It has been a busy fortnight in the oxyfuel community, starting with a meeting of the EU funded ‘Relcom’ project last week in Warsaw under the banner of one of the IFRF’s regular ‘ToTEM’ meetings. Closer to home, joint meetings at Imperial College London this week of the UK ‘OxyCAP’ research consortium and of the OxyCorr working group into oxyfuel research were organised by long-standing oxyfuel champion, Stanley Santos of IEA GHG.
The Relcom project, standing for ‘reliable and efficient combustion of oxygen/coal/recycled flue gas mixtures’ and involving 13 partners from academia and the energy industry, is now over halfway through its four year programme. Much of the research effort is focussed on the20 MWt PC pilot unit operated by Ciuden in Spain which, since the recent closure of the Schwarze Pumpe pilot, is Europe’s only large operational oxyfuel rig.
The Warsaw meeting was opened by an enlightening keynote from Gerry Hesselmann of Doosan Power systems, who began by outlining the history of oxyfuel combustion research since its origins in the early 90s. With experience from research facilities such as Doosan’s 40 MWt burner rig at Renfrew (still the largest single oxy-burner operated), initial challenges such as stabilising oxyfuel flames and automating the transition between air and oxy-firing are now well understood. However, Mr Hesselmann emphasised that such pilots can tell us much more about burner properties than they can about radiant heat transfer or burner interactions in a real boiler. In this respect, tests at the Ciuden unit should provide a useful complement to the Schwarze Pumpe results as, unlike the German plant, it is equipped with multiple burners and radiant superheater surface. Doosan have also completed a detailed FEED study for a large oxyfuel retrofit of a boiler at Young Dong in South Korea, which unfortunately appears to have been shelved before moving to a construction phase.
Reliably scaling up the experience from smaller oxyfuel pilots to full-scale plant will require accurate models which are tailored to the altered properties of oxyfuel combustion, and several of the Relcom funded research projects have sought to develop these models. A talk from Leeds University highlighted how the conventional ‘weighted sum of grey gases’ model of gas radiation can be inadequate for the highly radiative mixture of CO2 and water vapour found in oxyfuel boilers. Using a more accurate model of the radiative spectrum can in some cases produce a difference of up to 250ËšC in flame temperatures.
Other presentations looked at the various ways of reducing the energy penalty imposed by the capture process. An EDF study has used exergy analysis of a large oxyfuel plant to show how wasted thermal energy can be recovered to raise the process efficiency by over 2 percentage points. For example, the air compressors used in oxygen production reject large amounts of heat which can be put to use in heating feedwater within the plant’s steam cycle. A more straight-forward way of optimising oxyfuel efficiency, discussed by E-ON, is to recycle hot flue gases before they are cooled in a desulphurisation unit, also providing the significant benefit of much smaller flue gas treatment equipment than in conventional plant. However, this strategy also carries risks of concentrating corrosive sulphur species in the boiler and may limit the plant’s output under air-firing. Indeed, several means of optimising oxyfuel plant come with the disadvantage of limiting plant flexibility, either in load following or air-oxy switching, which may be undesirable for early plants at least.
Despite this, an emerging philosophy in carbon capture technologies is to design new processes with capture in mind from the start, rather than attempting to attach it to existing plant models. The pressurised oxyfuel concept presented by Italian utility Enel is an early example of this approach, having been tested at a 5 MWt plant since 2007. Pressurised combustion offers a number of advantages for oxyfuel, as the latent heat of water vapour can be recovered for feedwater heating, and air ingress to the boiler is reduced. Details of this process and others can be found in my report ‘Developments in oxyfuel combustion’ which is currently available in draft and will be published next month.
As hosts of the meeting, Polish research institute IEN provided equally informative tours of Warsaw city centre and their nearby research facility. Although the Institute’s large oxyfuel rigs are based at other sites, the Warsaw facility conducts oxyfuel tests in large drop tube furnaces, as well as biomass gasification and solid oxide fuel cell research. The city itself is surprisingly full of history, despite the tragedy of its almost complete destruction at the end of the war, having had its old centre entirely reconstructed in one the largest projects of the kind ever undertaken.
My thoughts on the Imperial College workshops on OxyCAP and OxyCorr will follow shortly.