Performance evaluation of process chains for the production of BioH2 from wood gas

Abstract

This work presents an evaluation of process chains to produce renewable high-purity hydrogen (BioH2) by processing wood gas derived from the commercial biomass steam gasification plant in Oberwart, Austria. Prior to this work the process chains had been successfully operated on long term test runs. During these test runs, high purity hydrogen was produced. The scope of this work was to evaluate them in terms of mass balances and energy consumption. For this purpose the process chains were modeled with the process simulation software IPSEpro according to the experimental set-up. The experimental data was implemented in the simulation set-up and characteristic values for each process chain were obtained as result. Process chain 1 used three steps of gas treatment: (1) Gas drying and cleaning in a low temperature gas scrubber using rapeseed oil methyl ester (RME) as solvent, (2) hydrogen enrichment through of a membrane module and (3) hydrogen purification by means of pressure swing adsorption (PSA). A total hydrogen recovery of 39 % "V" -_(-"BioH" -_"2" )/"V" -_("H" _"2" ",WG" ) was achieved, which corresponds to a specific hydrogen production (SHP) of 13.46 ("g" _(-"BioH" -_"2" ) " " )/("k" "g" _"woodchips" ). The compression steps of the membrane unit and the PSA units contributed most to the specific electricity demand of 1.12 ("kW" "h" _"el" )/-"N" "m" "3" -_(-"BioH" -_"2" ) . Gas drying and cleaning required 0.57 ("kW" "h" _"cool" )/-"N" "m" "3" -_(-"BioH" -_"2" ) of cooling power, whereas the heat demand for gas conditioning was of 0.15 ("kW" "h" _"heat" )/-"N" "m" "3" -_(-"BioH" -_"2" ) . For the second process chain a water-gas shift (WGS) unit was implemented before the gas scrubber of process chain 1 in order to gain additional hydrogen. After successful test runs with this configuration the membrane unit was skipped and process chain 3 was set up. Process chain 3 employed the following three steps: (1) catalysis of the WGS reaction, (2) gas drying and cleaning by means of a low temperature RME scrubber and (3) separation of high-purity hydrogen by PSA. The additional hydrogen produced by the WGS unit increased the hydrogen recovery to 129 % "V" -_(-"BioH" -_"2" )/"V" -_("H" _"2" ",WG" ) and the specific hydrogen production to 45.94 ("g" _(-"BioH" -_"2" ) " " )/("k" "g" _"WC" ). The omission of the compressor of the membrane unit decreased the electricity demand to 0.58 ("kW" "h" _"el" )/-"N" "m" "3" -_(-"BioH" -_"2" ) . On the other hand the heat demand for water evaporation in the WGS unit increased to 2.13 ("kW" "h" _"heat" )/-"N" "m" "3" -_(-"BioH" -_"2" ) . The downstream scrubber had to face a gas with higher temperature and water content, which resulted in a cooling demand of 1.71 ("kW" "h" _"cool" )/-"N" "m" "3" -_(-"BioH" -_"2" ) . As Process chain 3 showed possibilities for improvements, especially in terms of waste heat usage, an improved process set-up was simulated using the same units for gas treatment. The improvement measures led to an electricity demand of 0.42 ("kW" "h" _"el" )/-"N" "m" "3" -_(-"BioH" -_"2" ) (-28 %), a heating demand of 0.36 ("kW" "h" _"heat" )/-"N" "m" "3" -_(-"BioH" -_"2" ) (-83 %) and a cooling demand of 1.14 ("kW" "h" _"el" )/-"N" "m" "3" -_(-"BioH" -_"2" ) (-33 %).