Dynamic modelling and control of oxycombustion process
M.Sc.(Eng.), postgraduate student Laura Lohiniva
(Funded by Graduate School in Electronics, Telecommunication and Automation (GETA) 1.6.2010-2014)
Professor, D.Sc.(Tech.) Enso Ikonen
Docent, D.Sc.(Tech.) Jenö Kovács
Systems Engineering laboratory, Department of Process and Environmental Engineering, University of Oulu
in cooperation with R&D department of Foster Wheeler Energia Oy.
ABSTRACT
The research focuses on a new and challenging topic of modelling and control of oxycombustion.
The topic covers control and systems engineering in a wide range. From control and systems engineering point of view, the challenges include dealing with the multivariable nature, uncertainties and nonlinearities in the overall process as well as developing control that takes full advantage of the physicochemical knowledge related to chemical and thermodynamical phenomena, mass and energy balances etc. Oxycombustion is currently in demonstration phase.
METHODS
The main route to successful process control is via modelling of the plant and its dynamics. Models can be applied in advanced state estimation, subprocess control and plantwide control and optimization. The development of plant models is to be followed (partly in parallel) by development of local and coordinating process control. In addition to developing modern model-based controls (based on e.g. model-predictive approaches) the later stages of the research also permit an excursion into advanced methods of model-based state estimation and control (e.g. particle filters, finite controlled Markov chains).
OXYCOMBUSTION
Adoption of the Kyoto protocol has increased the urgency of reducing CO2 emissions in fossil fuel power plants. One option for carbon dioxide mitigation is CO2 capture and storage (CCS), ”separation of CO2 from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere” (IPCC, Special Report on CCS, 2005). Different types of CO2 capture systems include pre-, post- and oxyfuel combustion capture.
In oxycombustion, combustion air is substituted with an oxidant (mixture of O2 and recycled flue gas). Main flue gas compounds are CO2 and H2O, with smaller amounts of other gases (O2, N2, Ar, NOx and SOx). With high flue gas CO2 content the capture, compression and condensing of CO2 is easier and less expensive.
Oxycombustion can not be simply substituted for air combustion in existing fossil-fueled power plants due to differences in combustion characteristics. In order for oxycombustion to be utilized in existing plants, a thermal diluent is required to replace nitrogen in air: the oxygen produced in air separation is mixed with recycled flue gas to approximate the combustion characteristics of air firing.
In addition to combustion, the oxycombustion process involves an air separation unit (ASU) providing oxygen and a CO2 purification and compression unit (CPU). The overall efficiency of the plant is no longer only a matter of steam and power generation, but also the operation of ASU and CPU units. Consequently, monitoring and coordinating control of these three systems becomes important. No overall ASU-combustion-CPU model is available at the moment. For these reasons, the control solutions developed for air-firing units may not be directly applicable and new approaches are required.
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