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Dynamics of CO2 Capture Integrated in a Steel Mill

Dynamics of CO2 Capture Integrated in a Steel Mill

Guillermo Juan Martinez Castilla
Göteborg : Chalmers tekniska högskola, 2018. 86 s.
[Examensarbete på avancerad nivå]

Anthropogenic CO2 emissions are recognized to be the main factor contributing to climate change. Steel production accounts for 10% of the total CO2 emissions from fossil fuels worldwide, with especially high levels of emissions coming from the blast furnace processing due to the use of coke and coal as reducing agent and energy supply. Ongoing research and policy plans are trying to reduce such a large carbon footprint, with Carbon Capture and Storage (CCS) in the forefront of the possible solutions. Steel mills inherently operate with a surplus of energy, as the required reducing potential in the blast furnace generates an off-gas with a high heating value. This energy is often utilized in an integrated combined heat and power (CHP) plant, providing the energy required for the steel processing, power for the grid, and heat to local district heating networks. Due to the variations in the steel-making process and the integrated the CHP plant, a dynamic analysis is required for a successful implementation of CO2 capture and to evaluate the plant behavior, potential and controllability. This work analyzes the hourly transient events of an integrated steel mill during an entire year, performing its analysis and discretization while creating two scenarios: variations in available heat for the capture unit, and variations in the blast furnace gas flow to be treated in the capture unit. In addition, a dynamic model of the post-combustion capture plant is built, where the two scenarios are simulated. Three cases are considered for each scenario, based on the amount of heat available for the capture plant, i.e., the season of the year. The dynamic response of the process is studied and a control strategy is designed in order to optimize the performance towards maximum CO2 capture. A comparison between the dynamic and steady-state performance of the plant is also carried out. The data analysis shows the large amount of variations that the steel mill experiences: most of them related to the flared gas, available excess steam and blast furnace gas flow. Their magnitude and frequency reflects the importance of considering these disturbances. The amount of heat available for CCS can be doubled (in case of low heat available it can raise from 30 to 60 MW for some hours), or the blast furnace gas flow can drop to zero for periods of two to twelve hours, among other events. The simulation results show how the plant reacts to the disturbances found, and how different the responses are for each variable and case considered. The same variable can stabilize five times faster in summer (large amount of heat available) than in winter. Results also show that in the periods when there is no gas entering the absorber, the stripper keeps producing CO2 for eight to ten hours, due to the high loading of the solvent in steady-state conditions. The CO2 produced in the stripper is increased after implementing the control strategy, and the plant stabilizes several minutes faster. This thesis shows that the capture unit can handle the disturbances caused by the steel mill, even showing a potential increase in CO2 captured compared to the steady-state case. This work shows the large amount of hourly variations occurring within a steel plant and its integrated power plant, and how important is to take them into account when designing the capture plant. It also helps understanding the implications of the disturbances and how a dynamic model differs from off-design steady-state models.

Nyckelord: steel production, post-combustion, dynamic modelling, transient data analysis, process control

Publikationen registrerades 2018-09-20. Den ändrades senast 2018-09-20

CPL ID: 255968

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