SNG from biomass - Investigations of raw-SNG production from biogenic residues via aDFB gasification and fluidized bed methanation

Abstract

Heat generation accounts for a significant proportion of total energy consumption. Currently, this demand is largely covered by the combustion of fossil fuels. [1] Especially in hard to abate sectors, e.g., for provision of high-temperature heat in industrial processes, possible alternatives are under investigation. Suitable paths include the production of renewable gases by biomass-to-gas and power-to-gas technologies. However, every potential technology comes with certain challenges, e.g., the issues of gas transportation and storage, or electricity consumption. In the BioHEAT project, heat generation for industrial purposes from biogenic residues, so-called opportunity fuels, was investigated. Two pathways for heat generation were investigated: 1) by direct combustion of the product gas (PG) from advanced dual fluidized bed (aDFB) gasification, or 2) by the combustion of bioSNG (substitute natural gas) synthesised from this PG. For this purpose, two promising opportunity fuels were selected based on the availability in Austria and Poland, and the suitability for aDFB gasification: waste wood (WW) and corn cobs (CC). These opportunity fuels were then used for experimental investigation of raw-SNG production in pilot scale at TU Wien. A full process chain consisting of: 1) a 100kW aDFB gasifier, 2) gas cooling and cleaning units, and 3) a 10 kW fluidized bed methanation (FLBM) test rig was operated. During this full chain operation and subsequent stand-alone operation of the FLBM, an extensive parameter variation was conducted for optimization of methanation conditions and efficiencies. These parameter variations included variations of, e.g., methanation temperature or gas conditioning via water addition. Additionally, potential sector coupling with H2 production and its effect on the FLBM process was investigated. For further considerations, the experimental results of both aDFB gasification and FLBM in pilot scale were validated via process simulation in IPSEpro. Based on process efficiencies calculated by these simulations, the two possible pathways for providing heat are then further evaluated. The experimental results show that aDFB gasification with WW and CC is technically feasible, as steady operation was ensured for several hours. Since the elemental analysis of these residues is quite similar to typically used wood, the obtained PG compositions are in a similar range too. For WW, a PG composition of approximately 40/21/23/11/2/2 v.%db (H2/CO/CO2/CH4/C2H4/N2) was investigated, and approximately 42/20/25/9/2/2 v.%db (H2/CO/CO2/CH4/C2H4/N2) for PG from CC. However, increased effort for gas cleaning is necessary compared to aDFB gasification with softwood as especially NH3 concentrations were significantly higher. Downstream, the PG was successfully used for methanation in the FLBM pilot plant. At atmospheric pressure, a methanation temperature of 340°C and quite low amounts of water addition (≈ 15 v.%), raw-SNG with a composition of approx. 42/45/9/0.6/4 v.% (CH4/CO2/H2/CO/N2) was produced with PG from WW gasification. When methanation with H2 addition was investigated, the raw-SNG composition was improved to 54/14/27/0.5/5 v.%. This corresponds to an increase in carbon efficiency for FLBM from approx. 48 to 81% according to results from process simulation. Similar results were obtained for methanation with PG from CC. Nevertheless, neither of these raw-SNG compositions are in accordance with guidelines for feed-in into the gas grid in Austria and Poland. Hence, SNG upgrading must be performed. This upgrading was not investigated experimentally, but a suitable process chain will be presented.