Improving heat capture for power generation in coal gasification plants

Abstract

Improving the steam cycle design to maximize power generation is demonstrated using pinch analysis targeting techniques. Previous work models the steam pressure level in composite curves based on its saturation temperature alone. The present work examines the effect of including both sensible and latent heating of steam in the composite curve. It is shown that including sensible heating allows for better thermal matching between the process and steam system which results in improving the overall efficiency while minimizing the capital cost. Additionally, fixed steam headers, such as assumed in total site analysis, give no allowance for reheating before turbine expansion, which can be valuable to consider when optimizing the steam system for certain plant configurations. A case study using an integrated gasification combined cycle (IGCC) plant with carbon capture and sequestration (CCS) is analyzed to assess changes in steam cycle design on the plant efficiency and cost. In addition to improving the steam system within an IGCC plant to improve efficiency, losses within the radiant heat exchanger can also be reduced. Instead of using high temperature syngas, cooling from 1300°C to 760°C, to boil steam at 330°C, another heat transfer fluid can be used and heated to higher temperatures. Material constraints restrict the maximum allowable temperature of the heat transfer fluid. To maintain high heat transfer coefficients in the heat transfer fluid, a fluid with high thermal conductivity, such as a liquid metal, can be used and heated to high temperatures (~700°C). Liquid metals can then act as an intermediate heat transfer medium, absorbing heat from high temperature syngas and rejecting it to steam at temperatures in excess of 500°C. The use of liquid metals leads to a 0.75 point increase in plant efficiency. Gases, such as carbon dioxide and helium, are also considered as potential heat transfer fluids in the radiant heat exchanger. These gases can be at equal pressure to the syngas pressure in the radiant heat exchanger, reducing the tensile stress in tube walls, but their low thermal conductivities still necessitate high strength materials at high temperature. A Brayton power cycle with recuperation is considered in this work, absorbing heat from the hot syngas and rejecting it to steam. Over a range of different Brayton cycle pressure ratios and maximum temperatures, no improvement in plant efficiency was found with respect to the case where steam is boiled in the same sized heat exchanger.