Techno-economic study on bio-SNG and hydrogen production and recent advances in high temperature gas cleaning

The share of gasification and gas clean-up equipment for a plant producing synthetic fuels from biomass is in the range of 50-55 % of the total capital investment cost. Due to the stringent purity requirements, set by the downstream synthesis island, the gas clean-up needs to be carried out in several steps. The most important of these are a) high-temperature gas filtration b) reforming of hydrocarbon gases and tars in order to increase the yield of CO and H2, c) shift conversion to adjust the H2-CO ratio of syngas to meet the stoichiometric requirements of the downstream synthesis and d) gas cooling with effective heat integration and waste heat utilisation. Gas filtration is a key step in product gas cleaning. Effective filtration can be achieved with ceramic or metallic filter elements or by lower cost fibrous ceramic elements. These high temperature filter materials and filtration systems have been studied by VTT for cleaning of medium/high tar loaded gasification gas in various process applications. Tar content of product gas defines the temperature window in which the filter can be operated. In practice, product gas derived from fluidised-bed gasification of biomass is filtered at 500 - 600 °C, but research is on-going to achieve higher and thus more economical operation temperatures. Catalytic treatment, or reforming, of the gas offers a simple and economical way to solve gas clean-up problems related to tars and light hydrocarbons . Nickel catalysts are active in decomposition of these impurities but they are easily poisoned by sulphur compounds at temperatures below 900 °C and are easily deactivated due to coke deposits. Alternative tar decomposition catalysts are zirconia based and precious metal catalysts. Both are less prone to coking than nickel and can be operated at lower temperatures. The suitability of these catalysts was studied in various alternative configurations. The main finding is that optimal operation can be achieved by using a staged reformer so that zirconia based catalysts are used as pre-reformer layer before the nickel catalyst stage. The performance of the reformer can further be improved by using precious metal catalysts as a one layer in the staged reformer. Optimal operation of the reformer can be achieved by gradually increasing temperature in subsequent stages from 600 up to 1000 °C. The most important limitations set by the catalysts has also been studied and identified. The techno-economic feasibility of plants producing SNG or hydrogen was studied using Aspen Plus simulation. The effect of different catalytic reforming options to the thermodynamic efficiencies and production costs was examined and results will be presented. Detailed techno-economic assessments have indicated that in the case of plant processing 100 MWth of biomass, on the assumption of mature technology: 1) Gasification based technology for the manufacture of SNG has a levelised production cost of around 60 -70 /MWh. 2) Gasification based technology for the manufacture of hydrogen has a levelised production cost of around 50-60 /MWh. 3) Around 5 - 8 /MWh improvement in the production cost of SNG or hydrogen can be achieved by tailoring the right reforming process for a given product. Highest overall efficiencies from biomass to saleable energy products (of the order of 75 - 80 %) can be achieved when SNG or hydrogen are co-produced with district heat or process steam.