Development of 2nd Generation Oxyfuel CFB Technology – Small Scale Combustion Experiments and Model Development Under High Oxygen Concentrations

Abstract

The 1st generation oxyfuel CFB (Circulating Fluidised Bed) technology has been demonstrated up to 30MWth scale and the commercial concept for 300 MWe air/oxy flexible CFB power plant is available. Currently, the development of 2nd generation oxyfuel CFB technology is ongoing aiming to reduce significantly – around 50% – the overall efficiency penalty of CO2 capture in power plants compared to 1st generation concepts. The 2nd generation oxyfuel CFB plants are designed only to oxyfuel operation with CCS. The experimental results of the test campaigns with 0.1 MWth pilot scale CFB unit and laboratory scale BFB (Bubbling Fluidised Bed) unit under high oxygen concentrations of feed gas are presented. The pilot scale tests were carried out with Spanish anthracite and petroleum coke mixture and with Polish bituminous coal. Two Spanish limestone types were used for in- furnace sulphur capture. Test matrix of pilot scale CFB unit contained 11 test balances with varying feed gas O2 concentrations (between 21…42 vol-%) and O2 staging to primary and secondary gas feeds (primary gas O2 share 50…80 vol-%) at different bed temperature levels (820…920 °C). Combustion performance and emission formation was studied at air combustion and varying oxyfuel combustion conditions. The fuel impulse tests with laboratory scale BFB included 16 tests with Spanish anthracite and Polish bituminous coal in varying feed gas O2 concentrations (5…50 vol-%) with two fuel size fractions (0.5-2.0 mm and 4.0-8.0 mm) at constant bed temperature level (850 °C). The main objective was study how high O2 concentration effects on char reactivity and formation of nitrogen oxide emissions. A one dimensional model for laboratory scale BFB was used to further develop existing sub-models’ descriptions and parameters in the one dimensional model (1D-model) for pilot scale CFB in order to improve modelling capabilities in oxygen combustion conditions. Firstly the sub-models’ equations for different phenomenon – e.g. pyrolysis, char combustion and reactions of nitrogen species – were implemented and validated with bench scale experimental data. Secondly, the sub-model parameters found in bench scale modelling for char reactivity and NOx formation were implemented to the 1D-model of pilot scale CFB combustor. According to the results of the validation modelling against pilot scale experimental results, the combustion was successfully scaled up as the modelled temperature profiles and flue gas oxygen contents were well in line with measurements. Anyhow, further validation of the combustion model is needed by modelling of dynamic responses of pilot scale experiments. The pilot scale 1D-model was not able to predict NOx formation with the sub-model adapted from the bench scale model. Further model development and experimental work are needed with different particle size fractions at different operating conditions. The improved modelling capabilities under high oxygen concentrations can be utilized in the development and designing of 2nd generation oxyfuel CFB power plant concepts. Generally, the CFB technology appears to be ideally suited to oxyfuel combustion. The flexibility of the fluid-bed process offers an outstanding benefit for CFB in retaining uniform furnace temperature profiles, metal temperatures and local heat production rates in air-firing as well as in oxygen-firing and under varying load conditions. These benefits make also possible to apply higher oxygen concentrations which is a key element in the 2nd generation high efficiency oxyfuel CFB concepts.