The potential of small-scale SNG production from biomass gasification

Abstract

Synthetic natural gas (SNG) can be produced from biomass by thermochemical gasification and subsequent synthesis gas methanation and gas processing. For an industrial-scale process with high efficiency (up to 74 %; Rönsch et al. in VGB PowerTech 5:110–116, 2008), the large plant size is associated with a number of disadvantages such as a high biomass transportation volume and local environmental impacts. Small distributed SNG production units would minimize these negative aspects but are expected to cause lower efficiency. In order to show the potential of a small-scale SNG solution, different process chain configurations are simulated using Aspen Plus software. Combined heat and power generation via gasification and direct product gas conversion in a gas engine is compared to a SNG route, where the product gas is further cleaned, converted, and upgraded to SNG. Different gasification technologies (co-current fixed bed, countercurrent fixed bed, and dual fluidized bed) are evaluated. The SNG route is based on a dual fluidized bed gasification, subsequent methanation, and injection into the natural gas grid. As an outcome of the simulations, the efficiencies are calculated with special focus on heat integration and utilization. A maximized utilization of the released process heat results in a strong overall efficiency increase. Depending on the local heat utilization, gasification with subsequent methanation has an advantage compared to direct local power generation. The overall efficiency of the SNG option is found to be up to 73.9 %, which is within the range of the fluidized bed gasification option. The crucial factor for high efficiency, and thus for an economic operation, is the heat demand at the location. With even a small constant heat demand, the SNG solution becomes very competitive as some of the heat otherwise generated on-site is translated into chemical energy and carried to a power generation location elsewhere. It has been shown that SNG production subsequent of a small-scale fluidized bed gasifier can very well be efficient in both energetic and economic regards. The most important and crucial parameter is the heat utilization on-site and thus the local heat demand characteristics.