CONVERSION EFFICIENCY OF BIOGAS TO LIQUIDS FUELS THROUGH FISCHER-TROPSCH PROCESS M. T. Ashraf, J.R. Bastidas-Oyanedel, J. E. Schmidt Institute Center for Energy – iEnergy, Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emirates
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ABSTRACT: Demand for non-fossil liquid transport fuels is increasing and biogas to liquid fuels conversion is one possible way to produce liquid fuels. Conversion of natural gas to liquid fuels by Fischer-Tropsch synthesis (FTsynthesis) is an established process applied at industrial scale. Biogas plants have relatively lower throughput and methane concentration compared to natural gas. Conversion efficiency of carbon in methane to liquid fuel is an important parameter to benchmark conversion technologies. A biogas to liquid fuel conversion process using pressurized water scrubbing, dry methane reforming, and FT-synthesis is proposed. The process parameters are selected by optimization and sensitivity analysis. The process is simulated to calculate the carbon conversion and energy efficiency. The proposed process has a carbon conversion efficiency of 45% and energy efficiency of 30%. For the base case of 10,000 Nm3/h of dry biogas the process requires 7.08 MW of power in addition to 35 and 185 GJ/h of heating and cooling duties, respectively. From 4000 kg/h of methane in the biogas feed 1602 kg/h of FT crude is produced. Keyword: biogas, Fischer-Tropsch, liquid biofuel, reforming, clean synthesis gas, conversion
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INTRODUCTION
Our global industrialized society development has been based on fossil fuels, 66% of total energy consumption in 2012 came from fossil fuels [1]. Fossil fuels use results in increase of greenhouse gases (GHG) emission, with detrimental effects to our environment [2], [3]. Total global energy consumption has increased from 4,671 Mtoe (Millions of tonnes of oil equivalent) in 1973 to 8,980 Mtoe in 2012 [1]. In 2012, refined oil products, e.g. liquid fuels, accounted for 40.5 % of the total energy consumption, while bioenergy (biofuels and waste-toenergy) represented 12.4% [1]. In this environmental context, biofuels have the potential to mitigate GHG emissions [4]. Biofuels availability, to contribute meeting the projected fuel demands, is a global priority for a sustainable social and economic development, reducing GHG emissions. Biogas is produced by a biological process, anaerobic digestion, that uses organic matter, e.g. agro-residues, food wastes, animal manure, organic fraction of municipal solid waste and industrial wastes [5]–[7]. Table I shows the typical biogas composition. Biogas production advantages not only rely on the environmental viewpoint, reducing GHG emissions, but also on the robustness of the technology, its capacity to use diverse raw materials/applications, it can be use in remote locations [5], and its financial aspects [6]. For markets where there is a demand for sustainable liquid biofuels, biogas can be converted to liquid fuels using FT-synthesis. One processing route can be of following steps: 1) biogas cleaning to remove impurities 2) biogas reforming to produce synthesis gas (syngas) 3) syngas upgrading to remove CO2 4) Fischer-Tropsch synthesis (FT-synthesis) for liquid fuel production. The energy required for reforming can be provided by biomethane oxidation. The objective of this study is to evaluate feasibility of FT-synthesis process, which is conventionally used for natural gas to liquid fuel conversion, for relatively small scale biogas to liquid fuels conversion (10,000 Nm3/h dry biogas). A process topology is developed from the available technologies based on literature survey, and thermodynamic and first principles analysis. Carbon
conversion efficiency, heat and power requirement of the process are calculated by process simulation in Aspen Plus. The process is modeled for the base case of 10,000 Nm3/h of dry biogas with the nominal composition shown in Table I. Table I: Composition of biogas, adapted from [8] and [9]
Component CH4 CO2 N2 O2 H2 H2S NH3 Total Chlorine Siloxane
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Units vol% vol% vol% vol% vol% ppm ppm mg/Nm3 µg/g-dry
AD Biogas 53-70 30-50 2-6 0-5 NA 0-2000
