Biosyngas for Electricity Generation Using Fuel Cells - A Gas Quality Assessment (2018)

The increase in demand for energy and the issues related to climate change and pollution have necessitated the use of renewable energy and high efficiency energy conversion devices. Decentralized power generation becomes suitable for certain rural areas of India and Sub-Saharan Africa owing to the irregular and inadequate power supply available to these regions. The aim of the present work is to address these issues by adopting a high efficiency energy conversion device coupled with a fuel obtained from a renewable energy source, for stand-alone power generation. Gasification is a thermo-chemical process wherein a solid fuel like biomass or coal undergoes sub-stoichiometric oxidation and reduction reactions in a gasifier reactor with different reactants like air/pure oxygen/CO2/steam or a combination of them, generating a combustible gaseous mixture, having CO, H2, CH4, and CO2. In addition, condensable hydrocarbons such as tar - a complex mixture of aromatics and polycyclic aromatic hydrocarbons (PAH), particulate matter consisting of dust, soot, char and ash, some trace gases like hydrogen sulphide (H2S), hydrochloric acid (HCl), hydrogen cyanide (HCN), ammonia (NH3), etc. and alkali metals - mainly sodium and potassium, are generated. The combustible product gas, called producer gas/syngas can be used for power generation using I. C. engines, fuel cells, or gas turbines, or for synthesizing chemicals or liquid fuels. The present study describes the quantitative and qualitative analyses of tar present in the hydrogen-rich syngas obtained from a fixed bed downdraft oxy-steam biomass gasification system developed at Indian Institute of Science, Bangalore, while operating the system with coconut shells as fuel. The system generates hydrogen-rich syngas of average composition as: H2: ~45%; CO: ~16%; CH4: ~4% and CO2: ~35%. Further purification of the gas could help in generating pure hydrogen for Polymer Electrolyte Membrane Fuel Cell (PEMFC) applications. Solid oxide fuel cell is a fuel flexible, highly efficient energy conversion device with low emissions, which can be coupled with gasification systems for decentralized power generation. Researchers have reported that tar present in the syngas can cause degradation of solid oxide fuel cell anodes, necessitating the assessment of gas quality before using the syngas for solid oxide fuel cell applications. This paper describes the use of a method for sampling of tar which has adopted certain features of the Conventional Cold Solvent Trapping (CST)-Tar protocol, a well-established method. The analysis of tar using Gas Chromatography-Mass Spectrometry/Flame Ionization Detector (GC-MS/FID) and gravimetric analysis are discussed. Gravimetric tar may be defined as the evaporation residue of a part of a bulk solution at standard conditions provided in the Tar Guideline. Internal and external standards are used for the quantification of tar carried out using GC-MS/FID. The major components of the tar are identified using GC-MS while the quantification is carried out with GC-FID, using the reference compounds - naphthalene and phenol. The quantitative results obtained by gravimetric analysis and with GC-MS/FID are compared. Gravimetric tar concentration is about 4 mg. Nm-3 and the estimated value of tar using GC-MS/FID is about 60 mg. Nm-3 in the syngas. From the studies, it is found that the oxy-steam gasification system developed is ideal for clean syngas generation, making it a possible choice for decentralized power generation using solid oxide fuel cell and for chemical synthesis.