CO2 Capture and Power Production Integration; Optimization and Conceptual Design Studies

Abstract

The main contribution of this thesis is conceptual design consideration to improve the CO2 capture in absorption/stripping process. In the first stage the conventional configuration of capture plant is optimized. The important parameters for optimization are absorber and stripper height, rich and lean loading, stripper pressure and temperature.

The intercooling studies showed that intercooling can be an effective strategy to reduce the energy requirement of the capture plant. The theoretical best place for intercooling is defined about 1/4th of the column height from the bottom. Intercooling studies are done for two solvents, MEA and DEA. The effects of different parameters like lean loading, amine concentration, cooling temperature, lean amine temperature are investigated in chapter 2.

In chapter 3 the effect of heat distribution or inter-heating on the total energy requirement for CO2 stripping is investigated. The results show that inter-heating can have both negative and positive effects on the total energy requirement. If only one heat source at constant temperature exist, the inter-heating will increase the total energy requirement. If there are other heat sources at different temperatures, inter-heating is maybe beneficial. It depends on the energy price of the different heat sources and the temperature profile of the column. If some energy utilizes from hot process streams in the inter-heater, it can reduce the energy requirement. As a case study, utilizing heat from lean amine in the inter-heater is investigated. The simulation results show a saving up to 6.4 and 11.3 percent by one and two inter-heater respectively.

In chapter 4 a techno-economic study is given for five different configurations for post combustion CO2 capture of a flue gas with about 12 percent CO2 on wet basis, produced by a 150 MW coal power plant. These are the conventional configuration as a benchmark, splitstream, multi-pressure stripper, vapor recompression and compressor integration. Among these, the vapor recompression is the best configuration because it had the lowest total capture cost and CO2 avoided cost. The split-stream configuration with cooling of semi-lean amine is the second best.

In chapter 5, a self-optimizing concept control structure is designed for a post-combustion CO2 capturing plant. The focus of this work is to control of the plant with the aim of staying close to the optimal operating conditions. The cost function to be minimized is the energy demand of the plant. Firstly we define the degree of freedom and next we look for selfoptimizing variables for the remaining unconstrained variables. We found the temperature close to the top (tray no. 4) of the stripper to be a good CV for the remaining unconstrained degree of freedom. To validate the proposed structure, dynamic simulation is done and performance of the control structure is tested.

The dynamic behavior of the split-stream and the vapor recompression configurations are investigated, and the plant responses are compared with the conventional configuration as a benchmark. All process configurations are operationally feasible with the proposed control configuration. From these case studies we could conclude that the conventional configuration has the best dynamic behavior and is the most stable one. The vapor recompression configuration can handle disturbances better than the split-stream configuration.

Finally, a natural gas combined cycle (NGCC) power plant integrated to a chemical solvent absorber/stripping capture plant is investigated. The power plant capacity is about 430 MW. The capture ratio is 90% and the captured CO2 compressed to 110 bar. The study is done at both full load and part load conditions. CO2 capture causes 7.5 % reduction in the power plant net efficiency. 4.6 % of energy penalty is related to the steam extraction and 2.9% is related to electricity consumption. The integration of the power plant with the compression section of the capture plant for preheating the water to the HRSG cannot improve the net plant efficiency. In the part load investigation, results showed the net efficiency of the variable IGVs gas turbine is about 6.2% higher than the efficiency of the constant IGVs gas turbine at 50% load. Also the power plant with the throttled valve configuration for steam extraction has a better performance than the sliding configuration steam extraction.