Integrated oxygen-free gasification-gas turbine power concept: A low-emissions alternative for small-scale coal plants

A power plant concept with the potential to replace the large fleet of ageing, small-scale (< 50 MW_e), inefficient, polluting, and soon-to-be obsolete coal-fired power plants is proposed. The proposed plant comprises a bituminous coal-fed oxygen-free gasifier, low-temperature syngas cleanup system and an open-cycle gas turbine with heat recovery. Heat is supplied to the gasifier through combustion of a portion of the cleaned syngas produced by it. Since the proposed plant employs only a gas turbine, with no steam bottoming cycle, heat recovery from the gas turbine and its integration with the rest of the plant is crucial. A thermodynamic model of the plant has been created to assess its feasibility based on overall efficiency and emissions of CO₂, SO₂, mercury and particulates. The model comprises submodels for feedstock composition and enthalpy, as well as first-order thermodynamic models for each of the plant components including the gasifier, feedstock preparation, heat exchangers and steam generators, contaminant removal, combustors and turbomachinery. The results of the analysis show base case plant thermal efficiency of 38.2% on a HHV basis, which is roughly 5% points higher than that for a similarly-sized pulverized coal combustion (PCC) plants. Emissions of CO₂, SO₂, mercury and particulates per unit electrical energy produced in the base case are: 0.774 kg/kWh, 47.2 g/MWh, 2.37 g/GWh and 28.2 g/MWh, respectively. These values are well below emissions from similarly-sized PCC plants, which have been assessed using a spreadsheet model. The model of the proposed plant has been used to assess overall performance when torrefied pine wood is co-gasified with coal. Results show a slight decrease in plant efficiency with increasing co-gasification, with large decreases in CO₂, SO₂ and mercury emissions. Emissions of particulates increase slightly with co-gasification. Finally the model has been used to perform sensitivity analysis on the proposed system. Sensitivity analysis highlights the need for greater understanding of gasifier performance under a range of conditions.