The chemistry of carbon capture...

1 June 2009

The pressing need to reduce the efficiency penalty associated with CCS, particularly post combustion processes, has pushed the chemistry of carbon capture high up the power industry agenda in recent times.

As pointed out by Francois Giger of EDF at VGB’s recent “Power Plants in Competition” conference in Prague, amine based technologies – with a 10-15 percentage point efficiency penalty – are “generally considered as being the closest to industrial application” because the process is well established in the oil and gas business for sour gas sweetening and is also used for the production of food-quality carbon dioxide. But, as suggested by Giger, the fact they are so well proven could also be their downfall. He noted that, “considering the significant maturity of amine processes in the chemical industry, some specialists...fear that there might be only limited improvement in energy penalty to be expected” and added that the breakthrough we are looking for “will not be

delivered by incremental innovation but will

require a kind of ground breaking rupture.”

There are a growing number of concepts out there whose proponents believe they might

provide just the disruption that is needed.

Alstom, for example, has recently reported that positive preliminary data have emerged for its chilled ammonia process from the demonstration project being carried out at We Energies’ Pleasant Prairie facility. An Alstom press release reports 88-90% capture and over 99 % pure CO2 (although remains silent on estimated efficiency penalty, which is nevertheless a major driver behind the concept). The chilled ammonia technology will be further trialled at a second demo facility, being constructed at AEP’s Mountaineer power plant, which is due to start up later this year. Alstom says “Mountaineer will be the first integrated demonstration project that burns coal, cleans the flue gas, captures the CO2, compresses it, and sequesters the CO2” (the latter occuring at more than 8000 ft underground).

Meanwhile, returning to the Prague VGB conference, a very interesting paper by B Epple and

J Strohle of Technische Universitat Darmstadt described a retrofittable capture technology based on good old fashioned limestone. In this carbonate looping process the carbon dioxide in the flue gas is absorbed by lime (CaO) in a fluidised bed reactor (carbonator), resulting in the formation of limestone (CaCO3). This limestone is then calcined in another fluidised bed reactor (calciner), where the carbon dioxide for subsequent storage is released by raising the temperature above 850°C. CaO produced in the calciner is transferred back to the carbonator, creating a cycle (with an input of make-up limestone). A major attraction of the process is that, according to Epple and Strohle, the efficiency penalty looks like being 5-7 percentage points, which, as they say is “much lower than that of other CO2 capture technologies currently under evaluation.”

The mention of limestone being made from the carbon dioxide in flue gas raises the issue of whether a process could ever be designed in which CO2 is used on site to enable the power plant to make its own limestone for flue gas desulphurisation.

Doing something useful with the carbon dioxide rather than, rather inelegantly pumping it underground is of course a key motivation for RWE’s work on micro-algae at Niederaussem (see pp 17-19) – and another holy grail for those in the capture business, along with reducing the energy requirements of the capture process itself.

...and boiler tube failure

While we are on the subject of chemistry, another excellent paper at the VGB Prague (by Matshela Koko) touched on the problems Eskom has had with boiler tube failures in its coal fired plants. From October 2007 South Africa suffered a series of load shedding events, culminating in a national emergency on 24 January – Eskom’s nadir. The paper noted that on that day about half of the unplanned capacity shortfall was due to boiler tube failures.

The utility plans to address the issue in the four new major coal plants currently under construction and planned (Medupi, Kusile, Coal 3 and Coal 4) by paying more attention to water chemistry. Matshela Koko draws the following lesson from Eskom’s experience: “Design the plant chemistry first, then design the plant.” James Varley

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