The Brayton cycle based supercritical CO2 (sCO2) power plant is an emerging technology with benefits such as; higher cycle efficiency, smaller component sizes, reduced plant footprint, lower water usage, etc. There exists a high potential for its applicability in waste heat recovery cycles, either as bottoming cycles for gas turbines in a combined cycle or for industrial waste heat recovery in process industries such as iron & steel, cement, paper, glass, textile, fertilizer and food manufacturing. Conventionally steam Rankine cycle is employed for the gas turbine and industrial waste heat recovery applications. The waste heat recovery from a coke oven plant in an iron & steel industry is considered in this paper due to the high temperature of the waste heat and the technological expertise that exists in the author’s company, which has supplied over 50 steam turbines/ power blocks across India for various steel plants. An effective comparison between steam Rankine cycle and sCO2 Brayton cycle is attempted with the vast experience of steam power block technology and extending the high pressure-high temperature steam turbine design practices to the sCO2 turbine while also introducing the design of sCO2 compressor. The paper begins with an analysis of sCO2 cycles, their configurations for waste heat recovery and its comparison to a working steam cycle producing 15 MW net power in a coke oven plant. The sCO2 turbomachinery design follows from the boundary conditions imposed by the cycle and iterated with the cycle analysis for design point convergence. The design of waste heat recovery heat exchanger and other heat exchangers of the sCO2 cycle are not in the scope of this analysis. The design emphasis is on the sCO2 compressor and turbine that make up the power block. This paper highlights the design of a sCO2 compressor and turbine beginning from the specific speed-specific diameter (Ns-Ds) charts, followed by the meanline design. Subsequently, a detailed performance map is generated.
The relevance of this paper is underscored by the first of a kind design and comparative analysis of a Brayton sCO2 power block with a working Steam Power block for the waste heat recovery in the energy intensive iron and steel industry.