CO2-flue gas separation for a coal fired power plant

Udara S.P.R. Arachchige, Neelakantha Aryal

Telemark University College

3901, Porsgrunn, Norway

udara.s.p.arachchige@hit.no; aryalneel4all@yahoo.com

Prof. Morten C. Melaaen

Telemark University College/Tel-Tek

3901, Porsgrunn, Norway

Morten.C.Melaaen@hit.no

Abstract 

The CO₂ is one of the main pollutants for global warming and climate change effect. In order to carry on power generation by fossil fuel, CCS technologies are required to reduce the environmental impact by CO₂ emissions. Currently, chemical absorption is the preferred option for CO₂ capture by post combustion.The detailed description of the CO₂ removal process using mono-ethylamine (MEA) as a solvent for coal fired power plant is present in this paper. The model is implemented in Aspen Plus and possible chemical reactions were introduced using an electrolyte wizard. The rate based Electrolyte NRTL activity coefficient model is used in the Aspen Plus with 25^w/_w % MEA solution and 0.25 as lean CO₂ loading.

CO₂ removal efficiency was specified as a design specification option in Aspen Plus with the variation of Distillate rate in the stripper. With the help of sensitivity analysis, optimum parameters were selected for packed bed absorber and stripper column. The complete removal process with re-circulating solvent back to the absorber is implemented with the sequential modular method in Aspen Plus. The most significant cost related to CO₂ capture is the energy requirement for re-generating solvent, i.e. re-boiler duty. Thus, re-circulating solvent with make-up stream is added to the system to get lowest possible energy demand. The re-boiler duty requirement on 85% of CO₂ removal was calculated as 4120 kJ/kg CO₂ after several iterations. The temperature profiles are used to check the model behaviour.

 

 Fig 1: Process flow diagram for coal fired power plant

Technologies for Scrubbing

Three different types of scrubbing techniques are commonly used for carbon capture, listed below as;


1. Post Combustion Capture:
In post combustion capture, the CO₂ is removed after combustion of the fossil fuel — this is the scheme that would be applied to fossil-fuel burning power plants. Here, carbon dioxide is captured from flue gases at power stations or other large point sources. The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station.

2. Pre-Combustion Capture:
The technology for pre-combustion is widely applied in fertilizer, chemical, gaseous fuel (H₂, CH₄), and power production. In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The resulting syngas (CO and H₂O) is shifted into CO₂ and more H₂. The resulting CO₂ can be captured from a relatively pure exhaust stream. The H₂ can now be used as fuel; the carbon dioxide is removed before combustion takes place. There are several advantages and disadvantages when compared to conventional post combustion carbon dioxide capture. The CO₂ is removed after combustion of fossil fuels, but before the flue gas is expanded to atmospheric pressure. This scheme is applied to new fossil fuel burning power plants, or to existing plants where re-powering is an option. The capture before expansion, i.e. from pressurized gas, is standard in almost all industrial CO₂ capture processes, at the same scale as will be required for utility power plants.

3. Oxy-fuel Combustion:

In oxy-fuel combustion the fuel is burned in oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is recirculated and injected into the combustion chamber. The flue gas consists of mainly carbon dioxide and water vapor, the latter of which is condensed through cooling. The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored. Power plant processes based on oxyfuel combustion are sometimes referred to as "zero emission" cycles, because the CO₂stored is not a fraction removed from the flue gas stream (as in the cases of pre- and post-combustion capture) but the flue gas stream itself. A certain fraction of the CO₂ generated during combustion will inevitably end up in the condensed water. To warrant the label "zero emission" the water would thus have to be treated or disposed of appropriately. The technique is promising, but the initial air separation step demands a lot of energy.

Insight into CCS Technology

Carbon capture and storage (CCS), alternatively referred to as carbon capture and sequestration, is a means of mitigating the contribution of fossil fuel emissions to global warming. The process is based on capturing carbon dioxide (CO₂) from large point sources, such as fossil fuel power plants, and storing it in such a way that it does not enter the atmosphere. It can also be used to describe the scrubbing of CO₂ from ambient air as a geoengineering technique. Although CO₂ has been injected into geological formations for various purposes, the long term storage of CO₂ is a relatively new concept. The first commercial example was Weyburn in 2000.

An integrated pilot-scale CCS power plant was to begin operating in September 2008 in the eastern German power plant Schwarze Pumpe run by utility Vattenfall, in the hope of answering questions about technological feasibility and economic efficiency. CCS applied to a modern conventional power plant could reduce CO₂ emissions to the atmosphere by approximately 80-90% compared to a plant without CCS.The IPCC estimates that the economic potential of CCS could be between 10% and 55% of the total carbon mitigation effort until year 2100.

Capturing and compressing CO₂ requires much energy and would increase the fuel needs of a coal-fired plant with CCS by 25%-40%. These and other system costs are estimated to increase the cost of energy from a new power plant with CCS by 21-91%. These estimates apply to purpose-built plants near a storage location; applying the technology to preexisting plants or plants far from a storage location would be more expensive. Recent industry reports suggest that with successful research, development and deployment (RD&D), sequestered coal-based electricity generation in 2025 will cost less than unsequestered coal-based electricity generation today.

(source:Wikipedia)Read more in Wikipedia

CO2-flue gas separation for a gas fired power plant

Neelakantha Aryal, Udara S.P.R. Arachchige, Prof. Morten C. Melaaen

Abstract

Combustion of fossil fuel and natural gases are the main sources of CO₂ emission. The energy requirement, high capital and operating cost are the challenges regarding the CO₂ capture. Absorption is the most common and efficient technique for CO₂ removal today and one of the viable options for CO₂ capturing from power plants. In this paper, comprehensive flow sheet has been developed for post combustion CO₂ removal system with Aspen Plus. The CO₂ removal from 500MW natural gas-fired power plant flue gas was considered. The parameters and other operating conditions are selected to achieve 85% of CO₂ removal.

The rate based Electrolyte NRTL activity coefficient property method is used in the Aspen Plus with 25^w/_w % MEA solution and 0.25 as lean CO₂ loading. CO₂ removal efficiency was specified as a design specification option in Aspen Plus with the variation of Distillate rate in the stripper. The sensitivity analysis was performed to check the parameters’ effect on removal efficiency on the CO₂ removal process. As a result of sensitivity analysis, the optimum operating conditions and parameters were selected for absorber and stripper. The complete removal process with re-circulating solvent back to the absorber is implemented with the sequential modular method in Aspen Plus. The re-boiler duty requirement on 85% CO₂ removal was calculated as 4500 kJ/kg CO₂ after several iterations. Parameters’ effect on re-boiler duty was checked to optimize the process. Temperature and concentration profiles as well as CO₂ loading were analyzed to check the model behavior.

Key words: Gas fired flue gas, Aspen Plus, Simulation, Re-boiler duty