natural-gas HHV for air gasification. Thus, by using CO2/O2 as the gasification medium, the HHV of the producer gas can be further increased by 10% when compared to that obtained by air gasification.

Figure 12. Gas HHVs for the steam gasification of mesquite biomass at various S/F ratios.

mesquite at S/F = 0, 0.15, 0.3, and 0.45 on a dry, tar-free basis. The heating value of the produced gas was determined in each case from the gas composition measured using the mass spectrometer and the known heating values of the constituents of the gas mixture. The HHV of methane is 36264 kJ/Nm3, and the HHVs of CO and H2 are 11550 and 11700 kJ/Nm3, respectively.9 It was found that the HHVs of the gases produced followed the order HHVS/F=0.45 < HHVS/F=0.15 ≈ HHVS/F=0 < HHVS/F=0.3. The HHV of the gas first increased when S/F was increased from 0.15 to 0.3 because of the increased amount of H2 produced by the water−gas shift reaction. With the further increase in S/F to 0.45, the HHV of the gas decreased, and its value was lower than that of the gas obtained at S/F = 0.15. This is because, under high-S/F conditions, the lower gasification temperature resulted in the generation of less volatiles and more of the noncombustible gas CO2 (water−gas equilibrium shift). Figure 13 shows a comparison of the heating values of the gas mixtures obtained using different gasification media for gasifying pure mesquite and a blend of mesquite and coal. It can be observed that the use of the air/steam mixture increased the HHV of the gas mixture slightly when compared to the use of pure air. Using carbon dioxide/oxygen mixture resulted in a gas mixture with a high HHV in the range of 5500 kJ/Nm3 for mesquite gasification. This high HHV is due to the Boudouard reaction (CO2 + C → 2CO) taking place within the gasifier.

5. CONCLUSIONS The effects of using different gasification media for gasifying a woody species was studied using a small-scale fixed-bed gasification facility, and the following conclusions can be drawn from this study. 7467

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REFERENCES

(1) Salaices, E.: Catalytic Steam Gasification of Biomass Surrogates: A Thermodynamic and Kinetic Approach. Ph.D. Dissertation, The University of Western Ontario, London, Ontario, Canada. 2010. (2) Liming, H. Financing rural renewable energy: A comparison between China and India. Renewable Sustainable Energy Rev. 2009, 13, 1096−1103. (3) Chen, W.; Annamalai, K.; Ansley, R. J.; Mirik, M. Updraft fixed bed gasification of mesquite and juniper wood samples. Energy 2012, 41, 454−461. (4) Texas Brush Inventory; Soil Conservation Service, United States Department of Agriculture: Washington, DC, 1998. (5) Xu, W.; Li, Y.; Carraway, B. Estimation of Woody Biomass Availability for Energy in Texas; Texas Forest Service: College Station, TX, 2008. (6) Ansley, R. J.; Mirik, M.; Castellano, M. J. Structural biomass partitioning in regrowth and undisturbed mesquite (Prosopis glandulosa): Implications for bioenergy uses. Global Change Biol. Bioenergy 2010, 2, 26−36. (7) Gordillo, G. Fixed bed countercurrent low temperature gasification of dairy biomass and coal-dairy biomass blends using airsteam as oxidizer. Ph.D. Dissertation, Texas A&M University, College Station, TX, 2009. (8) Ptasinki, K. J.; Sues, A.; Jurascik, M. Biowaste-to-biofuel routes via gasification. In Biomass Gasification: Chemistry, Processes and Application; Badeau, J.-P., Levi, A., Eds.; Nova Science Publishers: Hauppauge, NY, 2009; pp 87−197. (9) Gordillo, G.; Annamalai, K. Adiabatic fixed bed gasification of dairy biomass with air and steam. Fuel 2010, 89, 384−391. (10) Lucas, C.; Szewczyk, D.; Blasiak, W.; Mochida, S. Hightemperature air and steam gasification of densified biofuels. Biomass Bioenergy 2004, 27, 563−575. (11) Lv, P. M.; Xiong, Z. H.; Chang, J.; Wu, C. Z.; Chen, Y.; Zhu, J. X. An experimental study on biomass air−steam gasification in a fluidized bed. Bioresour. Technol. 2004, 95, 95−101. (12) Gao, N. B.; Li, A. M.; Quan, C.; Gao, F. Hydrogen-rich gas production from biomass steam gasification in an updraft fixed-bed gasifier combined with a porous ceramic reformer. Int. J. Hydrogen Energy 2008, 33, 5430−5438. (13) Thanapal, S. S.; Annamalai, K.; Sweeten, J. M.; Gordillo, G. Fixed bed gasification of dairy biomass with enriched air mixture. Appl. Energy 2012, 97, 525−531. (14) Valero, A.; Usón, S. Oxy-co-gasification of coal and biomass in an integrated gasification combined cycle (IGCC) power plant. Energy 2006, 31, 1643−1655. (15) Mathieu, P.; Dubuisson, R. Performance analysis of a biomass gasifier. Energy Convers. Manage. 2002, 43, 1291−1299. (16) Worasuwannarak, N.; Sonobe, T. Kinetic analyses of biomass pyrolysis using the distributed activation energy model. Fuel 2008, 87, 414−421. (17) Chen, W. Fixed bed counter current low temperature gasification of mesquite and juniper biomass using air-steam as oxidizer. Ph.D. Dissertation, Texas A&M University, College Station, TX, 2012. (18) Eseltine, D. E. Effect of using inert and non-inert gases on the thermal degradation and fuel properties of biomass in the torrefaction and pyrolysis region. M.S. Thesis, Texas A&M University, College Station, TX, 2011.

Figure 14. Heating values of the gas mixtures obtained after removing CO2 and N2 from the producer gas.

(1) Both the peak and average bed temperatures decreased for air/steam gasification compared to air gasification. In addition, increasing the S/F ratio and ER value decreased the gasification temperature because of the endothermic char− steam reaction. (2) The main products for air/steam gasification were found to be CO, CO2, H2, N2, CH4, and C2H6. Increasing the S/F ratio led to an increase in the CO2 and H2 concentrations and a decrease in the CO percentage. The HHV of the gas mixture increased from 2800 to 3800 kJ/Nm3 when S/F was increased from 0.15 to 0.45. (3) Using a CO2/O2 mixture for gasification produced a gas with a much higher HHV because of the Boudouard reaction taking place under the CO2-rich conditions in the reactor. Because CO2 separation technology has been developed, the removal of CO2 from the product gas mixture in CO2/O2 gasification will increase the HHV of the product gases, and the HHV can reach almost 36% of the HHV of natural gas. (4) The optimum gasification conditions for gasifying mesquite to obtain a gas with a higher HHV would be to use an ER of 2.7 with a CO2/O2 mixture as the gasification medium and blending coal with the biomass to increase the percentage of fixed carbon in the fuel.

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ER = equivalence ratio HHV = higher heating value LHV = lower heating value Nm3 = standard cubic meter S/F = steam/fuel ratio Tpeak = peak temperature VM = volatile matter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1(979) 845-2562. Fax: +1(979) 845-3081. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from Texas AgriLife Research State Bioenergy Initiative Funding and the U.S. Department of Energy-NREL, Golden, CO. The authors also acknowledge ASME for their permission to publish this article.

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ABBREVIATIONS Cp = specific heat at constant pressure DAF = dry, ash-free 7468

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(19) Eseltine, D.; Thanapal, S. S.; Annamalai, K.; Ranjan, D. Torrefaction of woody biomass (juniper and mesquite) using inert and non-inert gases. Fuel 2013, 113, 379−388. (20) Cumming, J. W.; McLaughlin, J. The thermogravimetric behavior of coal. Thermochim. Acta 1982, 57, 253−272. (21) Annamalai, K.; Thanapal, S. S.; Lawrence, B.; Ranjan, D. Respiratory quotient (RQ) in biology and scaling of fossil and biomass fuels on global warming potential (GWP). Renewable Energy, manuscript submitted. (22) Annamalai, K.; Puri, I. K. Combustion Science and Engineering; Taylor & Francis Group, LLC: London, 2007. (23) Keoleian, G. A.; Volk, T. A. Renewable energy from willow biomass crops: Life cycle energy, environmental and economic performance. Crit. Rev. Plant Sci. 2005, 24, 385−406. (24) Lawrence, B.; Annamalai, K.; Sweeten, J. M.; Heflin, K. Cofiring coal and dairy biomass in a 29kWt furnace. Appl. Energy 2009, 86, 2359−2372. (25) Ghetti, P.; Ricca, L.; Angelini, L. Thermal analysis of biomass and corresponding pyrolysis products. Fuel 1996, 75, 565−573. (26) Ergüdenler, A.; Ghaly, A. E. A comparative study on the thermal decomposition of four cereal straws in an oxidizing atmosphere. Bioresour. Technol. 1994, 50, 201−208. (27) Giuntoli, J.; Arvelakis, S.; Spliethoff, H.; de Jong, W.; Verkooijen, A. H. M. Quantitative and kinetic thermogravimetric Fourier transform infrared (TG-FTIR) study of pyrolysis of agricultural residues: Influence of different pretreatments. Energy Fuels 2009, 23, 5695−5706. (28) Yang, H. P.; Yan, R.; Chen, H. P.; Lee, D. H.; Zheng, C. G. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007, 86, 1781−1788. (29) Fryda, L. A. Development of innovative systems of electricity production through biomass exploitation. Ph.D. Dissertation, National Technical University of Athens, Athens, Greece, 2006. (30) Thanapal, S. S. Gasification of low ash partially composted dairy biomass with enriched air mixture. M.S. Thesis, Texas A&M University, College Station, TX, 2010.

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