Enhancing the overall efficiency of a lignite-fired oxyfuel power plant with CFB boiler and membrane-based air separation unit

The power plant analyzed in this paper consists of the following systems: a steam turbine, a supercritical OXY-type circulating fluidized bed boiler fed with lignite characterized by a high moisture content (42.5%), and an air separation unit based on a 3-end type high temperature membrane and CO ₂ compression installation. The steam turbine gross power is equal to 600 MW, and both the live and reheated steam parameters are equal to 600 C/29 MPa and 620 C/5 MPa, respectively. With the assumed constant gross power of the analyzed plant, the thermal efficiency of the boiler and the power requirements of the equipment used in the above mentioned installations were calculated. These values and also the net efficiency of the analyzed plant were determined as a function of the oxygen recovery rate in the membrane (R). The net efficiency is lower by 7.26 percentage points in comparison with the reference system. The basic method to reduce the loss of net efficiency is to introduce an integrated system, both with a boiler and an ASU installation lignite drying system. This allowed for the reduction of the loss of net efficiency of up to 3.9 percentage points for the lignite dried to w = 20% and 3.3 percentage points at w = 10%. Increase in the intensity of the drying of the lignite not only causes an increase in the maximum system efficiency but also reduces the required membrane surface. A further reduction in the loss of efficiency is sought in the thermal integration of all installations with a steam turbine. This procedure which may improve efficiency by approximately 0.4 percentage point is intended to allow the closure of extractions in the steam turbine and an increase in turbine power.