Technical, environmental and economic analysis of co-firing of gasified biofuel in a natural gas combined cycle (NGCC) combined heat and power (CHP) plant

The goal of this study is to evaluate co-firing of gasified CO2 neutral biofuel in a retrofitted natural gas combined cycle (NGCC) combined heat and power (CHP) unit designed for natural gas operation. This type of situation may be particularly relevant if future CO2 abatement policies require that owners of natural gas fired CHP units reduce CO2 emissions. The study investigates the technical, economic and environmental performance of the CHP unit for different fractions of gasified biofuel in the unit's fuel mix. The power plant simulation program GateCycle was used for plant performance evaluation. The calculations are based on a large size (about 300 MWe) NGCC CHP plant that is planned to be built on the West coast of Sweden, producing electricity and district heating for the local energy utility company. The results from this study show that it is possible to co-fire up to 40% (energy basis) biofuel product gas in the CHP unit. At this level of product gas co-firing, the unit's electrical and total efficiencies decrease by approximately 2%-points, compared to operation on natural gas only. Global CO2 emissions can be reduced by approximately 400,000 tonnes/year. The total costs for the local utility company depend on the prevailing CO2 taxation rules, and on the assumed premium value of the renewable power produced. For the base case conditions assumed in the study, the cost of electricity (COE) for a natural gas fired NGCC CHP unit is 253 SEK/MWh, increasing to 306 SEK/MWh when a biofuel gasifier is integrated to the unit. A premium value for renewable electricity of 285 SEK/MWh is required for equal COE values to be achieved by the two systems.