Exergy analysis of biomass-to-synthetic natural gas (SNG) process via indirect gasification of various biomass feedstock

This paper presents an exergy analysis of SNG production via indirect gasification of various biomass feedstock, including virgin (woody) biomass as well as waste biomass (municipal solid waste and sludge). In indirect gasification heat needed for endothermic gasification reactions is produced by burning char in a separate combustion section of the gasifier and subsequently the heat is transferred to the gasification section. The advantages of indirect gasification are no syngas dilution with nitrogen and no external heat source required. The production process involves several process units, including biomass gasification, syngas cooler, cleaning and compression, methanation reactors and SNG conditioning. The process is simulated with a computer model using the flow-sheeting program Aspen Plus. The exergy analysis is performed for various operating conditions such as gasifier pressure, methanation pressure and temperature. The largest internal exergy losses occur in the gasifier followed by methanation and SNG conditioning. It is shown that exergetic efficiency of biomass-to-SNG process for woody biomass is higher than that for waste biomass. The exergetic efficiency for all biomass feedstock increases with gasification pressure, whereas the effects of methanation pressure and temperature are opposite for treated wood and waste biomass.     

Energy and exergy analysis of biomass gasification at different temperatures

Biomass is usually gasified above the optimal temperature at the carbon-boundary point, due to the use of different types of gasifiers, gasifying media, clinkering/slagging of bed material, tar cracking, etc. This paper is focused on air gasification of biomass with different moisture at different gasification temperatures. A chemical equilibrium model is developed and analyses are carried out at pressures of 1 and 10 bar with the typical biomass feed represented by CH_1.4O_0.59N_0.0017. At the temperature range 900–1373 K, the increase of moisture in biomass leads to the decrease of efficiencies for the examined processes. The moisture content of biomass may be designated as “optimal” only if the gasification temperature is equal to the carbon-boundary temperature for biomass with that specific moisture content. Compared with the efficiencies based on chemical energy and exergy, biomass feedstock drying with the product gas sensible heat is less beneficial for the efficiency based on total exergy. The gasification process at a given gasification temperature can be improved by the use of dry biomass and by the carbon-boundary temperature approaching the required temperature with the change of gasification pressure or with the addition of heat in the process.     

Exergy analysis of renewable light olefin production system via biomass gasification and methanol synthesis

The conceptual light olefin production system from biomass via gasification and methanol synthesis was simulated and its thermodynamic performance was evaluated through exergy analysis. The system was made up of gasification, gas composition adjustment, methanol synthesis, light olefin synthesis, steam & power generation and cooling water treatment. The in-depth exergy analysis was performed at the levels of system, subsystem and operation component respectively. The gasifier and the tail gas combustor were the main sources of irreversibility with exergy destruction ratios of 17.0% and 16.8% of the input exergy of biomass. The steam & power generation subsystem accounted for 43.4% of the overall exergy destruction, followed by 41.0% and 5.69% in the subsystems of gasification and gas composition adjustment respectively. The sensitivity evaluation of the operation parameters of gasifier indicates that the system efficiency could be improved by enhancing syngas yield and subsequent yield of light olefins. The overall exergetic efficiency of 30.5% is obtained at the mass ratios of steam to biomass and O₂-rich gas (95 vol%) to biomass (S/B and O/B) of 0.26 and 0.14 and gasification temperature at 725 °C.     

Effect of the type of gasifying agent on gas composition in a bubbling fluidized bed reactor

It is commonly accepted that gasification of coal has a high potential for a more sustainable and clean way of coal utilization. In recent years, research and development in coal gasification areas are mainly focused on the synthetic raw gas production, raw gas cleaning and, utilization of synthesis gas for different areas such as electricity, liquid fuels and chemicals productions within the concept of poly-generation applications. The most important parameter in the design phase of the gasification process is the quality of the synthetic raw gas that depends on various parameters such as gasifier reactor itself, type of gasification agent and operational conditions. In this work, coal gasification has been investigated in a laboratory scale atmospheric pressure bubbling fluidized bed reactor, with a focus on the influence of the gasification agents on the gas composition in the synthesis raw gas. Several tests were performed at continuous coal feeding of several kg/h. Gas quality (contents in H₂, CO, CO₂, CH₄, O₂) was analyzed by using online gas analyzer through experiments. Coal was crushed to a size below 1 mm. It was found that the gas produced through experiments had a maximum energy content of 5.28 MJ/Nm³ at a bed temperature of approximately 800 °C, with the equivalence ratio at 0.23 based on air as a gasification agent for the coal feedstock. Furthermore, with the addition of steam, the yield of hydrogen increases in the synthesis gas with respect to the water–gas shift reaction. It was also found that the gas produced through experiments had a maximum energy content of 9.21 MJ/Nm³ at a bed temperature range of approximately 800–950 °C, with the equivalence ratio at 0.21 based on steam and oxygen mixtures as gasification agents for the coal feedstock. The influence of gasification agents, operational conditions of gasifier, etc. on the quality of synthetic raw gas, gas production efficiency of gasifier and coal conversion ratio are discussed in details.     

Thermodynamic optimization of biomass gasification for decentralized power generation and Fischer–Tropsch synthesis

In recent years, biomass gasification has emerged as a viable option for decentralized power generation, especially in developing countries. Another potential use of producer gas from biomass gasification is in terms of feedstock for Fischer–Tropsch (FT) synthesis – a process for manufacture of synthetic gasoline and diesel. This paper reports optimization of biomass gasification process for these two applications. Using the non–stoichometric equilibrium model (SOLGASMIX), we have assessed the outcome of gasification process for different combinations of operating conditions. Four key parameters have been used for optimization, viz. biomass type (saw dust, rice husk, bamboo dust), air or equivalence ratio (AR = 0, 0.2, 0.4, 0.6, 0.8 and 1), temperature of gasification (T = 400, 500, 600, 700, 800, 900 and 1000 °C), and gasification medium (air, air–steam 10% mole/mole mixture, air–steam 30%mole/mole mixture). Performance of the gasification process has been assessed with four measures, viz. molar content of H₂ and CO in the producer gas, H₂/CO molar ratio, LHV of producer gas and overall efficiency of gasifier. The optimum sets of operating conditions for gasifier for FT synthesis are: AR = 0.2–0.4, Temp = 800–1000 °C, and gasification medium as air. The optimum sets of operating conditions for decentralized power generation are: AR = 0.3–0.4, Temp = 700–800 °C with gasification medium being air. The thermodynamic model and methodology presented in this work also presents a general framework, which could be extended for optimization of biomass gasification for any other application.     

Hydrogen-rich gas production from biomass steam gasification in an updraft fixed-bed gasifier combined with a porous ceramic reformer

This paper investigates the hydrogen-rich gas produced from biomass employing an updraft gasifier with a continuous biomass feeder. A porous ceramic reformer was combined with the gasifier for producer gas reforming. The effects of gasifier temperature, equivalence ratio (ER), steam to biomass ratio (S/B), and porous ceramic reforming on the gas characteristic parameters (composition, density, yield, low heating value, and residence time, etc.) were investigated. The results show that hydrogen-rich syngas with a high calorific value was produced, in the range of 8.10–13.40 MJ/Nm³, and the hydrogen yield was in the range of 45.05–135.40 g H₂/kg biomass. A higher temperature favors the hydrogen production. With the increasing gasifier temperature varying from 800 to 950 °C, the hydrogen yield increased from 74.84 to 135.4 g H₂/kg biomass. The low heating values first increased and then decreased with the increased ER from 0 to 0.3. A steam/biomass ratio of 2.05 was found as the optimum in the all steam gasification runs. The effect of porous ceramic reforming showed the water-soluble tar produced in the porous ceramic reforming, the conversion ratio of total organic carbon (TOC) contents is between 22.61% and 50.23%, and the hydrogen concentration obviously higher than that without porous ceramic reforming.     

Technical assessment of synthetic natural gas （SNG） production from agricul- ture residuals

This paper presents thermodynamic evaluations of the agriculture residual-to-SNG process by thermochemical conversion, which mainly consists of the interconnected fluidized beds, hot gas cleaning, fluidized bed methanation reactor and Selexol absorption unit. The process was modeled using Aspen Plus software. The process performances, i.e., CH 4 content in SNG, higher heating value and yield of SNG, exergy efficiencies with and without heat recovery, unit power consumption, were evaluated firstly. The results indicate that when the other parameters remain unchanged, the steam-to-biomass ratio at carbon boundary point is the optimal value for the process. Improving the preheating temperatures of air and gasifying agent is beneficial for the SNG yield and exergy efficiencies. Due to the effects of CO 2 removal efficiency, there are two optimization objectives for the SNG production process: (I) to maximize CH 4 content in SNG, or (II) to maximize SNG yield. Further, the comparison among different feedstocks indicates that the decreasing order of SNG yield is: corn stalk > wheat straw > rice straw. The evaluation on the potential of agriculture-based SNG shows that the potential annual production of agriculture residual-based SNG could be between 555×10 8～611×10 8 m 3 with utilization of 100% of the available unexplored resources. The agriculture residual-based SNG could play a significant role on solving the big shortfall of China’s natural gas supply in future.     

An experimental study on air gasification of biomass micron fuel (BMF) in a cyclone gasifier

Biomass micron fuel (BMF) produced from feedstock (energy crops, agricultural wastes, forestry residues and so on) through an efficient crushing process is a kind of powdery biomass fuel with particle size of less than 250 μm. Based on the properties of BMF, a cyclone gasifier concept has been considered in our laboratory for biomass gasification. The concept combines and integrates partial oxidation, fast pyrolysis, gasification, and tar cracking, as well as a shift reaction, with the purpose of producing a high quality of gas. In this paper, characteristics of BMF air gasification were studied in the gasifier. Without outer heat energy input, the whole process is supplied with energy produced by partial combustion of BMF in the gasifier using a hypostoichiometric amount of air. The effects of equivalence ratio (ER) and biomass particle size on gasification temperature, gas composition, gas yield, low-heating value (LHV), carbon conversion and gasification efficiency were studied. The results showed that higher ER led to higher gasification temperature and contributed to high H₂-content, but too high ER lowered fuel gas content and degraded fuel gas quality. A smaller particle was more favorable for higher gas yield, LHV, carbon conversion and gasification efficiency. And the BMF air gasification in the cyclone gasifier with the energy self-sufficiency is reliable.     

Exergy analysis of updraft and downdraft fixed bed gasification of village-level solid waste

The most commonly used for gasification of village-level solid waste is the fixed-bed gasifier, but there is no reasonable method to evaluate the gasification process. This paper attempts to find a gasifier that is most suitable for gasification of village-level solid wastes through exergy analysis method. Based on experimental data from literature, the exergy efficiencies and LHV(Low Heat Value) of product gas from updraft and downdraft fixed bed gasifier are studied in this paper. The results show that the updraft fixed bed gasifier has higher exergy efficiency, and the gas produced by the downdraft fixed bed gasifier has a higher heating value. Air gasification has higher exergy efficiency than steam gasification and pure oxygen gasification. The highest exergy efficiency at a gasification temperature of about 1000 °C and ER (Equivalence Ratio) value in the range of 0.33–0.36. The volatile content of gasification raw materials is higher, and the gasification efficiency is higher. Through the research of this paper, a new path to reasonably evaluate the gasification process is obtained.     

Characteristics of hydrogen-rich gas production of biomass gasification with porous ceramic reforming

In the present study, an updraft biomass gasifier combined with a porous ceramic reformer was used to carry out the gasification reforming experiments for hydrogen-rich gas production. The effects of reactor temperature, equivalence ratio (ER) and gasifying agents on the gas yields were investigated. The results indicated that the ratio of CO/CO₂ presented a clear increasing trend, and hydrogen yield increased from 33.17 to 44.26 g H₂/kg biomass with the reactor temperature increase, The H₂ concentration of production gas in oxygen gasification (oxygen as gasifying agent) was much higher than that in air gasification (air as gasifying agent). The ER values at maximum gas yield were found at ER = 0.22 in air gasification and at 0.05 in oxygen gasification, respectively. The hydrogen yields in air and oxygen gasification varied in the range of 25.05–29.58 and 25.68–51.29 g H₂/kg biomass, respectively. Isothermal standard reduced time plots (RTPs) were employed to determine the best-fit kinetic model of large weight biomass air gasification isothermal thermogravimetric, and the relevant kinetic parameters corresponding to the air gasification were evaluated by isothermal kinetic analysis.     

Exergy analysis of hydrogen production plants based on biomass gasification

Biomass gasification is a promising option for the sustainable production of hydrogen rich gas. Five different commercial or pilot scale gasification systems are considered for the design of a hydrogen production plant that generates almost pure hydrogen. For each of the gasification technique models of two different hydrogen production plants are developed in Cycle-Tempo: one plant with low temperature gas cleaning (LTGC) and the other with high temperature gas cleaning (HTGC). The thermal input of all plants is 10 MW of biomass with the same dry composition. An exergy analysis of all processes has been made. The processes are compared on their thermodynamic performance (hydrogen yield and exergy efficiency). Since the heat recovery is not incorporated in the models, two efficiencies are calculated. The first one is calculated for the case that all residual heat can be applied, the case with ideal heat recovery, and the other is calculated for the case without heat recovery. It is expected that in real systems only a part of the residual heat can be used. Therefore, the actual value will be in between these calculated values. It was found that three processes have almost the same performance: The Battelle gasification process with LTGC, the FICFB gasification process with LTGC, and the Blaue Turm gasification process with HTGC. All systems include further processing of the cleaned gas from biomass gasification into almost pure hydrogen. The calculated exergy efficiencies are, respectively, 50.69%, 45.95%, and 50.52% for the systems without heat recovery. The exergy efficiencies of the systems with heat recovery are, respectively, 62.79%, 64.41%, and 66.31%. The calculated hydrogen yields of the three processes do not differ very much. The hydrogen yield of the Battelle LTGC process appeared to be 0.097 kg (kg_{(dry biomass)})⁻¹, for the FICFB LTGC process a yield of 0.096 kg (kg_{(dry biomass)})⁻¹ was found, and for the Blaue Turm HTGC 0.106 kg (kg_{(dry biomass)})⁻¹.     

Exergy analysis of hydrogen production from biomass gasification

For a given set of operating conditions, the hydrogen production from biomass gasification can be improved through optimization of the operating parameters and efficiencies. The present approach can predict hydrogen production via biomass gasification in a range of 10–32 kg/s from biomass (sawdust wood). The biomass is introduced to a gasifier at an operating temperature range of 1000–1500 K. Also, 4.5 kg/s of steam at 500 K is used as gasification medium. Results indicate that improvement in hydrogen production from biomass steam gasification depending on the amount of steam and quantity of biomass feeding to the gasifier as well the operating temperature. Over the range of feeding biomass, the hydrogen yield reaches 80–130 g H₂/kg biomass while in the operating temperature examined, the hydrogen yield reaches 80 g H₂/kg biomass. On mole basis it is found that, in the first range of H₂ varies from 51 to 63% in the studied range of feeding biomass in existing 4.5 kg/s from steam while H₂ gets to 51–53% in existing of 6.3 kg/s from steam.     

The production of synthetic natural gas (SNG): A comparison of three wood gasification systems for energy balance and overall efficiency

The production of Synthetic Natural Gas from biomass (Bio-SNG) by gasification and upgrading of the gas is an attractive option to reduce CO₂ emissions and replace declining fossil natural gas reserves. Production of energy from biomass is approximately CO₂ neutral. Production of Bio-SNG can even be CO₂ negative, since in the final upgrading step, part of the biomass carbon is removed as CO₂, which can be stored. The use of biomass for CO₂ reduction will increase the biomass demand and therefore will increase the price of biomass. Consequently, a high overall efficiency is a prerequisite for any biomass conversion process. Various biomass gasification technologies are suitable to produce SNG. The present article contains an analysis of the Bio-SNG process efficiency that can be obtained using three different gasification technologies and associated gas cleaning and methanation equipment. These technologies are: 1) Entrained Flow, 2) Circulating Fluidized Bed and 3) Allothermal or Indirect gasification. The aim of this work is to identify the gasification route with the highest process efficiency from biomass to SNG and to quantify the differences in overall efficiency. Aspen Plus^® was used as modeling tool. The heat and mass balances are based on experimental data from literature and our own experience.Overall efficiency to SNG is highest for Allothermal gasification. The net overall efficiencies on LHV basis, including electricity consumption and pre-treatment but excluding transport of biomass are 54% for Entrained Flow, 58% for CFB and 67% for Allothermal gasification. Because of the significantly higher efficiency to SNG for the route via Allothermal gasification, ECN is working on the further development of Allothermal gasification. ECN has built and tested a 30 kW_th lab scale gasifier connected to a gas cleaning test rig and methanation unit and presently is building a 0.8 MWth pilot plant, called Milena, which will be connected to the existing pilot scale gas cleaning.     

Small‐scale production of synthetic natural gas by allothermal biomass gasification

The gasification of biomass can be coupled to a downstream methanation process that produces synthetic natural gas (SNG). This enables the distribution of bioenergy in the existing natural gas grid. A process model is developed for the small‐scale production of SNG with the use of the software package Aspen Plus (Aspen Technology, Inc., Burlington, MA, USA). The gasification is based on an indirect gasifier with a thermal input of 500 kW. The gasification system consists of a fluidized bed reformer and a fluidized bed combustor that are interconnected via heat pipes. The subsequent methanation is modeled by a fluidized bed reactor. Different stages of process integration between the endothermic gasification and exothermic combustion and methanation are considered. With increasing process integration, the conversion efficiency from biomass to SNG increases. A conversion efficiency from biomass to SNG of 73.9% on a lower heating value basis is feasible with the best integrated system. The SNG produced in the simulation meets the quality requirements for injection into the natural gas grid. Copyright © 2012 John Wiley & Sons, Ltd.     

Gasification characteristics of coke and mixture with coal in an entrained-flow gasifier

To enhance clean energy utilization and reduce greenhouse gases, various gasification technologies have been developed in the world. The gasification characteristics, such as syngas flow rate, compositions, cold gas efficiency and carbon conversion, of petroleum coke and mixture of petroleum coke and lignite were investigated in a 1 T/d entrained-flow gasifier (I.D. 0.2 m × height 1.7 m) with quencher as a syngas cooler. CO concentration was 31–42 vol% and H₂ concentration was almost 22 vol% in the gasification experiments of petroleum coke. In the case of mixture of petroleum coke and lignite, CO concentration was 37–47 vol% and H₂ concentration was almost 25 vol% due to synergy effect. The gasification of mixture resulted in higher syngas heating value and cold gas efficiency because of the higher H₂ and CO composition in syngas.     

Effect of temperature on the gasification of olive bagasse particles

A semi-batch fluidized-bed gasifier was used to investigate the experimental gasification process of olive bagasse particles, an olive oil industry residue. The effect of bed temperature was studied, in the range of 750 to 900 °C. The oxidant agent was air, fed at constant flowrate, and sand particles were used as bed material. The bagasse particles used had diameters within 1.25–2 mm and the biomass was characterized in terms of its higher heating value and ultimate analysis. During each run, several gaseous samples were collected to be further analysed by gas chromatography allowing the quantification of CO, CO₂, H₂, CH₄, O₂ and N₂. The reaction mechanism of the gasification process is determinant on the composition of the producer gas. Experimental results showed that higher bed temperatures favoured gas production as well as other gasification performance parameters. Best results were obtained for a bed temperature of 850 °C.     

Hydrogen production and CO2 fixation by flue-gas treatment using methane tri-reforming or coke/coal gasification combined with lime carbonation

The production of hydrogen and the fixation of CO₂ can be achieved by treatment of flue gases derived from fossil fuel fired power plants via catalytic methane tri-reforming or by coal gasification in the presence of CaO. A two-step process is designed to be carried out in two reactors: a) a catalytic gasifier or steam-reformer, operating exothermally at 900–1000 K, with inputs of the flue gas, a carbonaceous source, steam and air, as well as CaO from the calciner, and outputs of H₂, and of “spent” CaCO₃ to the calciner; b) a calciner, operating endothermally at 1100–1300 K, with inputs of spent CaCO₃ from the gasifier, make-up fresh CaCO₃, and outputs of CO₂, as well as of CaO, partly recycled to the gasifier and partly processed in a cement plant. Thermochemical equilibrium calculations along with mass/energy balances indicate that for flue-gas treatment by tri-reforming, CO₂ emission avoidance of up to ∼59% and fossil fuel savings of up to ∼75% may be attained when concentrated solar energy is supplied as high-temperature process heat for the calcination step, all relative to conventional H₂ production by coal gasification. If instead fossil fuel would be used to drive the calcination step, the CO₂ emission avoidance and the fuel savings would be only 20% and 67%, respectively. Estimated annual H₂ production from a coal-fired 500 MWe burner by the proposed flue-gas treatment using either CH₄-tri-reforming or coal gasification would amount to 0.7 × 10⁶ or 0.6 × 10⁶ metric tons H₂, respectively. Estimated fossil fuel consumption for H₂ production by tri-reforming or coke gasification would be 149 or 143 GJ fuel/ton H₂.     

Catalytic coal gasification for SNG manufacture

Exxon Research and Engineering Company is engaged in research and development on catalytic coal gasification (CCG) for the production of substitute natural gas (SNG). the catalysts being studied are the basic and weak acid salts of potassium. the use of a gasification catalyst allows the gasifier temperature to be reduced, reduces the tendency for swelling and agglomeration of caking coals and promotes gas phase methanation equilibrium. These features of the catalyst are utilized in a novel processing sequence which involves separation of product gas into methane (SNG) and a CO/H₂ stream which is recycled to the gasifier. the predevelopment phase of research on this process concept was completed in early 1978 and included bench-scale research on catalyst recovery and kinetics, the operation of a 6 in diameter × 30 ft long fluid bed gasifier and supporting engineering studies. As part of the engineering programme, a conceptual design has been developed for a pioneer commercial CCG plant producing SNG from Illinois No. 6 bituminous coal. the paper reviews the status of research and development on the CCG programme and describes the conceptual design and economics for the commercial scale CCG plant.     

Syngas production from catalytic gasification of waste polyethylene: Influence of temperature on gas yield and composition

The catalytic steam gasification of waste polyethylene (PE) from municipal solid waste (MSW) to produce syngas (H₂ + CO) with NiO/γ-Al₂O₃ as catalyst in a bench-scale downstream fixed bed reactor was investigated. The influence of the reactor temperature on the gas yield, gas composition, steam decomposition, low heating value (LHV), cold gas efficiency and carbon conversion efficiency was investigated at the temperature range of 700–900 °C, with a steam to waste polyethylene ratio of 1.33. Over the ranges of experimental conditions examined, NiO/γ-Al₂O₃ catalyst revealed better catalytic performance as a view of increasing product gas yield and of decreasing char and liquid yields in the presence of steam. Higher temperature resulted in more H₂ and CO production, higher carbon conversion efficiency and product gas yield. The highest syngas (H₂ + CO) content of 64.35 mol%, the highest H₂ content of 36.98 mol%, and the highest CO content of 27.37 mol%, were achieved at the highest temperature level of 900 °C. Syngas produced with a H₂/CO molar ratio in the range of 0.83–1.35, was highly desirable as feedstock for Fischer–Tropsch synthesis for the production of transportation fuels.     

Enhanced coal gasification heated by unmixed combustion integrated with an hybrid system of SOFC/GT

For clean utilization of coal, enhanced gasification by in situ CO₂ capture has the advantage that hydrogen production efficiency is increased while no energy is required for CO₂ separation. The unmixed fuel process uses a sorbent material as CO₂ carrier and consists of three coupled reactors: a coal gasifier where CO₂ is captured generating a H₂-rich gas that can be utilized in fuel cells, a sorbent regenerator where CO₂ is released by sorbent calcination and it is ready for capture and a reactor to oxidize the oxygen transfer material which produces a high temperature/pressure vitiated air. This technology has the potential to eliminate the need for the air separation unit using an oxygen transfer material. Reactors' temperatures range from 750 °C to 1550 °C and the process operates at pressure around 7.0 bar. This paper presents a global thermodynamic model of the fuel processing concept for hydrogen production and CO₂ capture combined with fuel and residual heat usage. Hydrogen is directly fed to a solid oxide fuel cell and exhaust streams are used in a gas turbine expander and in a heat recovery steam generator. This paper analyzes the influence of steam to carbon ratio in gasifier and regeneration reactor, pressure of the system, temperature for oxygen transfer material oxidation, purge percentage in calciner, average sorbent activity and oxidant utilization in fuel cell. Electrical efficiency up to 73% is reached under optimal conditions and CO₂ capture efficiencies near 96% ensure a good performance for GHG's climate change mitigation targets.