OPINION

March 2009 Filtration Industry Analyst5

Ken Sutherland looks at the realities of carbon capture and sequestra-

tion – the quantities involved, the likely processes, its future – and the place of filtration in it.

Let’s not kid ourselves – we shall be using fossil fuel sources for most of our energy sup-plies (and especially for the liquid fuels with-out which our cars, trucks, trains and planes will all stop running) well into the next cen-tury. This period will keep us going through the time when the oil runs out (about three quarters of the way through this century), through the use of gas by conversion to liquids (with natural gas probably lasting just into the 22nd century), into coal liquefaction (from the middle of this century forward for another hundred years at least), until, with any luck, nuclear fusion will have become the source of much of our energy. It will have to go this way because renewables just cannot cope with a base load, and nuclear fission stations, which can, are not being built fast enough.

So, given the continued high (and probably higher, as population and per capita usage both continue to grow) usage of carbonaceous materi-als, how can we offer the reduced carbon emis-sions yet sustainable consumption that we owe to our descendants. One major means would, of course, be to eliminate the carbon dioxide emissions coming from energy production and consumption processes, which make up a considerable proportion of the total amount of carbon that we dump into the atmosphere each year. The trouble is that the elimination process is currently very difficult to achieve, as difficult as getting the wind to blow continuously through a wind farm (or as getting planning permission for a nuclear power plant).

There is no getting away from the fact that, for every tonne of carbon that is completely burned, as oil or gas, as coal or biomass, there are produced 32

3 tonnes of

carbon dioxide, which disperse at once into the atmosphere, unless prevented from so doing. It is this carbon dioxide, emitted from power stations and process furnaces, that is now generally believed to be a major cause of global warming, and the resultant harmful changes to the world’s climate.

The numbers involvedThe weight of the atmosphere is colos-sal, about 5 quadrillion tonnes (that’s in American terms, ie 5 thousand million millions or 5 x 1015). On a dry basis, it contains close to 0.06% by weight of carbon

dioxide (0.0582% in 2007, to be precise). This means that the total amount of carbon dioxide in the atmosphere is just under 3 tril-lion (2.9 x 1012) tonnes.

It is calculated that all human activities add about 25 billion tonnes of carbon diox-ide to the atmosphere in a year (24.1 x 109 in 2002), just over a quarter of which (7 bil-lion tonnes) comes from the combustion of fossil fuels for energy generation.

The figure of 580 ppm (by weight) for 2007 is to be compared with 470 ppm in 1960, and a figure close to 420 ppm in 1750, before the start of the first industrial revolu-tion. This means an increase of only 50 ppm for the more than two centuries from 1750 to 1960, to compare with 110 ppm in the half century from 1960 to 2007. This is a stagger-ingly high change in the rate of growth in car-bon dioxide content. Higher rates are forecast: 2100 figures of 820 to 1470, depending on the scenario used in the calculation

These figures can be used to show that the total effective addition from 1960 to 2007 was 570 billion tonnes, or 12 billion tonnes per year. If this is now compared with the annual rate for 2002 of 24 billion tons, it appears that about half of the annual addi-tion does not stay there, but presumably is removed naturally from the atmosphere.

Natural removalIt is well known that carbon dioxide is absorbed by the surface layers of the earth: the sea, the soil and the plant matter growing on it. The surface area of the earth is about 510 million sq km, of which just under 150 million are land areas, and just over 360 are water covered. Capture by the seas is signifi-cant – absorption of carbon dioxide makes the oceans more acidic, and it is believed that the overall pH has fallen by 0.1 since pre-industrial times, but greater falls are pre-dicted (another 0.3 to 0.4 by 2100).

Schemes have been proposed to increase uptake of carbon dioxide: for example by “seeding” with ground limestone or haematite. The limestone would change the dissolved carbonate/bicarbonate balance, whereas the haematite adds iron as a growth promoter for algae, but neither scheme is likely to be used, as the risks of disturbance (and the costs) are too high: mankind has done enough damage to the seas, without risking more.

Much more obvious is the uptake by plants, as photosynthetic fixation of the car-bon to create growth (with the added advan-

tage of emission of a molecule of oxygen for every one of the dioxide absorbed). Carbon capture schemes involving plants are much more likely: reduction in the rate of defor-estation of the tropical rainforests in Africa, South America and South East Asia (paid for, if necessary, by the developed countries of the world) is a major need, not just to reduce carbon emissions. This can be supplemented by creating new forests, and perhaps by the new agriculture of biomass production des-tined for conversion to biofuels.

Plants take up more carbon dioxide than they need for growth. The rest is passed out through their roots into the soil, some being immediately returned to the atmosphere, but some is retained as silica complexes, and it is now accepted that the soil holds much more carbon than was real-ised for a long time. This must not be disturbed any more than is essential.

Rather more far-fetched “natural” ideas include the burial of a large proportion of the annual production of timber in a way that prevents its decomposition (which probably means very deep underground), either in its natural state, or after it has been carbonised to charcoal. Space limitations and the need for high involvement in labour probably discount this approach completely.

Capture systemsThe natural carbon removal methods just discussed will remove, as suggested above, upwards of one half of the annual discharges of carbon dioxide, leaving the rest to accu-mulate, eventually to our detriment. It does, of course, have some uses in industry, such as in gasifying beverages, or as a refrigerant, but these uses only delay its discharge to the atmosphere. Much more important would it be to find use for it, as the plant does, for conversion to organic materials. A great deal of industry relies on petrochemical sources for its raw material – how wonderful if this could come from catalytic conversion of carbon dioxide – especially after the oil runs out. Some work is being done on such proc-esses, but investment needs to be amplified greatly – the prize is worth it.

In the absence of consuming uses, the carbon dioxide must be taken out of emis-sions and stored safely. A major platform of waste treatment philosophy is that wastes should be treated in as concentrated state as possible, before they become dispersed into the environment. In practice, this means that treatment should be given as close as possible

Capture it if you can

OPINION

6Filtration Industry Analyst March 2009

to the points of origin of the wastes. In the case of carbon dioxide emissions, sadly, over half of the accumulating waste comes from distributed or mobile sources – the domestic fireplace or private car, and are most unlikely ever to receive carbon capture treatment. It is certainly not impossible, however, that the not too distant future will see free-standing capture plants – say the size of a transport container – established throughout built-up areas, working on the general atmosphere. This may be, in the end, the only way to control distributed discharges.

Obviously, carbon capture projects can only sensibly be applied at significant dis-charge sources, say power stations of more than 500MW output or similarly sized production plants. This means dealing with about 30% of the net discharges – but com-plete capture even of this amount (coupled with all of the other energy saving and pro-ducing schemes) would make a major impact on the global warming future.

It follows that carbon capture schemes will be applied to large emission sources, and such schemes are being developed along three different routes:

- pre-combustion systems- oxyfuel processes- post-combustion systemsaccording to where they fit in the carbon

dioxide generation process.Pre-combustion treatment involves con-

verting the carbonaceous material (primarily coal, but also natural gas or biomass) to a mixture of hydrogen and carbon dioxide, and then separating the two gases, the car-bon dioxide for storage and the hydrogen to be burned as the prime fuel. The conversion of coal or biomass will use established gasifi-cation processes, while methane conversion will be done by steam reforming.

In the oxyfuel process, the fuel is burned in the normal way, but instead of ambient air, pure oxygen is used as the combustion gas, so that the exhaust gas is a mixture only of water and carbon dioxide, with no nitrogen to make the gas separation stage more complicated.

The third scheme involves standard com-bustion of unconverted fuel, using ambient air for the purpose, producing an exhaust gas that is, ideally, a mixture of water, nitrogen and carbon dioxide, from which the dioxide has to be separated, probably by use of an absorbing solvent such as monoethanolamine.

The post-combustion scheme can be applied to any existing power station or fur-nace, as an additional array of equipment, but the pre-combustion and oxyfuel processes are really only suitable for new installations,

designed from the start to accommodate them. In equipment market terms, therefore, a company armed with post-combustion car-bon capture and storage (CCS) technology will have a much larger market available to it.

CCS plant componentsThe complete CCS plant will need to be able to take relatively pure carbon dioxide, to pre-pare it for transhipment, to move it possibly over some considerable distance, and finally to pump or otherwise inject it into a place of safe containment, where it should be possible to hold it for a considerable period of time (hun-dreds if not thousands of years). For two of the three proposed CCS schemes, the carbon dioxide will first have to be separated: from water for the oxyfuel process, and from water and nitrogen in the post-combustion process – separation being also required in the pre-combustion scheme, but as an integral part of the energy production process, where it must be separated from hydrogen.

The relatively pure dioxide will then have to be compressed, probably to the liquid state (the solid form being less easily moved over any significant distance) and pumped through a very secure pipeline to the site of its intended storage. This is most likely to be a cavity in an underground rock forma-tion, situated sufficiently far underground to retain it as a liquid without leakage, however slow. The cavity could be in an exhausted oil or gas deposit, or in an aquifer of salty water, or a salt mine, but wherever it is, it must be outside any zone of earthquake activity.

Problems and opportunitiesApart from any technical difficulties, the prime problem has to be that of cost. The actual cost generally varies with the person quoting it – a supporter of coal firing and CCS or an opponent. It is by no means negligible – it could effectively reduce the efficiency of a coal-fired power station equipped with CCS by 25%–40%. Costs in the region of US$30–50 per tonne of carbon dioxide stored are not at all unlikely. This, however, puts CCS proc-esses on a similar level to the carbon tax levels quoted for trading schemes. A moderately high tax level, firmly imposed, would give CCS a chance of competing on a cost basis.

The storage of carbon dioxide in places from which recovery is potentially possible means that should a process ever be com-mercialised for its conversion back to organic molecules, the feed for such a process would be ready and waiting.

The various CCS processes do, however, offer considerable opportunity to the filtration indus-

try. They involve sizeable gas filtration steps after gasification or combustion, and the membrane applications will be enormous – for gas/gas separations, and for membrane contactors. These are involved in the production of relatively pure carbon dioxide, but liquid filtration will also be necessary in an amine separation process and at the point of injection to the storage zone.

It is continually said by the opponents of coal firing that the processes involved in CCS are just not proven. This, of course, isn’t true, since most of them have been around for quite a while, and in their particular cur-rent applications are very well proven. The full panoply of processes, in the direct line of coal processing or carbon recovery, or on its fringes, includes the production of syngas (in use in South Africa by SASOL for decades), integrated gasification and combined cycle power generation (has been available for at least 30), combined heat and power (large residential areas in Scandinavia get their dis-trict heating from it), gas pipelining (natural gas lines throughout Europe and the USA), the separation of carbon dioxide from other gases (amine solvent separation has also been used for decades), hydrogen recovery from gas streams (also in wide use in oil refineries), production of pure oxygen (by splitting of air – for a century or more), enhanced oil recov-ery by re-injection of water or inert gas into depleted oil and gas reserves (almost as old as the oil business itself ), and so on.

Any competent chemical engineer could put together a process for generating energy that is emission-free, and the ancillary proc-esses to maximise the values present in coal. What is needed is the confidence to do so on a large enough scale – and to provide the funds for such a project.

There are compelling reasons why success should attend the use of CCS, and a great deal of positive statement is continually forthcoming – but little real money is being invested. The date for success with CCS is probably receding into the future about as fast as we are approaching it.

About the authorKen Sutherland has managed Northdoe Ltd, his process engineering and marketing con-sultancy, for over 30 years. Northdoe is large-ly concerned with filtration and related sepa-ration processes. He has written numerous articles and four books on separation proc-esses, most recently an A–Z of Filtration and the fifth edition of the Filters & Filtration Handbook, both for Elsevier. He can be contacted at: Tel: +44 (0)1737 218868 or by e-mail at: ken.suth@ntlworld.com