Apr-2009

Post-combustion capture of CO2

Case study of a generic amine-based absorption process. A lifecycle cost analysis for a power plant over a 40-year span clearly shows the advantages of structured packing over random packing

Abhilash Menon, Markus Duss and Christian Bachmann
Sulzer Chemtech

Article Summary

The past few years have seen an unprecedented interest in proving the techno-economic viability of CO2 capture technologies from power plants flue gas streams. The main challenge is removing concentrations of CO2 (typically 3.5–14 vol%) from large volume gas streams, which leads to very large column sizes. An important cost factor is the pressure drop per metre that can be saved inside the absorber. Structured packing offers an excellent solution in terms of both reducing the column’s dimensions (Capex) and providing a low pressure drop (Opex) over the absorber. The correct choice of gas and liquid distributor devices is also critical in the success of such large-scale industrial applications.

CO2 control initiatives
The past few years have seen significant interest worldwide in the handling of carbon dioxide (CO2). This is primarily linked to responsible utilisation of fossil fuels. Climate change and greenhouse gases (GHG) emissions have become a daily topic of discussion as world governments, politicians, industries, NGOs and the general public contemplate how to tackle this global problem. CO2 emissions from fossil fuel combustion are considered to be the primary contributor.

As a result, there has been considerable international interest in the development of technology for capturing and storing CO2. The current available technology is highly expensive, and there are many uncertainties linked to the costs and operation of a CO2 chain. For example, the cost of capturing CO2 from a coal-fired power plant amounts to approximately two-thirds of the cost of the entire CO2 chain, while transport and storage amount to approximately one-third (Norwegian Ministry, 2008). Capturing CO2 requires a lot of energy. The IPCC report (Norwegian Ministry, 2008) assumes that, if 90% of the CO2 from a power plant is captured, fuel consumption will increase by 11–40%, depending on the technology and the fuel. The report estimates that CO2 capture increases the production costs associated with power production by 20–85%. If the current level of research and development is maintained, the cost of capturing CO2 could be reduced by 20–30% over the next ten years.

Technically, there is no problem in separating CO2 at very low concentrations from flue gas streams. This is a well-established process within the gas processing, petrochemical and fertiliser industry. Gas absorption into chemical solvents such as amines is the most promising technology due to its capacity to handle large volumes of flue gas, plus it can be operated at a low temperature and pressure.

Flue gases from gas or coal-fired combustion plants typically contain some oxygen. Monoethanolamine (MEA)-based solvents with additives and corrosion inhibitors are commonly used in applications for flue gases. MEA-based processes look attractive due to their fast reaction rate with CO2 and the low cost of raw materials compared with other amines. However, MEA absorption processes are associated with high operating costs because of the significant amount of energy required for solvent regeneration, and severe operating problems such as corrosion, solvent loss and solvent degradation.

Post-combustion CO2 capture technology
Technology for capturing CO2 from gas and coal-fired power plants can be divided into three main categories: post-combustion, pre-combustion and oxy-fuel. Post-combustion entails separating CO2 from the exhaust gas of the power plant using chemical scrubbing. This technology, in principle, can be utilised in existing power plants without major modifications. Post-combustion is the most mature technology, although there is still significant uncertainty surrounding its use. The main problem is the low partial pressure of CO2 in the flue gas, which also contains other gases such as oxygen, water vapour and nitrogen. Separation methods are required to trap CO2 preferentially, so it can be compressed and stored in a sequestration site, complying with regulations for CO2 purity.

So far, none of these technologies have been subjected to large-scale testing in gas power plants. Therefore, there is much uncertainty associated with the use of the available technology for CO2 capture, particularly with regards to costs and performance.

Techno-economic design study
As previously noted, it is important to choose the right technology for mass transfer when it comes to applications such as CO2 capture, where pressure drop can be a significant factor in operating costs. While an industrial case study has been chosen to emphasise this point, it is not our intention to discuss process-specific parameters related to heat duty optimisation, energy/exergy analyses and so on, but rather to convey some essential concepts that must be understood and accounted for at an early stage in the design process.

Case study
Figure 1 provides a schematic overview of the amine-based scrubbing process for CO2 capture from a flue gas stream (eg, coal-fired power plant flue gas stream). The process conditions for this case study have been taken from Alin (2007). This study is performed on a 800 MW coal-fired power station, which is reasonable for a medium-sized power plant in Europe. The various process conditions, parameters and assumptions are summarised in Table 1. Simulations presented herein are performed using the commercially available tool ProTreat from OGT (2008). ProTreat is a rate-based modelling tool that uses mass transfer correlations readily available in literature for random packing (Onda, 1967) and structured packing (Bravo & Fair, 1992). The amine selected for this simulation is MEA, which is a widely used generic amine for sour gas treatment purposes. The simulations were run for a CO2 loading of 0.15 mol CO2/mol amine in the lean amine stream entering the absorber. Of course, the CO2 loading is directly coupled with the reboiler heat duty, but that discussion is outside the scope of this article.

The absorber is operated close to atmospheric conditions, which implies that pressure drop within the absorber is a critical factor when designing the column (ie, a higher pressure drop requires higher energy input into the fan, which feeds the flue gas into the absorber). This simple philosophy virtually rules out using trays as a mass transfer device in the absorber because of the significant pressure drop losses.