Fuels and electricity from biomass with CO2 capture and storage

Mass/energy balances and financial analysis are presented for (1) plants co-producing Fischer- Tropsch diesel and gasoline blendstocks plus electricity from biomass and (2) biomass integrated gasification combined cycle power plants. Plant designs with and without carbon capture and storage are analyzed. The feedstock is switchgrass. For plants with CO2 capture, we assume that the CO2 is stored in deep saline aquifers or used for enhanced oil recovery. Sustainably produced biomass is an essentially carbon-neutral energy source since the CO2 emitted from its use as energy is of recent photosynthetic origin. By capturing and storing below ground some carbon from biomass during its conversion to fuel or electricity, this biomass becomes a negative CO2-emitting energy supply. This paper summarizes and extends detailed analyses (1,2) of mass/energy balances and economics for plants that co-produce Fischer-Tropsch (FT) diesel and gasoline blendstocks plus electricity from biomass, with and without carbon capture and storage (CCS). Stand-alone biomass integrated gasification combined cycle (IGCC) power generation with and without CCS is similarly analyzed. For plants with CCS, overall economics are explored assuming the CO2 is stored in deep saline aquifers or used for enhanced oil recovery (EOR). The feedstock is switchgrass, a perennial grass native to the Great Plains of the USA that is a promising future bioenergy crop (3,4). Methodology We use Aspen Plus software to help design the FT and IGCC plants and calculate mass/energy balances. Some equipment components in our plants are not commercially available today but can be expected to be so in the 2010-2015 timeframe. To help understand the potential for these biomass conversion systems in this timeframe and beyond, our process simulations assume that key achievable technology advances are, in fact, realized. These include feeding of switchgrass to a pressurized fluidized-bed gasifier, reliable high-efficiency oxygen-blown fluidized-bed gasification, complete tar cracking in a separate vessel following gasification, and gas cleanup to specifications for downstream synthesis or gas turbine combustion. We consider a switchgrass feed rate of 5,669 tonnes per day (20% moisture as received), or 4535 dry tonnes per day. This scale of bioenergy conversion is larger than most prior analyses have considered, although biomass processing facilities this size and larger are operating commercially today (e.g., some Brazilian sugarcane mills). When switchgrass is produced as an agricultural crop, average transport distances will be relatively modest for delivering feedstock to conversion facilities of the size we consider here. In earlier work we have shown that up to very large conversion plant sizes, the impact on overall economics of increasing average delivered feedstock costs with increasing plant sizes is more than offset by scale-economy gains in the capital cost of the larger conversion facilities (5). All plants include chopping of the switchgrass, followed by oxygen-blown fluidized-bed gasification, gas cooling and gas cleanup. In our IGCC design with CCS (IGCC-C), CO2 is removed with Rectisol technology following water gas shift (WGS), after which the remaining hydrogen-rich syngas is burned in a gas turbine combined cycle. With CO2 venting (IGCC-V), no