Comparative Study of Temperature Impact on Air Gasification of

Mar 21, 2017 - Department of Bioresource Engineering, McGill University, Quebec H9X ... ABSTRACT: A parametric study of the gasification of six biomas...
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Comparative Study of Temperature Impact on Air Gasification of Various Types of Biomass in a Research-Scale Down-draft Reactor Edris Madadian,*,† Valerie Orsat,† and Mark Lefsrud† †

Department of Bioresource Engineering, McGill University, Quebec H9X 3V9, Canada ABSTRACT: A parametric study of the gasification of six biomass feedstocks (switchgrass, hardwood, softwood, fiber, cardboard and chicken manure), as representative of different types of biomass, has been performed on an experimental, pilot-scale 10 kWth down-draft gasification facility. A comparison was made of the performance of the gasifier as a function of feedstock, in terms of the producer gas production and composition. The variation of producer gas composition was analyzed in the range 600−1000 °C. The results indicated that switchgrass, as representative of energy crops, has greater potential to yield the main components of the producer gas (i.e., hydrogen and carbon monoxide) in comparison with the other feedstocks. However, this did not guarantee the greatest suitability of the switchgrass for downstream applications due to low molar ratios of the ratio of hydrogen to carbon monoxide (H2/CO), which measured at 1.01. To enhance the utility efficiency of producer gas, the downstream engines such as combustion engines require an adjusted molar ratio of H2/CO to utilize certain types of fuels which typically ranges from 1.5 to 3. By means of the catalytic water−gas shift reaction, an important portion of the CO content in the cracked gas can be used for additional hydrogen generation. The temperature variation of the down-draft reactor showed that the CO concentration increased with an increase in gasification temperature followed by a drop of temperature dependent on the different biomass feedstocks. Conversely, CO2 concentration follows an opposite trend that means an initial decreasing trend followed by an increase as the temperature increases. H2 concentration follows a direct relationship with the gasifier temperature. The concentration of CH4 varies very slightly with the increase in gasifier temperature. The results of this study showed that there were significant differences between the energy crops and chicken manure mixed with wood chip, in terms of composition. In general, the variations in producer gas components were smaller at higher temperatures whereas H2/CO showed greater variation between individual feedstocks.

1. INTRODUCTION The effect of growing population of the globe is an increased worldwide energy demand and is projected to rise sharply over the coming years.1 The worldwide fuel demand is recorded at 29.75 billion barrels of equivalent oil in 2011 and projected to be 34.90 billion barrels in 2030.2 The high level of environmental pollution is principally derived from excessive consumption of fossil fuels over the past few decades. As a consequence, greenhouse gas emissions have considerably increased in the earth,3 which served as a stark reminder of the task facing politicians as the governments committed, in the Paris agreement on climate change 2015, to keep the increase in global average temperature to well below 2 °C above preindustrial levels. In this context, there is a critical need toward development of reliable renewable systems. This appeal is being supported by aspiring energy policies such as the European Union to cover one-fifth of its current energy consumption with the renewable resources by 2020.4 Replacing the current global demand for fossil fuels with the existed generation of biofuels would likely increase the chance of extinction of the species and the habitats and threaten global food security through doubling of the human share of net primary productivity.5,6 High-performance extraction and conversion of hydrocarbons from lignocellulosic feedstock is pivotal to effective application of biofuels. From an ecological point of view, biofuels derived from agricultural residues, and lignocellulosic biomass particularly perennial species of wood may prove to be more acceptable than grain and grass feedstocks. © 2017 American Chemical Society

Biomass is available in many varieties, consisting of energy crops, forestry and agricultural residues, municipal solid waste (MSW), and animal wastes. Perennial energy crops such as switchgrass and miscanthus, and forest and agricultural residues are potential biomass feedstocks for bioenergy production.7 Second generation biofuels derived from agricultural material, such as straw, have a high potential to contribute to covering the primary energy demand.8 Wood pellets, which represent forest residues, are mainly densified material rejected by wood product manufacturers. By densification of the residues derived from agricultural and forestry sectors, significant amounts of solid fuel are supplied for producing biofuels and contributing to bioeconomy while mitigating the adverse environmental impact by reducing greenhouse gas emissions.9 MSW generation keeps growing due to the continuous urbanization and improvements in the living standards.2,10 The green energy derived from animal waste is considered to be carbon-neutral because animal feed is sourced from plants.11 Theoretical calculations indicate that thermochemical processes (e.g., gasification) are inherently more productive in energy conversion than anaerobic digestion because most of the carbons can be utilized.12 The conversion of biomass can be achieved through one of two major routes: biochemical conversion (fermentation, an/ Received: December 29, 2016 Revised: March 12, 2017 Published: March 21, 2017 4045

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Energy & Fuels Table 1. Proximate and Ultimate Analysis of Different Types of Biomass Feedstock

proximate analysis (dry)

ultimate analysis (as received)

feedstock

wood pellet (hardwood)

wood pellet (softwood)

fiber pellet (pulp and paper waste stream)

switchgrass pellet

chicken manure (unpelletised)

cardboard briquette

moisture content (mass %)

3.2 ± 0.41

6.5 ± 0.32

4.2 ± 0.57

3.4 ± 0.51

26.1 ± 0.83

6.8 ± 0.33

volatile matter (mass %) fixed carbon (mass %) ash content (mass %) carbon (%)

77.9 ± 5.74 16.5 ± 0.74 1.4 ± 0.24 43.52

76.1 ± 6.33 17.5 ± 0.63 1.1 ± 0.27 46.54

85.7 ± 6.72 3.5 ± 0.34 4.2 ± 0.18 44.94

80.9 ± 8.44 11.9 ± 0.52 3.8 ± 0.27 48.21

31.2 ± 4.12 6.4 ± 0.37 14.4 ± 0.61 41.19

81.2 ± 5.47 8.3 ± 0.40 2.4 ± 0.23 44.29

hydrogen (%) nitrogen (%) sulfur (%) oxygen (%) HHV (MJ kg−1)

7.0 0.23 0.08 43.53 18.31

6.8 0.12 0.05 40.29 17.52

5.9 0.18 0.19 38.46 17.09

5.8 0.32 0.05 38.45 20.34

8.5 24.83 10.29 24.17 12.85

6.8 0.52 0.44 37.63 17.73

formation of the main components of the producer gas (i.e., CO, H2, H2O, CO2, and CH4) along with other light hydrocarbons (HCs), nitrogen, and byproducts (tar and char), as shown in reaction 1.26 It is worth mentioning that the nitrogen content can be as high as 50% in the resulting producer gas due to the use of air as gasifying agent.

aerobic digestion) and thermochemical conversion (pyrolysis, gasification and liquefaction).13 Biochemical conversion may be conducted on two broad pathways: chemical decomposition and biological digestion. Biological processes are essentially microbial digestion and fermentation. A biochemical process converts biomass to ethanol and methane.14 There are two fundamental approaches toward the thermochemical conversion process, the first of which is gasification and the second is liquefaction. The former refers to the process of hydrocarbon breakdown into lighter products. The latter refers to liquefying biomass feedstock by high-temperature pyrolysis, high-pressure liquefaction, ultrapyrolysis, or supercritical extraction. The output of both pathways results in production of enriched energy products from waste.15 Gasification is the most effective process for the production of hydrogen from biomass.16 Gasification utilizes various types of carbon-based feedstocks, such as natural gas, coal, petroleum, petcoke, biomass, and industrial waste.16 The gasification process consists of the thermochemical conversion of carbonaceous material into fuel gas rich in CO and H2, called syngas. However, due to the presence of impurities in the syngas, thereafter it is called a producer gas. The composition of producer gas is affected by gasification conditions, such as temperature, equivalent ratio, and pressure.17,18 There are several operational factors, such as gasifying agent, temperature, pressure, and equivalence ratio (ER), which affect the efficiency of the gasification and ultimately the quality of producer gas. In addition, the biomass feedstock characterization from proximate and ultimate analysis would also influence the performance of gasification.19,20 Recent studies showed that feedstock composition and moisture content along with the type of gasifier and most importantly, temperature affect the gasification process considerably.9,21,22 High oxygen and moisture content adversely affect the lower heating value of the producer gas ( 800 °C) favors an increase in gas yield due to a high decomposition of lignocellulosic feedstock particularly cellulose and hemicellulose. To trim the producer gas for further downstream applications, several re-forming processes could be served. For temperatures between 750 and 800 °C, steam re-forming and water−gas reaction favor higher H2 concentration and lower CH 4 concentration. However, the Boudouard reaction (reaction 2)13,27 favors higher CO concentration in the producer gas at a higher temperature between 850 and 900 °C.19,28 As mentioned earlier, gas composition depends on feedstock composition, type of gasification reactor, gasification agents, and operating condition.23 C + CO2 → 2CO + 172 kJ mol−1

(2)

The present study systematically compares the yields of gases from the gasification of switchgrass, hardwood, softwood, fiber, cardboard, and chicken manure.

2. METHODOLOGY 2.1. Characterization of Biomass Feedstock. Five types of feedstock were selected as representative of different categories of biomass. Gasification experiments were conducted using switchgrass, hardwood, softwood, fiber, cardboard, and chicken manure. The biomass materials, according to the American Society of Testing Materials (ASTM), were proximately analyzed for moisture content,29 ash content,30 fixed carbon,31 and volatile matter in the wood pellet.32 An ultimate analysis was conducted to determine the biomass composition in mass percentage of carbon, hydrogen, and oxygen33 as well as nitrogen33 and sulfur34 (Table 1). Wood Pellet (Hardwood and Softwood). Pelletized woody biomass from Valfei Products (hardwood and softwood, Shawinigan, Quebec, Canada) was used as representative of forest residues. The wood pellet production consists of using recycled hardwood remnants and was densified cylindrically, 4046

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Figure 1. Schematic view of the GEK double layer down-draft gasifier. GEK compartments (a) external vessel of the reactor that minimizes heat loss during the gasification process, (b) internal vessel with air heating lines attached to preheating the air before entering to the reactor, and (c) crosssection of internal vessel where the fuel and gasifying agent (i.e., air) react and produce synthesis gas. The initial ignition is made through ignition port and then mixes with the air from coming out of the air nozzles. The thermocouples record the temperature across the reactor, and the ash generated is removed through the ash trap at the bottom.

bed charcoal impregnated with 60 mL of gasoline through an ignition port. This external heat initiated the combustion process around the air nozzle. The hot gas moved upward to reach the raw material and dry the feedstock. The dried material moved down through the reactor and pyrolysis occurred above 600 °C. The pyrolysis products (i.e., charcoal, tar, and pyrolysis gas) moved downward to crack in the higher temperature combustion zone where CO2 and H2O are produced. The combustion gaseous product lost a molecule of oxygen, in the reduction zone, to produce CO and H2 as the main components of the producer gas. The air, as the gasifying agent, entered the main reactor via an air inlet in the middle of the chamber, in the combustion zone. The air entered in the reactor with an equivalent ratio that varied from 0.1 to 0.5. The generated ash was discharged at the end of each single run to avoid ash agglomeration. The effluent of the reactor was sampled three times after each replication and analyzed with a gas chromatograph equipped with a thermal conductivity detector (TCD) and mass spectrometer for quantification of H2, CO, CO2, H2O, and N2 in the range of concentrations of interest. The gas sampling was conducted using sample bags, which were taken every 20 min for a total of three samples per experiment. The sample bags were filled according to SKC Inc. guideline.37 Gas samples were taken after primary and secondary filtration performed in cyclone column (to capture tars) and charcoal filtration system (to remove excess water and particulates). To facilitate the producer gas analysis in the gas chromatograph, tars and moisture were allowed to condense in the impingers sitting in an ice bath (−20 °C). The impingers contained isopropyl alcohol (IPA) solvents to absorb tars. The experimental procedure involved collection and recording of temperature readings along the gasifier bed using five Type-K thermocouples. The thermocouples were mounted in the middle section at five different locations along the length of the gasifier where drying (T1), pyrolysis (T2), combustion (T3), and reduction (T4) processes took place. The heights of these locations from the lid of the reactor are 15, 25, 40, and 50 cm, respectively. Temperature readings were collected using USB based gasifier control unit (GCU) (Model v3.02, All Power

averaging 8 mm and 30 mm in diameter and length, respectively. Fibrous Pellet. The fibrous feedstock were sampled from the pulp and paper waste stream that consists of cellulose fibers present in hardwood and softwood trees. The biomass feedstock was pelletized with 5 mm in diameter and length varying from 25 mm to 40 mm using a pellet mill machine. The feedstock was supplied from a company in Stanstead, Quebec, Canada. Cardboard Briquette. Food grade waxed cardboard and gable top cartons from municipal solid waste stream were collected, shredded, and pressed using a briquetting machine in the average dimension of 50 mm in diameter and 40 mm in length. The material was received from a company in Quebec City, Canada. Chicken Manure. Chicken manure, as representative of animal waste, was collected from a chicken barn in Quebec City, Canada. The moisture content of feedstock was measured at 48% and was dried to 26% using sun drying and then mixing with wood chip with the ratio of 50/50. Switchgrass. Switchgrass (Panicum virgatum) is a perennial warm-season bunchgrass native to most of North America east of the Rocky Mountains and extends north to 55° N latitude in northeastern Canada.35,36 The switchgrass bale was supplied from a farm in Ottawa, Canada. The switchgrass was pelletized, by the supplier, with a pellet mill machine, with 6 mm in diameter and average length of 30 mm. 2.2. Experimental Setup. The process of gasification is carried out using a down-draft gasifier where the feedstock and the gas flow is concurrent. The Gasifier’s Experimenter’s Kit (GEK, Level 4, Model V3.1.0), with the nominal thermal output of 10 kWth, was purchased from All Power Laboratories (Berkeley, California) and assembled at McGill University. The main reactor is a double-layer vessel (cylinder-shaped, height of 0.4 m and diameter of 0.35 m) equipped with nozzles, where the gasifying agent enters the reactor, and gas pipes to transport the gaseous products into the purification system (Figure 1). Briefly, biomass was fed into the down-draft reactor in batch mode. The feeding rate varied depending on the flowability properties of the feedstock. The process started with ignition of 4047

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Figure 2. Average change in producer gas composition with respect to temperature profile during gasification of (a) switchgrass and (b) chicken manure.

Figure 3. Average change in producer gas composition with respect to temperature profile during gasification of (a) hardwood and (b) softwood.

any change. CO2 showed a decreasing trend; conversely, H2 increasingly reached a maximum of 30% and then continued with a slight positive slope. According to Le Chatelier’s principle and dynamic equilibrium, There is a compatibility between an increase in temperature and endothermic methane re-forming reaction products. As a consequence, higher char conversion and more H2 production is achieved.40 The sum of other impurities such as nitrogen oxide (NOX), ethylene (C2H2), and ethane (C2H6) drastically dropped from 27% to 10% and then started increasing at 800 °C to 19%. The presence of noxious gases is because of using air as gasifying agent. It should be noticed that the values of N2, NOX, C2H2, and organic compounds were estimated by calculation and are named as “others” in the results. In the case of chicken manure, H2 showed an increasing trend at a much lower concentration up to 14% at 1000 °C. CO concentration peaked slightly to 20% at 800 °C and then decreased with the same slight slope; on the contrary, CO2 decreased to almost 12% at 700 °C and increased to 18% at higher temperatures. The concentration of CH4 did not change significantly and ranged 16% to 18%. The other gas

Laboratories, Berkeley, California) every 2 s, and the readings were automatically stored in a computer. These results were obtained at an air biomass ratio of 0.3 (i.e., the optimum air-fuel ratio) and different reactor bed temperatures.

3. RESULTS AND DISCUSSION 3.1. Effect of Temperature on Producer Gas Composition. The gasification temperature controls the equilibrium of the chemical reactions.38,39 The gas compositions were recorded at five different instant temperatures of 600, 700, 800, 900, and 1000 °C during the operation and at an airbiomass ratio of 0.3. All the feedstock reached temperatures over 1000 °C whereas the average bed temperature measured was different for each feedstock. The use of air as gasifying medium causes partial oxidizing of biomass in an exothermic process that aids to drive the endothermic reactions. As shown in Figure 2, switchgrass showed great potential in producing CO and H2. The concentration of CO increased (37% to 41%) with increasing gasifier temperature to almost 700 °C and then decreased to 35% until 900 °C where it increased again. Similarly, CH4 increased to 13% up to 800 °C and then decreased to 7% and continued at this level without 4048

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Figure 4. Average change in producer gas composition with respect to temperature profile during gasification of (a) fiber and (b) cardboard.

Figure 5. Temperature profile of different types of biomass feedstock within down-draft gasifier. The was higher temperature dispersion observed in combustion and reduction zones, where the temperature peaked, than drying and pyrolysis zones.

14%. The other components dropped from 36% to 25% from 600 to 1000 °C. Cardboard H2 concentration of producer gas increased from 5.2% to 25% whereas CO dropped to 16% after peaking to 22% at 700 °C (Figure 4). With the same trend, CO and CH4 dropped from 18.6% to 13%. CO2 decreased to 20% and then increased to 29% at the higher temperature of 1000 °C. The other components also decreased continuously to finally reach the same concentration as CO at 16%. As mentioned earlier, the other gaseous products include NOX, C2H2, C2H6, N2, and organic compounds estimated by calculation:

components, however, formed a major part of the producer gas in the range from 38% to 45%. Hardwood and softwood pellets showed almost identical trends in changing the composition of the producer gas with respect to the temperature profile. As can be seen in Figure 3, CO concentration in hardwood increased from 22%, peaked to 29% at 800 °C, and then dropped to 17% at 1000 °C. CO2 decreased from 14% to 11% at 700 °C and increased to 23% at 1000 °C. The hydrogen content of producer gas increased from 7% to 26%. Likewise, the CH4 decreased at 700 °C from 17% to 9%. The other components of the producer gas drastically decreased to 24% by increasing H2 and CO2 at higher temperatures. The softwood trends were the same as the hardwood except CO, which had fluctuations from 25% to 20%. Fiber H2 increased from 5.1% to 23% (Figure 4). The concentration of CO decreased from almost 25% to 23% in from 600 to 1000 °C, with a slight increase at 700 °C where it reached 26%. Conversely, CO2 decreased to almost 13% at 700 °C and then increased to 19% at 1000 °C. The methane content of the producer gas decreased slightly from 16% to

other gasses (mol %) = 100 − [H 2 + CO + CO2 + CH4]

where H2, CO, CO2, CH4 are mole fractions of hydrogen, carbon monoxide, carbon dioxide and methane gases in percentage. Perhaps most importantly, gasification of biomass leads to the formation of tars. These liquid byproducts of gasification are condensable organic compounds that can foul catalysts, plug lines, and lower the efficiency of the process. Although we 4049

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Figure 6. Producer gas composition of different types of biomass feedstock at different bed temperatures. Switchgrass as representative of energy crops showed higher hydrogen and carbon monoxide than other feedstocks.

takes place followed by a slight drop in the reduction zone (T4) where the reducing environment led to a decreased amount of oxygen. Among feedstocks, the highest temperature profile belonged to switchgrass at 502, 915, and 851 °C at pyrolysis, combustion, and the reduction zones, respectively. The chicken manure demonstrated the lowest temperatures of 364, 783, and 685 °C, at the mentioned zones, respectively. The reason for this difference can be due to the high moisture content of the manure even after drying and mixing with wood chips (50/50). Comparing softwood to hardwood, we find that hardwood has a slightly higher temperature within the reactor that can be attributed to the lower moisture content. Moreover, the cardboard showed a very close temperature profile to paper fiber while having a higher moisture content. In spite of the higher moisture content of cardboard, this similarity between the temperature profiles of cardboard and paper fiber could be due to the better flow of cardboard briquette than fiber pellet in the reactor. The amount of energy input needed for dewatering the biomass increases with the moisture content,42 and the result is lowering the temperature. Madadian et al.9 reported on the bridging problem due to the pelletized geometry of woody biomass in the down-draft reactor. They concluded that less bulky material increased the chance of material bridging near the throat of the reactor and stopped the downward flow of feedstock. 3.3. Effect of Temperature on Producer Gas Production. The influence of reactor temperature was studied by recording composition of produced producer gas corresponding to values of the oxidation zone temperature for each biomass feedstock. The average of these gas composition values recorded over the steady operation period was used to calculate the calorific value of producer gas. Operation of the pilot facility required long run times to obtain the steady-state results that are relevant for continuous operation. More than an hour was required for the temperatures and product concentrations to come to steady state after the conditions were set. Once the process was deemed to be at steady state, data were typically collected at that condition. As shown in Figure 6, the highest bed temperature was 915 °C from switchgrass where the highest amount of hydrogen and CO was produced (29.6 and 37.2%). Both types of wood (hard and soft) produced similar amounts of producer gas

did not measure the tar content, the higher moisture content in some feedstock such as chicken manure could intensify the amount of tar generated after each run and increased the chance of clogging the pipes and junctions. The moisture in the structure of the fuel has undesirable effect on the gasification process. Because the high amount of moisture in fuel uses the energy and results in decreasing the temperature of gasification, which results in a less efficient process. In terms of energy efficiency, it obviates the need (capital, energy and time) for feedstock dewatering and drying as needed for other conversion technologies such as direct combustion and pyrolysis.2,41 Furthermore, the content of H2 exhibits an increasing trend with temperature. This is an expected result because most H2 production reactions are endothermic. The content of CH4 decreases with temperature because higher temperatures increase the steam re-forming reaction of CH4. The content of CO first increased and then decreased with temperature in most cases. 3.2. Temperature Profile of Different Biomass Feedstock. The average temperature profile of each biomass feedstock was recorded during steady-state conditions of the gasification process. Due to different operational duration for each feedstock, the temperature was recorded in four identical time frames. This makes the temperature independent of the time in the graphs. The graphs are presented on the basis of the global steady-state condition of the system after starting the operation. These profiles are provided under the designated conditions including the pretreatment of feedstock (moisture percentage, particle size and component of feedstock) and operating conditions (pressure and equivalence ratio). As shown in Figure 5, the temperature profiles have a ballistic behavior within the reactor. The average temperature in every zone is recorded. The initial location was called T0 and is always set at the ambient temperature of 25 °C because the feedstock just enters into the reactor and there is no exposure to an external source of heat. Starting the ignition increases the temperature in the combustion zone and quickly influences the upper zones of drying (T1) and pyrolysis (T2). The average temperature in the drying zone was recorded below 100 °C for all feedstock. However, the pyrolysis zones ranged between 300 to 500 °C. The temperature of all feedstock peaked in the combustion zone (T3) where the main exothermic reactions 4050

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Figure 7. Hydrogen to carbon monoxide ratio of different biomass feedstock at different bed temperatures. H2 and CO were individually measured through producer gas composition analysis.

production of hydrogen. It is after 800 °C that the CO production dropped, as indicated in section 3.1. Cardboard, hardwood, softwood, switchgrass, fiber, and chicken manure produced the greatest to smallest H2/CO ratio in this order. Cardboard briquette showed a great potential to produce higher H2/CO especially at higher temperatures. This can relate to the shape of the feedstock, which has a smoother flow in the reactor. On the basis of Fourier’s law, the greater area increases the conductive heat transfer. Hence, we hypothesized that the larger surface area of the cardboard briquette could result in higher conductivity, which increases the carbon conversion rate. As for switchgrass, the H2/CO ratio is the lowest before 800 °C and it increased to the values of 0.82 and 1.01 by reaching the higher temperatures of 915 °C (average bed temperature) and 1000 °C (instantly). This relatively low ratio is due to the incomplete water−gas shift reaction (reaction 2) and excess of CO, which did not convert to H2 and CO2. Therefore, the catalytic reactions are required to enhance the hydrogen content of the producer gas to adjust the H2/CO ratio for appropriate the downstream applications.

components except for H2, where it was 3% higher in hardwood. This similarity can be mainly due to comparable bed temperatures 894 and 873 °C for hardwood and sofwood, respectively. As for fiber and cardboard, the amounts of H2 and CO generated in cardboard were negligibly 5.3% and 2.1% more than the ones in cardboard. This result was expected considering both feedstock originated from the pulp and paper stream. However, the greater flowability of cardboard briquette than fiber pellet due to higher bulk density9 can be a reason for greater char conversion and producing more hydrogen and carbon monoxide. In addition, the lower moisture content and higher lignin content of the fiber pellet might be the reason for generating the higher temperature from the fibrous feedstock. In the case of chicken manure, the high moisture content of feedstock did not allow the entire thermal energy spend to decompose the biomass. Therefore, a part of the energy was used to remove the water content from the manure. The temperature of the manure reactor only rose to 645 °C, which is normally considered a low-temperature gasification process. As a result, the heat required to move forward gasification endothermic reactions is not supplied sufficiently. Lack of adequate heat to move the water−gas (reaction 3)13,43 and steam re-forming (reactions 4 and 5)13,27 reactions forward led to lower production of hydrogen. −1

C + H 2O ↔ CO + H 2 + 131 kJ mol

(3)

CH4 + H 2O ↔ CO + 3H 2 + 206 kJ mol−1

(4)

CH4 + 0.5H 2O → CO + 2H 2 − 36 kJ mol−1

(5)

4. CONCLUSION As a basis for investigating biomass gasification and to demonstrate the key characteristics of biomass thermochemical conversion, different types of feedstock (switchgrass, wood, paper fiber, cardboard, and chicken manure) were gasified in a pilot scale fixed bed reactor. The highlights of this study can be listed as below: • The results showed that switchgrass as a representative of energy crops have great potential in producer gas yield; however, the H2/CO might not be necessarily suited for downstream applications. The reason can be due to incomplete vital reactions such as water−gas shift reactions that result in the generation of CO2 and H2 from carbon monoxide and water vapor. • Although it is important how much H2 and CO is generated, their ratio is much more important, which can be adjusted by using catalytic reactions downstream from the gasification main process.

3.4. Effect of Temperature in H2/CO Molar Ratio. The ratio of hydrogen to carbon monoxide was calculated in the range 0.15−1.39 for all cases under different temperatures. The amounts of H2 and CO were recorded through producer gas composition analysis explained in the Methodology section. As shown in Figure 7, there is an increase in the H2/CO ratio with an increase in the temperature profile within the reactor. The standard deviation between H2/CO ratios from 600 to 1000 °C calculated at 0.067, 0.069, 0.093, 0.147, and 0.252, which shows a greater difference between the ratios increased for different feedstock at higher temperatures. The difference can relate to the effect of the higher temperatures that favors higher 4051

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• Feeding the producer gas into the catalytic water−gas shift reaction can be a solution to enhance the H2/CO ratio. • The CO concentration increases with an increase in gasification temperature followed by a slight drop. However, CO2 concentration follows an opposite trend. H2 concentration increases with the increase in gasifier temperature. The concentration of CH4 varies slightly with the increase in gasifier temperature. • Gasification does not individually guarantee the sustainability of purpose grown feedstocks and it requires an integrated approach toward postgasification to valorize the produced producer gas and promote greater use of the biomass feedstock. • Defining the optimum physicochemical characteristics for the feedstock, which is being conducted by the author, along with the results of the present study provide applicable information package for using in biorefineries.

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AUTHOR INFORMATION

Corresponding Author

*E. Madadian. E-mail: [email protected]. Tel: +1 (438) 995 3056. ORCID

Edris Madadian: 0000-0003-2540-9825 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from BioFuelNetwork Canada for BFN Project #29, Valfei Products for supplying the wood pellets, and EMISPEC Co. in Quebec City for supplying the cardboard briquettes.

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DOI: 10.1021/acs.energyfuels.6b03489 Energy Fuels 2017, 31, 4045−4053