Steam co-gasification of coal and biomass derived chars with synergy effect as an innovative way of hydrogen-rich gas production

The main results of the experimental work on steam co-gasification of Polish hard coal and Salix Viminalis blends in a fixed bed reactor under atmospheric pressure and at the temperature of 700, 800 and 900 °C are presented in the paper. The effectiveness of co-gasification of coal/biomass blends of 20, 40, 60 and 80% w/w biomass content was tested in terms of gas flows, composition, carbon conversion and chars reactivity. A synergy effect in the co-gasification tests, consisting in increase in the volume of hydrogen produced, when compared to the tests of coal and biomass gasification, was observed at all tested temperatures. The observed synergy effect was attributed to the catalytic effect of K₂O present in blend ash (6-10% wt). Moreover, in co-gasification of blends of 20 and 40% w/w biomass content, increase in the total gas yield was observed, when compared to the tests of coal and biomass gasification at all tested temperatures. In tests of co-gasification of blends of higher biomass content (i.e. 60 and 80% w/w), a slight decrease in the total volume of product gas was observed, when compared to the tests of coal and biomass gasification. Nevertheless, higher ratio of biomass in co-gasification makes it still an attractive option in terms of CO₂ emission reduction and increase in hydrogen production.     

Effect of fuel blend composition on hydrogen yield in co-gasification of coal and non-woody biomass

In this study, torrefaction of sunflower seed cake and hydrogen production from torrefied sunflower seed cake via steam gasification were investigated. Torrefaction experiments were performed at 250, 300 and 350 °C for different times (10–30 min). Torrefaction at 300 °C for 30 min was selected to be optimum condition, considering the mass yield and energy densification ratio. Steam gasification of lignite, raw- and torrefied biomass, and their blends at different ratios were conducted at downdraft fixed bed reactor. For comparison, gasification experiments with pyrochar obtained at 500 °C were also performed. The maximum hydrogen yield of 100 mol/kg fuel was obtained steam gasification of pyrochar. The hydrogen yields of 84 and 75 mol/kg fuel were obtained from lignite and torrefied biomass, respectively. Remarkable synergic effect exhibited in co-gasification of lignite with raw biomass or torrefied biomass at a blending ratio of 1:1. In co-gasification, the highest hydrogen yield of 110 mol/kg fuel was obtained from torrefied biomass-lignite (1:1) blend, while a hydrogen yield from pyrochar-lignite (1:1) blend was 98 mol/kg. The overall results showed that in co-gasification of lignite with biomass, the yields of hydrogen depend on the volatiles content of raw biomass/torrefied biomass, besides alkaline earth metals (AAEMs) content.     

Steam gasification of co-pyrolysis chars from various types of biomass

Co-pyrolysis of two different types of biomass among apple tree branch (ATB), knotweed stem (KWS), seaweed (SW) and rice straw (RSt) was conducted to obtain co-pyrolysis char (co-char), and then the steam gasification of those co-chars was compared with the steam co-gasification of the physically mixed individual biochars to investigate the synergetic effect resulted from alkali and alkali earth metal (AAEM) in each biomass involved. It is found that the silica species in the RSt had negative effect on the activity of co-char due to the formation of alkali silicate compounds. However, combination of RSt with some non-woody biomass such as SW also showed promoting effect. In particular, the gasification of the co-char from the combination of various biomass with low or no silica content showed improved gasification efficiencies due to the synergetic effect AAEM species in the co-char from the different biomass. Therefore, the biomass selection should play a significant role in the co-pyrolysis of different biomass in the two-stage gasification system.     

Steam gasification of energy crops of high cultivation potential in Poland to hydrogen-rich gas

In the paper energy crops of considerable cultivation potential in Poland, namely: Salix viminalis, Helianthus tuberosus, Sida hermaphrodita, Spartina pectinata, Andropogon gerardi and Miscanthus X giganteus were tested in terms of steam gasification reactivity of biomass chars, as well as yields and composition of product gas in steam gasification and lime-enhanced steam gasification in a laboratory scale fixed bed reactor at 650 °, 700 ° and 800 °C.The highest value of reactivity for 50% of carbon conversion, R₅₀, was observed for Sida hermaphrodita, regardless the process temperature.Application of CaO for in-situ CO₂ capture in steam gasification of biomass chars resulted in hydrogen content increase at 650 °C to the levels comparable with the ones reached at 800 °C without carbonation reaction. Also hydrogen and total gas yields increased in tests of lime-enhanced gasification.     

Syngas production from chemical looping gasification of rice husk-derived biochar over iron-based oxygen carriers modified by different alkaline earth metals

Oxygen carrier (OC) is a key factor in chemical looping gasification (CLG). Iron oxide is a promising OC due to advantages regarding cost and environment; however, its reactivity with biochar requires improvement to increase syngas production. The novelty of this work is to compare the performance of alkaline earth metals (AEMs: Ca, Ba, and Mg) ferrites and iron oxide OCs for enhancing syngas production from CLG of biochar. The thermogravimetric analyzer and fixed bed system were used to investigate OCs performance. The results demonstrated that the AEMs ferrites have good gasification performance, and BaFe₂O₄ showed highest syngas yield through solid-solid reaction. Furthermore, CLG was conducted under steam addition, and the results revealed that all AEMs ferrites had improved the performance compared to iron oxide, as syngas yield increased by 39.2%, 15.7%, and 13.8% for BaFe₂O₄, CaFe₂O₄, and MgFe₂O₄, respectively. The highest syngas produced by BaFe₂O₄ reached 0.07 mol/g of biochar.     

Experimental study of dual fixed bed biochar-catalytic gasification with simultaneous feed of O2-steam-CO2 for synthesis gas or hydrogen production

The catalytic gasification of biochar was investigated in the presence of a Ni/SiO₂ catalyst in a fixed bed reactor with an O₂-steam-CO₂ gas feed. The effects of operating temperature, catalyst nickel loading and composition of O₂-steam-CO₂ feed gas on biochar carbon conversion and gas products were investigated. The results indicate that the highest biochar carbon conversion could be obtained at approximately 800 °C, whilst the 10% Ni/SiO₂ catalyst was shown to produce the greatest syngas yields. The presence of O₂ in the feed gas can result in slightly more CO in the gas product, whilst a higher steam content leads to more H₂ in the gas product. The CO₂ offered a benefit as an adjusting agent for achieving a desired H₂/CO ratio. No evidence of coke deposition on the catalyst was found under any of the tested conditions.     

Steam catalytic gasification of municipal solid waste for producing tar-free fuel gas

In the paper, a two-region municipal solid waste (MSW) steam catalytic gasification process was proposed. The gasifier was composed of two individual reactors: one is the gasification reactors and the other is the catalytic reactor. The MSW was initially gasified and the produced tar was gasified in the gasification reactor, and further, the tar not gasified entered the catalytic reactor together with the fuel gas and was catalytically decomposed to fuel gas. The influences of the catalysts, steam and temperature on the content of tar, dry gas yield and composition, and carbon conversion efficiency were studied. The results indicated that under the optimum operating conditions, the dry gas yield can be up to 1.97 Nm³/kg MSW and the tar in the product can be completely eliminated. The concentration of hydrogen, carbon monoxide and methane in the fuel gas produced was 50.8%, 9.32% and 13.3%, respectively.     

Steam gasification of biochars derived from pruned apple branch with various pyrolysis temperatures

In this study, the gas production behavior from the steam gasification of the biochar derived from the pruned apple brunch was investigated using a fixed-bed reactor. The optimal biochar obtained at the pyrolysis temperature of 550 °C was gasified under different operating conditions for the hydrogen rich gas production. The experimental results indicated that high reaction temperature and high water flow rate were both beneficial to the hydrogen gas yield, but excess steam had a negative impact contrarily. Besides, the small size particles (0.5–1.0 mm) showed better performance in the hydrogen gas production at the low water flow rates (0.05–0.20 g/min); while the large size particles (1.0–2.8 mm) showed better performance at the high water flow rates (0.25–0.30 g/min). The suitable operating conditions for the gasification of the biochar were determined as the reaction temperature of 850 °C, water flow rate of 0.25 g/min, and particle size of 1.0–2.8 mm.     

Catalytic conversion of Venice lagoon brown marine algae for producing hydrogen-rich gas and valuable biochemical using algal biochar and Ni/SBA-15 catalyst

This study investigated the catalytic behavior of two different types of materials: (i) algal biochar and (ii) 15 wt% Ni impregnated on SBA-15 support (Ni/SBA-15), in the thermochemical decomposition of Venice lagoon brown marine algae (Sargassum). First, non-catalytic pyrolysis tests were conducted in a temperature range of 400–800 °C in a dual-bed slow pyrolysis reactor. The optimum temperature for maximized liquid yield was at the temperature of 700 °C. Biochar catalyst exhibited excellent catalytic activity toward producing aromatic compounds via Diels-Alder-type reactions. However, Ni/SBA-15, because of interconnected pores provided easy passage for reactant and product during the catalytic pyrolysis process and resulted in an improvement in total gas yield (25.87 mmol/g Sargassum) and hydrogen-rich gas production (8.54 mmol/g Sargassum). The catalytic performances of both biochar and Ni/SBA-15 catalysts were compared to biochar-based catalysts derived from red and green macroalgae. High specific surface area, large pore volume, highly ordered pore structure, and narrow pore size distribution make SBA-15 a promising catalyst support in pyrolysis of biomass.     

Experimental study on syngas production by co-gasification of coal and biomass in a fluidized bed

The main results of an experimental work on co-gasification of a Chinese bituminous coal and two types of biomass in a bench-scale fluidized bed are reported in the present study. Experiments were performed at different oxygen equivalence ratio, steam/carbon ratio and biomass/coal ratio. In addition, stabilization of co-gasification process was investigated. It was found that a relatively low oxygen equivalence ratio favors the increase of syngas yield (CO + H₂). There is a maximum value in the curve of syngas yield versus steam/carbon ratio. Moreover, the content of H₂ in gas increases with the increase of biomass ratio while that of CO and syngas yield decrease. A continuous stable operation can be gained.     

From coal towards renewables: Catalytic/synergistic effects during steam co-gasification of switchgrass and coal in a pilot-scale bubbling fluidized bed

Recent environmental sharp curbs on fossil fuel energy systems such as coal power plants due to their greenhouse gas emissions have compelled industries to include renewable fuels. Biomass/coal co-gasification could provide a transition from energy production based on fossil fuels to renewables. A low-ash coal and switchgrass rich in potassium were selected on the basis of previous thermogravimetric studies to steam co-gasify 50:50 wt% coal:switchgrass mixtures in a pilot scale bubbling fluidized bed reactor with silica sand as the bed material at ∼800 and 860 °C and 1 atm. With the switchgrass added to coal, the hydrogen and cold gas efficiencies, gas yield and HHV of the product gas were enhanced remarkably relative to single-fuel gasification. The product gas tar yield also decreased considerably due to decomposition of tar catalyzed by switchgrass alkali and alkaline earth metals. Switchgrass ash therefore can act as inexpensive natural catalysts for steam gasification and assist in operating at lower temperatures without being penalized by an increase in product tar yield. An equilibrium model over-predicted hydrogen and under-predicted methane concentrations. However, an empirically kinetically-modified model was able to predict the product gas compositions accurately.     

Preliminary investigation on hydrogen-rich gas production by co-steam-reforming of biomass and crude glycerin

Hydrogen production from renewable biomass is of great interests. Co-steam-reforming of biomass and crude glycerin with the ratio of 1:1(w/w) was investigated in a fixed-bed gasifier aiming at improving biomass to hydrogen conversion, focusing on the effects of temperature, pressure, H₂O/C ratio and Ca/C ratio on producing gas composition. The results show that high temperature and low pressure favors hydrogen production. With temperature increasing from 650 to 825 °C, H₂ yield shows a linear increase from 0.053 mol/kg to 0.059 mol/kg. Both H₂ yield and its mole fraction increase obviously with the increasing H₂O/C ratio. No influence on gas production is found for Ca/C ratio > 1. The study reveals the optimum condition for producing hydrogen is: temperature of 700-750 °C, pressure of 0.1 MPa, H₂O/C ratio of 1.7-2.25 and Ca/C ratio of 1. Our experimental analysis shows co-steam-reforming of biomass and crude glycerin for hydrogen production is feasible and promising.     

Catalytic hydrothermal co-gasification of canola meal and low-density polyethylene using mixed metal oxides for hydrogen production

Canola meal is a low-value agricultural residue obtained after oil extraction from canola, the utilization of which requires further attention. On the other hand, plastic waste disposal is also another leading issue that creates severe environmental challenges. Supercritical water gasification is considered an environmentally friendly technology to produce hydrogen from plastic residues and organic wastes. This study deals with hydrothermal co-gasification of canola meal and plastic wastes (i.e., low-density polyethylene) while exploring the influence of temperature (375–525°C), residence time (15–60 min) and plastic-to-biomass ratio (0:100, 20:80, 50:50, 80:20 and 100:0) on hydrogen yield. Maximum hydrogen yield (8.1 mmol/g) and total gas yield (17.9 mmol/g) were obtained at optimal temperature and residence time of 525°C and 60 min, respectively. A change in the gas yield with variable plastic-to-biomass ratio showed synergistic effects between both feedstocks. The trend of catalytic performance towards improving hydrogen yield was in the following order: WO₃–TiO₂ (18.5 mmol/g) > KOH (16.9 mmol/g) > TiO₂ (9.5 mmol/g) > ZrO₂ (7.8 mmol/g) > WO₃–ZrO₂ (7.4 mmol/g).     

Catalytic steam gasification of Mengdong coal in the presence of iron ore for hydrogen-rich gas production

Hydrogen-rich gas production from catalytic steam gasification of coal was investigated in the presence of iron ore in a vertical fixed bed reactor. The addition of iron ore significantly promoted the H₂ yields. The effects of operation parameters (upper zone temperature, lower zone temperature, steam concentration, and iron to coal ratio) on the yield of selected gaseous products (H₂ and CO) during catalytic steam gasification, were studied using Taguchi method. The results of signal-to-noise ratio indicated that steam concentration and iron to coal ratio were the most important parameters in determining the yield of H₂ and CO, respectively. Semi-quantification X-ray diffraction analysis of iron ores indicated that increase in steam concentration intensified the oxidization of low valence iron compounds to Fe₃O₄. In addition, formation of Fe₃O₄ was also favored with increasing reaction temperatures (600 °C–900 °C). However, the formation of Fe₃O₄ was inhibited at higher reaction temperature (1000 °C) due to the destruction of porous structures of the iron ore.     

Investigation into kinetic compensation effects for the production of hydrogen-rich gas during gasification of sub-bituminous coal char in varying gas environments

This study has focused on the evolution of kinetic parameters, i.e., E_app and lnA_app and kinetic compensation effects (KCE) of char consumption and CO, CO?, and H? formation during gasification of Collie coal char in varying gas environments (15%H?O–Ar, 8%H?O–Ar, and 0.4%O?+8%H?O–Ar). The instantaneous rate was obtained continuously during experiments by measuring the product gas composition using a quadrupole mass spectrometer. Observed variations of E_app, lnA_app, and the extent of KCE suggest that the decrease in steam partial pressure from 15%H?O to 8%H?O surrounding the coal char particles appears to affect the path of CO? and H? formation from surface water-gas shift (WGS) reaction in 15%H?O–Ar to char-H?O heterogeneous reaction in 8%H?O–Ar, at higher conversion (conversion >0.5). However, H? formation displays strong KCE and increased H? molar flux in 0.4%O?+8%H?O–Ar when compared to gasification of char with steam only (8%H?O–Ar and 15%H?O–Ar). In contrast, the introduction of 0.4%O? to 8%H?O–Ar dominates the char gasification with only consumption of carbon active sites present on the char surface. That resulted in weak KCE for char consumption and decreased CO/CO? (molar flux ratio) compared to the gasification of char in 15%H?O–Ar and 8%H?O–Ar. Furthermore, the results indicate that the residual char obtained from the faster diffusion-controlled char-O? heterogeneous reaction in the 0.4%O?+8%H?O–Ar acts as an activated char precursor for subsequent kinetics-controlled char-H?O heterogeneous reactions. This enhances the migration of H radicals generated from the H?O molecule to the inner char matrix in 0.4%O?+8%H?O–Ar with an increased H? formation rate than gasification of char with steam only.     

The economic value of biochar in crop production and carbon sequestration

This paper estimates the economic value of biochar application on agricultural cropland for carbon sequestration and its soil amendment properties. In particular, we consider the carbon emissions avoided when biochar is applied to agricultural soil, instead of agricultural lime, the amount of carbon sequestered, and the value of carbon offsets, assuming there is an established carbon trading mechanism for biochar soil application. We use winter wheat production in Eastern Whitman County, Washington as a case study, and consider different carbon offset price scenarios and different prices of biochar to estimate a farm profit. Our findings suggest that it may be profitable to apply biochar as a soil amendment under some conditions if the biochar market price is low enough and/or a carbon offset market exists.     

Hydrothermal gasification of microalgae over nickel catalysts for production of hydrogen-rich fuel gas: Effect of zeolite supports

A series of Ni catalysts with different zeolites were prepared by wet impregnation method and used to catalyze supercritical water gasification (SCWG) of microalgae for production of hydrogen-rich fuel gas under conditions of 430 °C, 60 min, ρ_H₂O = 0.162 g/cm³, 2 g/g Ni/zeolites. Compared with noncatalytic SCWG, the presence of Ni/zeolite could increase the hydrogen gasification efficiency and carbon gasification efficiency by promoting water–gas shift and steam reforming reactions which are mainly affected by the amount of strong acid sites and Ni, respectively. The highest carbon gasification efficiency (CGE) and hydrogen gasification efficiency (HGE) of 23.61% and 23.55% were achieved with Ni/HY (Na₂O, 0.8%). The gaseous produced mainly consisted of H₂ and CO₂. The H₂ content in the gaseous products varied from 27.15 to 40.51% depending on the Ni/zeolites and increased with increasing the SiO₂/Al₂O₃ molar ratio of HZSM-5, which is 2.3–3.6 times higher than that of produced without catalyst. The H₂ yield varied between 2.57 and 3.61 mmol/g depending on the Ni/zeolites and increased from 2.19 to 5.61 mmol/g with increasing the SiO₂/Al₂O₃ molar ratio from 50:1 to 170:1, which is 3.6–7.8 times higher than that of produced without catalyst. Coke formation, surface area loss, and sintering of Ni could decrease the activity of the Ni/zeolites.     

Steam reforming of technical bioethanol for hydrogen production

Essentially all work on ethanol steam reforming so far has been carried out using simulated bioethanol feedstocks, which means pure ethanol mixed with water. However, technical bioethanol consists of a lot of different components including sugars, which cannot be easily vaporized and steam reformed. For ethanol steam reforming to be of practical interest, it is important to avoid the energy-intensive purification steps to fuel grade ethanol. Therefore, it is imperative to analyze how technical bioethanol, with the relevant impurities, reacts during the steam reforming process. We show how three different distillation fractions of technical 2nd generation bioethanol, produced in a pilot plant, influence the performance of nickel- and ruthenium-based catalysts during steam reforming, and we discuss what is required to obtain high activity and long catalyst lifetime. We conclude that the use of technical bioethanol will result in a faster catalyst deactivation than what is observed when using pure ethanol–water mixtures because of contaminants remaining in the feed. However, the initial activity of the catalysts are not affected by this, hence it is important to not only focus on catalyst activity but rather on the lifetime of the catalyst.     

Comparative analysis of biomass and coal based co-gasification processes with and without CO2 capture for HT-PEMFCs

With the seasonal availability and low energy density of biomass and the high environmental impact of coal, the co-gasification of biomass and coal is an alternative approach facilitating a trade-off between renewable and non-renewable resources. The aim of this study was to investigate hydrogen production from the co-gasification of biomass and coal integrated by means of the sorption-enhanced water gas shift reactor (G-SEWGS) for a high temperature proton exchange membrane fuel cell (HT-PEMFC). The effects of the gasifier temperature, the steam to fuel ratio (S/F ratio), and the equivalence ratio (ER) on the hydrogen production performance and environmental impact of the G-SEWGS were theoretically analysed and compared with the conventional gasifier integrated with the water gas shift reactor (G-WGS) and the sorption-enhanced gasifier integrated with the water gas shift reactor (SEG-WGS). As compared to the conventional water gas shift reactor, the addition of a CaO sorbent in the modified water gas shift reactor not only reduces the amount of the CO₂ emission but also leads to an increase in the hydrogen concentration and hydrogen content. The G-SEWGS provides better performance in terms of its fuel processor efficiency and CO₂ emission than the G-WGS and the SEG-WGS. Also, the problem of sulphur compound in the hydrogen-rich gas can be reduced by using of the sorption-enhanced water gas shift reactor (SEWGS). The best system exergy efficiency, which was around 22% for the power generation, was determined from the HT-PEMFC integrated with the G-SEWGS. The main exergy destruction of around 70% of the total loss was caused by hydrogen production processes.