Article pubs.acs.org/EF
Cite This: Energy Fuels 2017, 31, 13824−13833
Cleaner Production of Hydrogen and Syngas from Catalytic Steam Palm Kernel Shell Gasification Using CaO Sorbent and Coal Bottom Ash as a Catalyst Muhammad Shahbaz,†,§ Suzana Yusup,*,† Abrar Inayat,‡ David Onoja Patrick,† Muhammad Ammar,† and Angga Pratama† †
Biomass Processing Lab, Centre of Biofuel and Biochemical Research, Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia ‡ Department of Sustainable and Renewable Energy Engineering, University of Sharjah, 27272 Sharjah, United Arab Emirates § Department of Chemical Engineering, University of Gujrat, Gujrat, Pakistan ABSTRACT: The catalytic-sorbent based steam gasification of palm kernel shell is examined in a pilot-scale integrated fluidizedbed gasifier and fixed-bed reactor using coal bottom ash as a novel catalyst for cleaner hydrogen and syngas production. It enhances tar cracking and enriches hydrogen composition as an effective catalyst. The effect of temperature, steam/biomass ratio, CaO/biomass ratio, and coal bottom ash wt % is evaluated for hydrogen yield, syngas composition, lower and higher heating values, and efficiency of carbon conversion, gasification, and cold gas. Steam is the most influential factor, and it enhances the hydrogen yield from 35.7 to 79.77%. The CaO/biomass ratio and coal bottom ash have a positive impact on hydrogen and syngas yield with CO2 sorption and catalytic effect, respectively. The enhancement of hydrogen and syngas composition is due to the catalytic effect of Fe, Al, Mg, and Ca contents present in coal bottom ash as detected by X-ray fluorescence (XRF). The metals content of Fe, Al, Mg, and Ca increased the hydrogen content by enhancing tar cracking, methane reforming, and water gas shift reaction. Maximum hydrogen and syngas production of 79.77 and 85.55 vol %, respectively, were achieved at a temperature of 692 °C with 1.5 steam/biomass ratio and 0.07 wt % coal bottom ash.
1. INTRODUCTION The ample dependence on fossil fuels to meet the increasing demand for energy has raised serious concerns on energy crises and environmental issues like global warming, climate change, and greenhouse gas emission.1 These concerns have become the driving force for research on new, clean, and environmentally friendly fuels from renewable sources to ensure sustainability.2 Cleaner hydrogen and syngas production processes have significant potential to meet the growing energy demand and settle the environmental issue. It is predicted that the future energy trade will be based on hydrogen economy due to its wide application in transport as a clean fuel, electricity generation through the fuel cell, intensive energy density, energy carrier, and safe transportation.3 Syngas could be used as a fuel and also has the potential to be converted into many conventional fuels like methane, biodiesel, ethanol, etc.4 Both syngas and hydrogen are conventionally obtained from fossil fuels like crude oil, coal, and natural gas. The abundant availability of biomass, renewability, the reduction in carbon footprints, and sustainable life cycle make it a promising source for hydrogen and syngas production.5 Biomass gasification is a promising path for hydrogen and syngas production.6 It has drawn a lot of attention in the area of thermochemical and biological processes due to its high conversion of chemical energy of the original fuel from solid to gas.7 Biomass is converted into gaseous products in the reactor by the utilization of steam, carbon dioxide, air, or a combination of these known as gasification agents.8,9 Steam is a preferable gasification agent. It has the advantages of a higher yield of N2-free syngas with © 2017 American Chemical Society
high H2 content which makes it economical at both lab and commercial scales.8 The fluidized bed gasifier has some advantages over other gasifiers. It has a lower gasification temperature and higher reaction rate and conversion efficiency due to better solid−gas contact and effective mixing.10 Hydrogen and syngas have been obtained from steam gasification of biomass in a fluidized bed due to the following complex reactions scheme. combustion reaction
C + O → CO
− 111 kJ/mol (R1)
combustion reaction
C + O2 → CO2
− 283 kJ/mol (R2)
Bourdouard reaction
C + CO2 ↔ 2CO
+ 172 kJ/mol (R3)
methanation reaction
C + 2H 2 ↔ CH4
− 75 kJ/mol (R4)
methanation reaction
2C + 2H 2O → CH4 + CO2
+ 103 kJ/mol water gas shift reaction
(R5)
C + H 2O ↔ CO + H 2
+ 131 kJ/mol
(R6)
Received: October 21, 2017 Revised: November 28, 2017 Published: November 30, 2017 13824
DOI: 10.1021/acs.energyfuels.7b03237 Energy Fuels 2017, 31, 13824−13833
Article
Energy & Fuels water gas shift reaction − 41 kJ/mol
(R7)
carbonation reaction
CO + CaO ↔ CaCO3
− 170.5 kJ/mol
steam reforming + 206 kJ/mol
high temperature of 1000 °C.20 A few studies have been reported for steam gasification of palm oil waste using CaO as a sorbent. Inayat et al.21 investigated gasification of EFB with CaO sorbent and reported 76.5 vol % of hydrogen at a high steam/biomass of 2.5. The production of syngas through the gasification of palm oil waste provides a sustainable solution to palm oil waste management and cleaner fuel production. Even though biomass gasification is studied extensively, it can be concluded from the above discussion that only a few studies were conducted on the steam gasification of palm oil waste using CaO as a sorbent. In the case of catalytic steam gasification of PKS, only conventional Ni and Fe based catalysts are used that are expensive and increases the total cost of the process. Furthermore, the use of coal bottom ash in steams gasification has not been investigated. The replacement of coal bottom ash with expensive conventional catalyst such as Ni and Fe can reduce the cost of the process that helped to commercialize the process. The objective of the present study is to evaluate the effect of coal bottom ash as a prospective catalyst and CaO as a sorbent for hydrogen and syngas production in a pilot-scale gasification system. Coal bottom ash was mixed with biomass and a gasification agent in a fluidized bed gasifier while CaO was placed in a downstream reactor. Along with coal bottom ash and CaO, the impact of critical parameters like temperature and steam/biomass ratio have been also studied in the steam gasification of integrated sorption enhanced steam gasification system. A range of parameters were selected on the basis of previous studies (650−750 °C temperature, 0.5−1.5 wt/wt steam/biomass, 0.5−2 wt/wt CaO/biomass, and 0.07 wt % coal bottom ash to biomass weight).2 The effects of parameters were also evaluated based on carbon conversion, cold gas and gasification efficiencies, and lower and higher heating values of product gases.
CO + H 2O ↔ CO2 + H 2
(R8)
CH4 + H 2O → CO + 3H 2 (R9)
Gasification technologies currently used have some issues affecting their commercial application. They result in poor quality of syngas, lower H2 content, and additional cost for the removal of unwanted components from product gas which effect the overall economy of the process. The use of a catalyst improves the gas quality by converting higher hydrocarbons into lighter gases.3 In addition, it controlled the economy of the process by reducing the gasification temperature and improving the reaction rate.11 The conventional catalysts used in the gasification process are Ni, dolomite, olivine, and alkaline earth metals.12,13 Each type of catalyst has some pros and cons. Some are good for tar reduction, and others have short active life and regeneration problems for expensive catalyst and enhance the particulate content in the product gas.12 Many researchers have attempted to gouge out efficient and cost-effective catalyst for biomass gasification system. In the current work, waste material is used as a catalyst in the gasification process. Coal bottom ash is the waste generated from the combustion of coal in power plants. It has very limited applications mostly in construction industries, and its dumping process is costly.14 It contains many toxic elements which pollute water bodies and the atmosphere when it is dumped. Thus, a sustainable utilization of coal bottom ash is needed. In recent years, many researchers have detected a reasonable amount of alkaline metals, especially Ca and Al in coal bottom ash.15 Alkaline metal oxides and CaO are conventional catalysts used in biomass gasification.16 Xiong et al.15 used coal bottom ash as a bed material for the pyrolysis of coal and noticed a good effect on tar reduction compared to sand. In literature, only the use of biomass ash as a catalyst has been reported for biomass gasification. Umeki et al.17 reported that the use of biomass ash can maintain the activity of the catalyst in biomass gasification. It could be concluded that very little work has been done on the utilization of ashes in biomass gasification, and coal bottom ash has still not been thoroughly investigated. CaO is a well-investigated component as a catalyst and adsorbent for CO2 capture in biomass gasification. The sorption of CO2 occurs via the carbonation reaction. It lowers the partial pressure of CO2 within the system thus shifting the thermodynamic equilibrium of water gas shift reaction toward H2 and CO production. The use of CaO for sorption of CO2 is well-documented in the literature. Guxoinet al.18 used CaO in steam biomass gasification and reported a 51.1 vol % increase in H2 and a 28.4 vol % decrease in CO2 content. They also noted that the optimum temperature range of 923−973 K is needed for CaO activity. Palm oil waste is a major biomass source in Malaysia. Malaysia and Indonesia produce about 86% of the total world palm oil production.19 The research work on the utilization of palm oil waste for fuel production increased substantially in the past decade, especially for air gasification. Hydrogen production of about 12−38 vol % was observed for gasification of EFB at a
2. METHODOLOGY 2.1. Biomass Material. The palm kernel shell (PKS) utilized for the steam gasification was supplied by Kilang Sawit Felcra Nasarudin Sdn. Bhd. The biomass went through sun drying for 4−5 days, and further drying was done in an oven to ensure the removal of free bond moisture. In order to achieve the desired particle size of 0.75 mm, the material was passed through the grinding and sieving process. The particle size of 0.75 mm was found to be optimum in this operation as described in an earlier publication.2 In order to find the elemental and component composition and determine the heating value of PKS, standard operating procedure was adopted as detailed in a previous publication.2 The proximate and ultimate analysis is given in Table 12 2.2. Coal Bottom Ash Catalyst. The coal bottom ash used as a catalyst in this process was acquired from the boiler of TNB
Table 1. Proximate and Ultimate Analysis of PKS2 moisture proximate analysis (dry mass fraction basis) volatile matter (%) fixed carbon (%) ash content (%) ultimate analysis (dry mass fraction basis) C (%) H (%) N (%) S (%) O (%) (by difference) HHV (MJ/kg) 13825
9.70 80.81 14.25 4.94 48.78 5.70 1.01 0.21 44.3 18.82
DOI: 10.1021/acs.energyfuels.7b03237 Energy Fuels 2017, 31, 13824−13833
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Energy & Fuels Janamanjung Sdn Bhd power plant Selangor, Malaysia. The presence of alkaline oxides and their elemental content were identified by performing X-ray fluorescence (XRF) analysis as shown in Table 2.
Table 3. Physical Properties and Chemical Composition Quicklime chemical composition by XRF CaO MgO Fe2O3 Al2O3 K2O3 SiO surface properties adsorption average pore width (4 V/A by BET): (m2/g) pore volume (Barrel−Joyner−Halenda, BJH): (cm3/g) pore width particle density (kg/m3) bulk density (kg/m3)
Table 2. Physical Properties and Chemical Composition CBA chemical composition by XRF SiO Fe2O3 CaO Al2O3 MgO K2O3 surface properties adsorption average pore width (4 V/A by BET): (m2/g) pore volume (Barrel−Joyner−Halenda, BJH): (cm3/g) pore width particle density (kg/m3) bulk density (kg/m3)
44.1 24.3 13 9.21 1.88 1.25 51.01 0.01 33.012 Å 1400
88.5 8.94 0.522 0.498 0.0353 0.726 0.673 0.007 172.553 Å 3090 1101
diameter of the reactor were 250 and 15 cm, respectively. Silica sand was used as the bed material in the fluidized bed. Biomass was mixed with coal bottom ash and supplied at 1 kg/h continuously using N2 gas to ensure forward flow. The feeder was cooled using a cooling jacket to avoid the thermal degradation of biomass before entering into the gasifier. Steam generated in the boiler was further heated in a superheater to 350 °C and supplied at 0.5 to 1.5 kg/h to the gasifier for steam gasification. The gases produced in the first reactor pass through the dolomite bed in the second reactor. The height and diameter of the fixed bed reactor were 250 and 15 cm. Both reactors were made of Inconel 625 material to allow usage up to 850 °C. Uniform heating was done by eight electrical heaters, and thermocouples were used to determine the temperature profile. All of the piping system was wrapped with insulation tape to evade clogging caused by the condensation of tar. Tiny solid particles were removed from the product gas in the cyclone separator. The temperature of the gases was reduced to 40 °C by with the help of a water scrubbing system. The product gas was passed through the separator to remove any residual water. The volumetric flow was measured at the exit of the separator. The gas was then moved to the gas sampling unit, and its composition was measured using an online gas analyzer (Teledyne 7500). The gas analyzing unit measures the gas composition after every 6 and 60 min and was used for all experimental runs. Before the start of each run, the system was purged with N2 gas.
The presence of Fe2O3, Al2O3, and MgO in good percentage shows its potential to catalyze the process. The detection of these oxides was also reported by Xiong et al.15 The presence of alkaline metal has shown the catalytic effect in gasification as reported in the literature.22,23 The surface texture and particle shape were determined using scanning electron microscopy (SEM; model Zeiss Supra 55 VP) as shown in Figure 1. The surface was found to be made of coarse and
Figure 1. SEM images (a) 1500× (181 μm) and (b) 500× (542 μm). irregulars shaped particles. The pore size, pore volume, and other properties were determined using physiosorption test. The BET surface area was 51.01 m2/g. The Barrel−Joyner−Halenda (BJH) bases volume of pores was 0.01 cm3/g, and the average pore width was 33.012 Å. 2.3. CaO Adsorbent. The material used in this study as an adsorbent is dolomite (CaO) obtained from Kinetic Chemical Sdn. Bhd., Malaysia. The dolomite was also ground into the desired particle size of 0.250 mm. The properties of adsorbent are given in Table 3. The (XRF) analysis showed that the CaO content was 88.5%, which was acceptable for adsorption application.24 The presence of other compounds MgO, Fe2O3, and Al2O3 also has a good impact on the gasification process. The physio-sorption test was done in order to find the pore properties. The BET surface area was found to be 0.6734 m2/ g. The porosity properties are associated with pore volume and pores size that is important for carbonation reactions. The pore volume (Barrel−Joyner−Halenda, BJH) was 0.007 cm3/g. The average BJH pore size was 172.553 Å. The particle density and bulk density are given in Table 3. 2.4. Equipment and Experimental Setup. The pilot-scale gasification system used for gasification of palm kernel is shown in Figure 2. The gasification setup consists of one cylindrical fluidized bed reactor and one fixed bed reactor with externally jacketed heaters, cyclone separators, biomass feeding system, steam generation system, water scrubbing system, water treatment system, and online gas analysis system. In order to maintain fluidized bed conditions, a perforated plate was used for the flow of air and steam. The length and
3. RESULTS AND DISCUSSION 3.1. Effect of Steam/Biomass Ratio. Steam is the most promising gasifying agent for hydrogen and syngas production in the gasification system. In the current work, steam gasification of palm kernel shell in the presence of CaO sorbent and coal bottom ash as an alkaline oxide rich catalyst has been investigated. The effect of steam/biomass ratio varies from 0.5 to 1.5 (w/w) on product gas composition, lower and higher heating values, carbon conversion efficiency, gasification efficiency, and cold gas efficiency and is shown in Table 4. The temperature and biomass flow rate were 692 °C and 1 kg/h, respectively. CaO was used as a sorbent, and the sorbent/ biomass ratio was 1.42. Coal bottom ash used for the process was 0.07% of biomass weight. From Table 4 hydrogen production significantly increased due to increase in the steam to biomass ratio. Hydrogen production was 35.7 vol %, at 0.5 steam/biomass ratio, and it rose to 62.56 vol % at a steam/biomass ratio of 1. The maximum hydrogen production achieved was 79.7 vol % at a steam/biomass ratio of 1.5. The enhancement in hydrogen yield by the introduction of steam was due to the activeness of methane reforming reaction R9, water gas shift reaction R7, and char gasification reaction R6. In this study, the palm oil waste gasification was done in the 13826
DOI: 10.1021/acs.energyfuels.7b03237 Energy Fuels 2017, 31, 13824−13833
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Figure 2. Process flow diagram for catalytic-sorption based gasification system.
Steam promoted the steam-methane reforming reaction and reduced methane formation in the product. Inayat et al.21 also observed the reduction in CH4, CO, and CO2 when steam was added in a modeling and experimental study of palm oil waste (EFB) steam gasification. An optimum steam/biomass ratio is important for syngas production while excessive steam reduces the gasification temperature.27 The lower yield of CO2 shows the sorption activity of CaO in the process. The higher syngas yield is also supported by the presence of Al2O3, Fe2O3, and MgO in coal bottom ash for catalytic steam gasification. The Fe2O3 and Al2O3 enhanced the char gasification.28 The lower and higher heating values of the gases are shown in Table 4. The lower heating value (LHVgas) decreases with an increase in steam/biomass ratio from 13.64 to 12.64 MJ/Nm3. The main contributors in lower heating values are H2, CO, and CH4 as shown in eq 1.
Table 4. Experimental Results at Different Steam/Biomass Ratios parameters steam/biomass ratio (w/w) biomass feed flow rate (kg/h) temperature (°C) CaO/biomass ratio (w/w) coal bottom ash % (wt) gas composition (vol %) H2 CO2 CO CH4 syngas (H2 + CO) lower heating value (MJ/Nm3) higher heating value (MJ/Nm3)
values 0.5 1 692 1.42 0.07
1 1 692 1.42 0.07
1.5 1 692 1.42 0.07
35.75 20.39 25.55 18.31 61.3 13.64 15.07
62.56 12.06 10.59 14.77 73.15 13.38 15.20
79.77 5.48 5.93 8.81 85.7 12.5 14.44
LHVgas = (30 × CO + 25.7 × H 2 + 85.4 × CH4)0.0042
(1)
The drop in LHVgas was due to the reduction of CH4 and CO contents as they have the larger contribution in LHV of gases. The system performed well in terms of heating values as seen from the small fall in heat content. The decrease in heating value of gas for steam gasification of PKS was also observed by Khan et al.3 The higher heating value (HHVgas) decreased from 15.07 to 14.28 MJ/Nm3 with an increase in steam to biomass ratio as calculated using eq 2.
presence of CaO. Khan et al.3 reported an increase in hydrogen yield as a result of an increase in steam/biomass ratio for in situ steam catalytic gasification of PKS. Furthermore, the use of steam enhances the hydrogen yield by shifting the equilibrium of water gas shift reaction R7 toward the forward direction as reported in the literature.25 The trend of CO, CO2, and CH4 gas production with the variation of steam supply had good agreement with the previously reported literature.26 Syngas production increased when the steam/biomass ratio increased from 62.5 to 85.5 vol %. It can be observed from Table 4 that the CO content of syngas decreased and hydrogen content increased with the increase in the steam/biomass ratio. At a steam/biomass ratio of 0.5, the content of CO was high (about 25.1 vol %), and it decreased to 5.93 vol % at a higher steam/biomass ratio of 1.5. The drop in CO yield is due to the dominance of the water gas shift reaction R7 that consumed the CO produced in steam methane reforming reaction R9. The functionality of these reactions was also reported in previous studies for steam biomass gasification.25 Methane production decreased from 18.3 to 8.81 vol % in steam gasification of palm kernel shell.
HHVgas = (H 2 × 30.52 + CO × 30.18 + CH4 × 95)0.0041868 (2)
The increase in HHVgas is due to an increase in H2 contents in the product gas. The drop in heating value was due to the drop in high-energy content gases (CH4 and CO) at a higher steam/biomass ratio of 1.5. It is observed that the drop in CO and CH4 content was higher than the increase in H2 content. A similar trend was also observed for palm waste gasification by Li et al.29 Lower heating values of biomass and product gas helped to determine the cold gas efficiency in order to evaluate the performance of the gasification system along with carbon conversion and gasification efficiencies. 13827
DOI: 10.1021/acs.energyfuels.7b03237 Energy Fuels 2017, 31, 13824−13833
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and dropped to 71.5 vol % at 750 °C as shown in Table 5. A similar trend was noticed for syngas yield in which a higher syngas composition (85.71 vol %) was obtained at 692 °C. The second constituent of syngas (CO) decreased from 13.12 to 8.80 vol % when the temperature increased. The product gas composition was the result of endothermic and exothermic reactions R1−R7 and R9. The activeness of the water gas shift reaction R6 set the trend of hydrogen and syngas yield in the temperature range from 650 to 692 °C. A similar trend was observed for catalytic steam gasification of palm oil waste for a temperature range of 600−675 °C.3 The use of CaO to adsorb the CO2 produced during the gasification process is also important in determining the final syngas composition. The CO composition varied from 5.83 to 8.39 vol % as shown in Table 5. Its lowest value was at 692 °C which shows the effectiveness of CaO as a sorbent material. The effect of CaO on gases composition in steam gasification was also reported for a temperature range of 600 to 670 °C in literature.31 A significant reduction in methane formation from 14.98 to 8.80 vol % was noticed when the temperature was increased from 650 to 692 °C as shown in Table 5. The temperature range from 650 to 800 °C is found to be good in fluidized bed gasifier for H2 production with CaO sorption.32 This was due to the activeness of the endothermic methane reforming reaction that contributed to the enrichment of H2 and CO content in the syngas. Shen et al.25 recorded a decrease in methane yield with an increase in the temperature from 650 to 700 °C in biomass steam gasification using the fluidized bed. The endothermic tar cracking reaction was also favored by higher temperatures and enhanced the hydrogen concentration in the product array.33 In the current study, it was noticed that CO and CO2 contents increased at a higher temperature of 750 °C while the H2 content decreased. The calcination of CaCO3 at 750 °C boosted the CO2 formation. The calcination of CaCO3 at 727 °C was also reported in the literature.34 Inayat et al.21 observed the increase in hydrogen yield for in situ steam gasification of palm oil wastes (empty fruit bunches). The increase in hydrogen yield with temperature was also observed for pine sawdust steam catalytic gasification with CaO sorbent.35 The lower and higher heating values were determined using the molar content of CO, H2, and CH4 on the basis of eqs 1 and 2. The lower heating value decreased slightly from 12.88 to 12.5 MJ/Nm3 with an increase in temperature from 650 to 692 °C as shown in Table 5. The heating value increased with a further increase in temperature. A similar profile of LHVgas with temperature was reported in the literature for catalytic steam gasification of palm kernel shell with CaO. The HHVgas increased significantly with an increase in temperature from 650 to 692 °C as depicted in Table 5. Gupta et al.36 also reported a similar trend for the heating values for steam gasification of palm residues in semibatch type reactor. Carbon conversion efficiency is defined as the ratio between mol of carbon contained in gaseous products (CO, CO2, and CH4) to mol of carbon in biomass feed,32 whereas the gasification efficiency is calculated based on the mass of product gases (CO, CO2, H2, and CH4) to the mass of biomass feed.37 The carbon conversion efficiency was lower than the gasification efficiency as shown in Figure 4. It is depicted from Figure 4 that both carbon conversion efficiency and gasification efficiency increased with an increase in temperature. The increase in carbon conversion efficiency at higher temperature was due to an increase in CO and CO2 content
Figure 3 shows the effect of changes in the steam/biomass ratio on the carbon conversion, cold gas, and gasification
Figure 3. Effect of steam/biomass on carbon conversion, gasification, and cold gas efficiencies.
efficiencies. Both gasification and cold gas efficiencies are higher at steam/biomass ratio of 1 and increased first and then decreased with the increase in steam/biomass ratio. The drop in gasification efficiency is due to a decrease in CH4, CO, and CO2 content. Whereas, the cold gas efficiency decreased due to an increase in the steam/biomass ratio from 75.69% to 69.74% because it is related to LHV of gases that decreased with increases in the steam/biomass ratio. The gasification efficiency is higher than carbon conversion efficiency because it is related to the mass of gases, whereas the carbon conversion efficiency is calculated on the basis of mol of product gases. Detournay et al.30 also noted similar trends in catalytic steam gasification of biomass in a fluidized bed for steam/biomass ratio range of 0.5−2.0 (w/w). 3.2. Effect of Gasification Temperature. Temperature has a capital role in the gasification process. In this study, the palm kernel shell steam gasification was carried out at three different temperatures of 650, 692, and 750 °C. The effect of temperature on product gas composition, lower and higher heating values, carbon conversion efficiency, gasification efficiency, and cold gas efficiency is shown in Table 5. The adsorbent/biomass ratio of 1.42 and steam/biomass ratio of 1.5 (w/w) was used with 0.07% coal bottom ash as a catalyst. The hydrogen and syngas composition was sensitive to the temperature. The hydrogen production increased from 66.6 to 79.7 vol % as the temperature increased from 650 to 692 °C Table 5. Experimental Results at Different Bed Temperatures parameters temperature (°C) biomass feed flow rate (kg/h) steam/biomass ratio (w/w) CaO/biomass ratio (w/w) coal bottom ash % (wt) gas composition (vol %) H2 CO2 CO CH4 syngas (H2 + CO) lower heating value (MJ/Nm3) higher heating value (MJ/Nm3)
values 650 1 1.5 1.42 0.07
692 1 1.5 1.42 0.07
750 1 1.5 1.42 0.07
66.67 7.83 13.11 14.98 79.78 12.88 14.32
79.77 5.48 5.93 8.80 85.7 12.5 14.44
71.31 8.39 10.35 10.94 81.66 12.79 14.64 13828
DOI: 10.1021/acs.energyfuels.7b03237 Energy Fuels 2017, 31, 13824−13833
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the effect of trimetallic (Fe, Al, and Ni) catalyst on these reactions and on hydrogen yield. The effect of a catalyst on tar cracking was also observed for Al2O3 and NiO-based catalysts used. The XRF analysis showed the presence of these oxides in coal bottom ash. Effect of coal bottom ash on tar reduction was also noticed in the pyrolysis of coal.15 Second, the effectiveness of the water gas shift reaction and the steam reforming reaction was due to the enhancement in the conversion of solid biomass into gaseous products as a result of catalyst usage.38 The alkali metals, Ca and K, enhance gasification reactivity by catalytic action.22 The effect of Febased catalyst was also observed by Zakir et al.39 in steam catalytic gasification of PKS and palm oil fronds. The hydrogen yield slightly decreased with further increase in coal bottom ash up to 0.10 wt %. This was due to a slight increase in the yield of CH4, CO, and CO2 resulting from a slight decrease in the water gas shift reaction and capacity of CaO adsorbent. 0.07% was found to be an optimum value for hydrogen and syngas production. Very little variation was noticed in lower and higher heating values with the change in wt % coal bottom ash. Both slightly decreased from 13.018 to 12.018 MJ/Nm3 and 14.84 to 14.325 MJ/Nm3, respectively, as shown in Table 6. The decrease in LHVgas and HHVgas of the gases was due to a decrease in CH4 and CO contents of the product gas. Both carbon conversion and gasification efficiency decreased with an increase in coal bottom ash as shown in Figure 5. The drop in carbon
Figure 4. Effect of steam/biomass on carbon conversion, gasification, and cold gas efficiencies.
of the product gas. A similar observation was reported for PKS steam gasification.3 The increase in gasification efficiency might be due to the increase in molar gases production at the higher temperature. A similar increase in gasification efficiency was also reported by other researchers.30 The cold gas efficiency decreased up to 70.3% with an increase in temperature up to 750 °C. The trend of cold gas efficiency was related to the lower heating values of biomass and product gas. The cold gas efficiency dropped due to a decrease in CO and CH4 content at 692 °C. The slight increase in efficiency at higher temperature was due to increase in CO content. 3.3. Effect of Catalyst. The use of catalysts in biomass gasification is always significant in terms of product gas composition, reduction of tar, and higher gasification reactivity.12 In this study, a new material, coal bottom ash, was used as a catalyzed because of Fe2O3, Al2O3, CaO, and MgO present in it as identified by XRF analysis and also as reported by Xiong et al.15 The effects of percentages of coal bottom ash used (0.02, 0.07, and 0.10) on product gas composition, lower and higher heating value, carbon conversion, gasification, and cold gas efficiencies are given in Table 6. The hydrogen production is increased by about 11 vol % Table 6. Experimental Results at Different Catalyst wt % parameters coal bottom ash wt (%) biomass feed flow rate (kg/h) temperature (°C) CaO/biomass ratio (w/w) steam/biomass ratio (w/w) gas composition (vol %) H2 CO2 CO CH4 syngas (H2 + CO) lower heating value (MJ/Nm3) higher heating value (MJ/Nm3)
Figure 5. Effect of coal bottom ash % on carbon conversion, gasification, and cold gas.
values 0.02 1 692 1.42 1.5
0.07 1 692 1.42 1.5
0.10 1 692 1.42 1.5
68.16 9.32 10.37 12.13 78.53 13.08 14.84
79.77 5.48 5.93 8.81 85.7 12.5 14.44
75.68 7.91 7.22 9.17 82.9 12.37 14.23
conversion efficiency was due to the enhancement of the water gas shift reaction. The carbon conversion efficiency was based on the mol of carbon containing gases and carbon of biomass. The catalyst enhanced the conversion of solid carbon into gaseous product and also enhanced tar cracking which enriched the hydrogen content of syngas. Chin et al.38 obtained a lower amount of residue after catalytic gasification compared to noncatalytic gasification which was an evidence of the impact of catalysts on conversion of solid into gases. In our previous study, the conversion of biomass feed increased with increase in wt % coal bottom ash.2 The higher value of gasification efficiency (63.59%) compared to carbon conversion efficiency (47.71%) was due to the fact that gasification efficiency was calculated from the total mass of all product gases while the total mol of product gas was used for carbon conversion efficiency. The cold gas efficiency slightly decreased from 72 to 68.11% with an increase in wt % coal bottom ash. It is related to the lower heating value of gases and biomass and has similar behavior with LHVgas.
with an increase in coal bottom ash wt % from 0.02 to 0.07%, as shown in Table 6. It can also be observed that CO and CO2 and CH4 contents decrease with an increase in coal bottom ash. The enrichment in hydrogen yield was due to the activeness of water gas shift reaction R7 and steam reforming reaction R9 which enhanced hydrogen production. Li et al.29 also explained 13829
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Energy & Fuels 3.4. Effect of CaO/Biomass Ratio. CaO is used as an adsorbent for CO2 sorption in biomass steam gasification. It also has a catalytic effect because it enhances the water gas shift reaction and steam methane reforming reaction in gasification.18 The effect of adsorbent to biomass ratio on product gas, lower and higher heating values, carbon conversion efficiency, gasification efficiency, and cold gas efficiency is shown in Table 7. The hydrogen production increased from
The lower heating value decreased from 13.58 to 12.50 MJ/ Nm3 with an increase in CaO/biomass ratio from 0.5 to 1.5 due to the reduction in CH4, CO, and CO2 contents as shown in Table 7, whereas a slight increased in LHVgas was observed when CaO/biomass ratio was increased further. The trend was similar to that observed for higher heating value (HHVgas). A value of 15.40 MJ/Nm3 was obtained at 0.5 CaO/biomass ratio due to higher content of CH4. This is similar to the range reported in literature.40 Figure 6 shows that carbon conversion
Table 7. Experimental Results at Different CaO/Biomass Ratios parameters CaO/biomass ratio (wt %) biomass feed flow rate (kg/h) temperature (°C) coal bottom ash wt (%) steam/biomass ratio (w/w) gas composition (vol %) H2 CO2 CO CH4 syngas (H2 + CO) lower heating value (MJ/Nm3) higher heating value (MJ/Nm3)
values 0.5 1 692 0.07 1.5
1.42 1 692 0.07 1.5
2 1 692 0.07 1.5
63.67 8.83 13.10 13.98 76.77 13.58 15.40
79.77 5.48 5.93 8.81 85.7 12.5 14.44
65.23 9.05 12.5 13.12 77.73 13.33 15.14
Figure 6. Effect of CaO/biomass ratio on carbon conversion, gasification, and cold gas efficiencies.
66.67 to 79.5 vol % when CaO/biomass ratio increases from 0.5 to 1.42 wt/wt. This fell to 71.2 vol % with a further increase in the ratio to 2. CaO is produced at a higher temperature due to decomposition of CaCO3. CaO captured the CO2 through carbonation reaction R7 in which CaO reacted with the produced CO2 and formed the CaCO3. This phenomenon is reported by many researcher.40 The effect of CaO is in two stages; at the first stage, enrichment of hydrogen production occurred due to its catalytic effect on methane reforming and tar cracking reactions that have previously been reported in the literature.41 Second, the increase in hydrogen production is attributed to the functionality of CaO to capture the CO2. The CO2 adsorption caused the equilibrium of the water gas shift reaction and steam methane reforming to be shifted to the forward direction, ultimately enhancing the H2 contents in the final syngas. It can also be observed from Table 6 that CO and CO2 contents are reduced from 13.11 and 7.83 vol % to 5.93 and 5.48 vol %, respectively. CaO sorption and catalytic effects were also noticed when it was applied on CO and CO2 yield.18 The reduction of CH4 from 13.98 to 8.81 vol % advocated the steam reforming reaction. A similar observation was reported in the steam gasification of PKS with CaO at temperature 675 °C.40 The sensitivity of hydrogen and syngas yield was also observed in reported literature.42 A lower production of hydrogen and syngas in the second stage indicated that after a certain value of CaO, its catalytic and sorption effect decreases. This is supported by the slight increase in CO and CO2 content at a CaO/biomass ratio of 2 as shown in Table 7. This trend was reported for CaO/biomass ratio above 1.5.31 Khan et al.40 also found that 1.5 is the maximum ratio for the effective influence of CaO in the biomass steam gasification process. In the current study, this maximum limit was 1.42 because some amount of CaO was present in coal bottom ash along with Fe2O3 and other compounds.
efficiency and gasification efficiency increase with an increase in CaO/biomass ratio from 0.5 to 1.42. The carbon conversion efficiency and gasification efficiency dropped to 29.2 vol % and 38.2 vol %, respectively at a higher ratio of 2. The lower value of carbon conversion efficiency compared to gasification efficiency was due to the fact that it was measured based on moles of carbon containing gases. The same trend was also reported in literature.40 On the other hand, it first decreased from 73.12 to 69% with an increase in CaO/biomass ratio up to 1.42. Further increase in the ratio resulted in an increase in cold gasification efficiency. 3.5. Comparative Study. 3.5.1. Hydrogen Gas Composition. The comparison of hydrogen and syngas production in this study with the other reported literature is given in Table 8. In the present study, the maximum hydrogen and syngas compositions obtained were 79.98 and 84.5 vol %, respectively, at 692 °C, 1.5 steam/biomass ratio, 1.5 adsorbent to biomass ratio, and 0.07% coal bottom ash. Han et al.42 reported 60 vol % hydrogen and 68% syngas production at 1.33 steam/biomass ratio and relatively higher temperature of 740 °C for in situ gasification of sawdust in a fluidized bed. Inayat et al.21 reported 76.1 vol % hydrogen for in situ steam gasification of EFB. Similarly, 76.2 vol % of hydrogen was reported for cogasification of PKS with polyethylene at 800 °C for a steam/biomass ratio of 1 and adsorbent to biomass ratio of 1.25.27 Li et al.29 produced 52.12 vol % hydrogen from palm oil steam gasification with trimetallic catalyst and 40.6 vol % with dolomite at 800 °C and a steam/biomass ratio of 1.33. In both cases, CO2 contents were higher than 20 vol %. The hydrogen production was lower (48.2 vol %) for ficus virines biomass gasification at S/B = 1, A/B = 0.5, and 750 °C as reported by ref 43. The lower hydrogen content was due to lower S/B and A/B ratios. A maximum hydrogen content of 82.2 vol % was achieved in steam gasification of PKS at 675 °C, S/B = 2, and A/B = 1 using a commercial Ni catalyst.40 The results showed that the system worked well both for hydrogen and syngas 13830
DOI: 10.1021/acs.energyfuels.7b03237 Energy Fuels 2017, 31, 13824−13833
Article
Energy & Fuels Table 8. Comparative Study of Hydrogen Production H2 and syngas yield
biomass palm oil waste
79.98%, 84.5 vol %
palm kernel shell/ polyethylene
76.20%
palm oil waste
52.12%
empty fruit bunches
76.12
saw dust
60 vol %, 67%
palm kernel shell
82.2%
ficus virens
48.2
conditions T = 692 °C S/B = 1.5 A/B = 1.42 T = 800 °C S/B = 1 A/B = 1.25 T = 800 °C S/B = 1.5 A/B = T = 750 °C S/B = 3 A/B = 1 T = 740 °C S/B = 1.33 A/B = 0 T = 675 °C S/B = 2 A/B = 1 T = 750 °C S/B = 1 A/B = 0.5
Table 10. Comparative Study for Carbon Conversion and Gasification Efficiency
ref material
present study
palm kernel shell
Moghadam et al.27
palm kernel shell biomass
Li et al.29
palm kernel shell palm kernel shell palm oil waste
13.58 13.14 10.72
gasification efficiency 75.2% (T = 692 °C, S/B = 1, A/B = 1.42) 42.95% (T = 675 °C, S/B = 1, A/B = 1)
ref present study Khan et et al.40 Xu et al.44
53.2% (T = 800 C, S/B = 1.2, A/B = 0)
Hu et al.11
Inayat et al.21
gasification system with Ni catalyst. A lower carbon conversion efficiency (24%) was observed for steam catalytic gasification at a relatively higher temperature (722 °C), S/B ratio of 2, and A/ B ratio of 1.44 A gasification efficiency of 53.2% was reported for steam gasification of apricot at a relatively high temperature (800 °C) and S/B ratio of 1.2.11 The present steam gasification system using CaO and catalyst coal bottom ash performed better in terms of carbon conversion efficiency and gasification efficiencies.
Han et al.42
Khan et al.40
Zhang et al.43
4. CONCLUSION In this study catalytic-sorption based steam gasification of palm kernel shell has been analyzed at the pilot scale using coal bottom ash as an alkaline metal oxide rich catalyst. The steam/ biomass ratio enhanced the hydrogen and syngas yield remarkably (from 35.7 to 79.98 vol % and 61.10 to 85.78 vol %, respectively). Steam has a positive impact on carbon conversion efficiency and gasification efficiency but not on the cold gas efficiency and lower and higher heating values. The temperature appeared to be a significant factor affecting compositions of syngas and hydrogen. The maximum values for carbon conversion efficiency of 47.03%, gasification efficiency of 64.04%, and higher heating value of 14.64 MJ/ Nm3 at 750 °C. The CaO/biomass ratio from 0.5 to 1.42 has a positive impact on hydrogen, syngas yield, carbon conversion efficiency, and gasification efficiency. Coal bottom ash has shown a catalytic effect due to the presence of Fe2O3, Al2O3, and MgO content by increasing the hydrogen production from 69.79 to 79.99 vol % with varying content from 0.02 to 0.07%. The comparative study also indicates that coal bottom ash is an efficient catalyst and CaO is an efficient CO2 sorbent in gasification.
Table 9. Comparative Study of Lower Heating Values lower heating value, MJ/Nm3
59.9% (T = 692 °C, S/B = 1, A/B = 1.42) 41.42% (T = 675 °C, S/B = 1, A/B = 1) 24% (T = 722 °C, S/B = 2, A/B = 1)
apricot stones
production using coal bottom ash as a catalyst. A Ni catalyst was used to obtained high hydrogen content gas,3 but this work attained comparable values using a less-expensive coal bottom ash waste and at the same time overcoming the problem of agglomeration of bed material which is common in in situ processes. 3.5.2. Lower Heating Values. The lower heating value of syngas for the present study is compared with the literature data for biomass obtained from the same source, as shown in Table 9. It can be seen from Table 6 that Janfen et al.29
material
carbon conversion efficiency (%)
parameters
ref
T = 692 °C, S/B = 1.5, A/B = 1.42 T = 675 °C, S/B = 1.5, A/B = 1 T = 800 °C, S/B = 1.33, A/B = 1
present study Khan et al.3 Li et al.29
reported lower heating values of 10.72 N m3 for steam gasification of palm oil wastes at 800 °C. Khan et al.3 reported that a lower heating value was 13.14 MJ/Nm3 for in situ steam gasification of PKS at 675 °C at a S/B ratio of 1.5 and adsorbent/biomass ratio of 1. In the current study, the product gas has a lower heating value of 13.58 MJ/Nm3 at a temperature of 692 °C, steam/biomass ratio of 1.5, and adsorbent/biomass ratio of 1.42. The system performed better than previous studies in terms of lower heating value. 3.5.3. Carbon Conversion Efficiency. Table 10 exhibits the comparison of carbon conversion and gasification efficiency with previously reported studies. The maximum carbon conversion efficiency was 59.9% and gasification efficiency of 75.2% at a S/B ratio of 1, A/B ratio of 1, and 692 °C. Khan et al.40 reported the carbon conversion efficiency of 41.2% and gasification efficiency of 42.9% at a relatively low temperature of 675 °C and S/B and A/B ratios of 1 in the catalytic steam
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Suzana Yusup: 0000-0003-2137-8435 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research project is funded by the Ministry of Higher Education Malaysia under the Long-term Research Grant Scheme (LRGS) and (KETHHA). The authors would like to thank Universiti Teknologi PETRONAS for providing facilities to conduct this research work. 13831
DOI: 10.1021/acs.energyfuels.7b03237 Energy Fuels 2017, 31, 13824−13833
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Energy & Fuels
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