Polygeneration of Fischer-Tropsch fuels and electricity by hybridised solar gasification of coal - a pseudo-dynamic process model

Abstract

Presented is a comparative energetic and environmental performance analysis of a solar hybridised gasification, coal to liquids polygeneration system and a non-solar reference. Using AspenPlus and HYSYS software, the reference system was configured, assuming the integration of a pressurised, Shell entrained flow gasifier with a Fischer Tropsch liquids (FTL) polygeneration facility. The hybrid plant assumes the feasibility of integrating an atmospheric, continuously operational, directly irradiated, oxygen blown hybrid solar reactor with this polygeneration facility. To mitigate the diurnal and stochastic impacts of the solar-boosted production of syngas, the hybrid polygeneration model was also assumed to be configured with a pressurised syngas storage plant. Here, the dynamic operation of the polygeneration system was modeled using a pseudo-steady state approximation for two, six day time-series of validated solar insolation model data. The two time-series were selected to represent a period in 'summer' characterised by a steady, low cloud period of solar insolation, and a period in 'winter' marked by intermittent solar insolation and high cloud. Using these data, a MATLAB model was used to predict the maximum steady rate of liquids that could be produced for the given solar insolation scenario. For the summer time series, the hybrid solar gasification system was shown to improve the steady rate of liquids production by 32% and decrease the source-to-wheel (STW) GHG emissions per GJ FTL by 26%, relative to the reference. For the winter time-series, only a marginal improvement in liquids production was predicted .This result followed from the assumed difference in gasification temperatures between the hybrid system and the reference. On a total energetic output basis, GHG emissions were shown to decrease relative to the reference by 17-21 percentage points for the summer-time series, but increase by 18 percentage points for the winter time-series. This increase in GHG emissions follows the large parasitic impact of SG compression on the hybrid system's net electrical output. Additional sensitivity analyses identified the potentially significant energetic and environmental value of either operating the hybrid gasifier at just 2 bar-g instead of 1 bar-g or incorporating an O2 storage system to reduce the parasitic load of the hybrid system's ASU. Importantly, the present analysis found the difference in the steady output from the FT reactor and electricity generating plant between the summer and winter time-series to be within the feasible operational limits of the respective plant components. This provides assurance as to the feasibility of developing a solar hybridised C2L polygeneration facility using commercially available plant components.