Operation of power cycles with integrated CO₂ capture using advanced high-temperature technologies

Abstract

One of the routes for CO2 capture from power plants is to remove carbon content of the fuel before combustion takes place, known as pre-combustion CO2 capture. A typical pre-combustion CO2 capture method consists of two stages of Water Gas Shift (WGS) reactors at two different temperature levels, followed by a CO2 capture unit. The CO2 capture process is usually based on physical absorption at low temperature. A novel pre-combustion CO2 capture technology, so-called Sorption Enhanced Water Gas Shift (SEWGS), combines both the WGS reaction and CO2 capture in one single unit, at elevated temperatures. The equilibrium-controlled WGS reaction is hence enhanced towards higher conversions of CO into CO2. The CO2 is adsorbed simultaneously on the solid adsorbent. This thesis deals with dynamic performance assessment of an Integrated Gasification Combined Cycle (IGCC) power plant, incorporating a SEWGS process for pre-combustion CO2 capture. This is to examine how well the IGCC with SEWGS can perform under load variations. Syngas from a coal gasifier is sent to the SEWGS system after going through solids and H2S removal units. A multi-train SEWGS system treats the feed syngas being produced continuously in a gasifer of the IGCC. Each SEWGS train consists of eight reactors, working in parallel and packed with a mixture of the WGS reaction catalyst and CO2 adsorbent. Each SEWGS reactor undergoes a fixed sequence of processing steps, repeated in a cyclic manner, based on a Pressure Swing Adsorption (PSA) process. The PSA process steps consist of feed, rinse, three pressure equalization, depressurization, purge and repressurization steps. The SEWGS reactors are interacting with each other in all the cycle steps, except during depressurization and purge step. The interconnection between the reactors is carried out using valves. Steam is assumed to be extracted from the steam cycle and used as the rinse and purge gas. A H2-rich stream is produced during the feed step, where the WGS reaction and CO2 adsorption take place simultaneously. Part of the H2-rich being produced during the feed step is used for the repressurization step. A CO2-rich gas is recovered during the depressurization and purge step. The H2-rich product is used as a fuel in a GT within the IGCC power plant. Cyclic operation manner of the SEWGS process means that the system is inherently dynamic and therefore studying dynamic performance of such a system is necessary, particularly when such a process is incorporated into a power plant. Also, the SEWGS system dynamic characteristic at different flow rates of feed syngas is interesting for further investigation of the load-following performance of the IGCC power plant at different GT load levels. A one-dimensional, non-isothermal, homogeneous dynamic model of a PSA-based SEWGS system of multiple dispersed plug-flow reactors has been carried out. Operation schedule of the SEWGS system, including aspects such as transition from one PSA processing step to another for a given reactor and switching of the connections between the reactors using interconnecting valves, is implemented by the modeling approach. The designed SEWGS process gives a CO2 recovery rate of 95%, with around 99% purity of the recovered CO2. The H2-rich product purity achieved is around 81%. The H2-rich stream flow rate produced from a SEWGS train, is found to undergo a periodic fluctuation of around ±33%, due to using part of the H2-rich product stream repeatedly during the re-pressurization step. While, the GT requires a smooth fuel heat input (flow rate, composition) at any given load of operation, it is essential to dampen the H2-rich product flow rate fluctuations as much as necessary. A schedule is developed to initiate operation of the trains with time lags and evaluate its impact on improving the H2-rich fuel fluctuation. Two different scheduled operation schemes are applied and time lags between the operation of trains are optimized. The fluctuations of the H2-rich stream flow rate are decreased from ±33% to ~±14% and ~±11% for the first and second operation scheme, respectively. A closed-loop control strategy including a buffer tank followed by a control valve, before the GT is implemented to further smooth out the fluctuations in the H2-rich fuel flow rate and composition. The control system is also designed to control the H2-rich fuel at full-load and part-load operations of the GT, complying with the fuel flow rate and heating value requirements of a modern GT. Performance simulation of the IGCC integrated with the SEWGS system, incorporating the fuel control strategy is first carried out at full-load operation of the GT. For evaluating part-load performances, four different cases, introducing various load change strategies for the GT and gasifier are studied. Step/ramp changes of the GT and gasifier, unplanned/planned GT load changes and same/different GT and gasifier load change occurrence time are all addressed through these four cases. Simulation results indicate that the designed control strategy functions properly and is able to control the H2-rich fuel as per GT requirements at different part-loads, while keeping the buffer tank pressure within the desired range. Dynamic characteristics of the SEWGS system is revealed from the SEWGS simulations at different feed syngas flow rates and compared with those of the gasifier and GT. Using the buffer tank between the SEWGS and the GT, improves part-load operation flexibility of the GT. Smooth operation and load-following capability of the IGCC integrated with the SEWGS system is achievable, depending on the load change strategy, taking into account the limited load gradient of the SEWGS and gasifier units compared to the GT.