More in

Energy materials processing: Filtration and the fuels of the future

The processing of the fossil energy materials – coal, oil and gas – is an important industrial sector for the employment of separation equipment. To these fossil fuels have been added the renewable biofuels as a sustainable alternative. Ken Sutherland reviews the range of applications for filtration and other separation equipment in the whole field of fuels processing, with the emphasis on biofuels production.

Clean coal processing could provide future opportunities for filtration.
Clean coal processing could provide future opportunities for filtration.
Oil refineries employ a wide range of filtration and sedimentation equipment.
Oil refineries employ a wide range of filtration and sedimentation equipment.
Natural gas production well.
Natural gas production well.
/any increase in nuclear power programmes would create good filtration business.
/any increase in nuclear power programmes would create good filtration business.
Biomass can be used as a solid fuel.
Biomass can be used as a solid fuel.
A refining and transesterification plant for biodiesel.
A refining and transesterification plant for biodiesel.

In the production of usable fuels from the earth's fossil energy reserves, there is a very definite two-stage process, combining extraction of the energy material from the earth with its subsequent treatment to prepare it for efficient combustion. These two processes – extraction and processing – can be undertaken in totally different parts of the world. Both are usually carried out in large facilities, with extraction at the source of the fuel, and processing relatively near to the end-use marketplace.

The arrival on the scene of biofuels, as a viable alternative for energy production, has changed this two-stage system somewhat, with the ‘extraction’ stage now replaced by the growing of the basic vegetable materials, which can be done on a smaller scale to suit a local market (although the economies of scale still apply).

The use of filtration and other separation equipment in the processing of fossil fuels is reviewed in this article, with emphasis on the increasingly important range of biofuels.

The future for energy

The end for fossil fuels on a global scale is frequently foretold, but has not occurred yet, even though the currently forecast lifetimes, at least for oil and gas, are quite short. In its June 2010 issue, the BP Statistical Review of World Energy gave the 2009 lifetimes (measured by the ratio of proven resource to current production rate) as 45.7 years for oil and 62.8 years for gas, which do not bode well for the future of a fossil fuel based global economy. However, it should be noted that these two lifetime figures have not changed much for the past several years, i.e. fresh fuel resources are being discovered at rates sufficient to allow for fuel consumption at a growing rate – a situation that is expected to continue for some time yet. It is also true that the reserves of coal are twice or three times greater than those for oil and gas, with a much lower investment in finding new reserves.

These comments on lifetime concern have paid no heed to the phenomenon of global warming, which is blamed very much on over-use of fossil fuels. International actions to reduce the consequent climate change may well shorten the life of these fuels, unless there are abatement moves that do not result in reductions of consumption.

On balance, these comments suggest that the world economy cannot continue to grow sustainably without some significant changes in the way in which energy materials are employed. These changes will include:

• A strong move in coal usage towards clean coal production and its combustion in cleaner processes;

• The production of oil and gas from the less easily worked reserves, such as shale gas, tar sands, oil shales and coal bed methane;

• A major investment in carbon dioxide capture and sequestration (CCS) processes associated with all fossil fuel usage;

• Renewed emphasis on efficient use of energy, especially in residential promises, but also in industrial operations;

• A resurgence of nuclear power generation, which is largely free of carbon emission during operations (although certainly not without its own waste problems);

• Investment in all of those renewable energy techniques that are indirect converters of solar power – hydroelectricity, wind power, and wave and tide power; and

• Rapid development of the biomass conversion processes – which are effectively carbon-neutral over their life cycle - away from the use of food raw materials as input, towards the use of agricultural and food processing wastes, and of crops grown specifically for biofuel production.

A major policy question affecting the sector as a whole is the need to choose between use of the materials as energy source, for which purpose they are obviously very well suited, apart from the problem of carbon emissions, or as feedstocks for a very wide range of chemicals, for which they are almost uniquely suited.

Coal processing

The processing of mined coal already provides a significant market for screens, filters and centrifuges, especially in the treatment of coal washings. This segment will grow significantly if there is a serious return to the use of coal, in one of the ‘clean coal’ systems, which will probably use some kind of chemical treatment to remove the sulphurous and other impurities mixed in with the coal. This will give scope for the usage of filters and centrifuges in a fairly abrasive, and probably corrosive, environment – needing metallic media in some quantity.

Coal will only grow again if carbon capture and sequestration systems are installed. These systems require fine filtration of the carbon dioxide before its re-injection, but this is discussed later, under gas processing.

A major problem for coal, in the overall energy material context, is that it cannot replace oil as a vehicle fuel. This can be overcome by converting the coal to liquid fuels by hydrogenation. Such conversion (with several key filtration applications) is one of the main clean coal processes, using established processing techniques – as is gasification.

Oil processing

Once crude oil has been extracted from its underground reservoir (or from reserves of tar sands) it is sent, sometimes over long distances, by pipeline or marine tanker, to a petroleum refinery, where a very wide range of purification and transformation processes take place. These processes use a range of filtration and sedimentation equipment, with a strong emphasis on liquid/liquid separations. The main refinery processes are:

• The refining of crude oil by distillation, to produce the various commercial grades of oil;

• The cracking and reforming of oil fractions and residues to produce petrochemical raw materials;

• The blending of oil fractions to produce further products such as lubricating oils; and

• The treatment of process off gases for the recovery of valuable components.

Refinery separation duties include recovery of catalyst from cracker and reformer product streams, furnace exhaust filtration, and the general clarification of liquid products, especially of lubricating oils and similar products. Many refining wastes are oil/water mixtures, and the classic application of the lamellar separator is in the separation of residual oil from water.

A return to a coal-based economy is feasible, employing coal refineries that convert the solid coal to gases, or to liquid fuels – with similar downstream treatment technologies.

One important application (for gas separating membranes) is in the recovery of gaseous hydrogen from cracker and reformer off gases. There is much talk within renewable energy circles of the use of hydrogen as a major energy source for the future, to be used in fuel cells in automobile and other engines. This is misplaced, because hydrogen is not an energy source, but a means of moving energy from one place to another (rather like electricity). At present, and probably for some time to come, it costs more to generate hydrogen than the value that is recoverable by its combustion – so recovery from refinery by-products will be a growing need.

Gas processing

Natural gas produced from dedicated wells, or in association with crude oil, will normally leave the well head in a reasonably clean state, and will only require subsequent filtration, if at all, at the point of use.

The exciting application (from a filtration point of view) is the processing of gas streams for carbon dioxide removal, and its sequestration. This will probably be by compression and injection of gases into underground strata – to store the gas for the long term, or to improve oil production rates. This can be into the gas cap over a reservoir – when the need for filtration is low, or directly into the rock formation, either as an enhanced oil recovery process or as a sequestration method for carbon dioxide disposal.

The direct injection of gases will require that they be free from suspended solids, at least down to 2 μm. This will be done in the same sort of microfilters as are used for engine intakes, using V-block mini-pleat filter panels, for example.

Other gas treatment processes in this sector include:

• The condensation of natural gas into LNG for convenient shipment (and storage);

• The increasingly important conversion of gas into liquid fuels; and

• The recovery of methane from secondary sources, such as landfill sites, old or undeveloped coal workings, and sludge digestion.

Nuclear fuel processing

Nuclear energy offers itself as a reasonable source of power, free of carbon emissions during operation, but with serious problems – public attitude, a satisfactory answer to the radioactive waste disposal problem, and high capital cost. There are some signs of a return to favour, but the sector is plagued by fear of terrorism.

Were large nuclear power problems to be established again, then the fuel processing and reprocessing stages would create good filtration business: for inlet and exhaust air filtration, and for liquid filtration under severe operating conditions – for which metal media filters are well suited.

Renewable energy systems

Much has been said about expansion in renewable energy systems as the key to sustainable energy production, but they can only ever be a top-up to established sources, given that the visual intrusion of large wind farms, for example, can be accepted. Even in 2020, direct renewable sources (wind, tide, and wave) are not expected to exceed 2% of the total. Greater success is forecast for the use of biomass materials as a fuel, discussed in the next section – the main problem for biomass being its need for land, which puts it in competition with food crops.

In broad terms, the electro-mechanical renewable systems (direct solar energy capture, wind power, and wave and tidal power) do not involve filtration or sedimentation equipment.

Biofuel processing

From the point of view of energy material generation, the processing of materials derived from vegetable matter (and to a much lesser extent, from animal by-products) is of two kinds:

• The combustion of biomass as a solid fuel, and

• The conversion of the biomass to liquid fuels.

The use of whole biomass as a solid fuel can employ specially grown woody crops (such as willow) or agricultural wastes such as straw (or even chicken manure), fed in bulk onto the grate of a suitably designed furnace. The resultant ash can be used as a fertilizer, and the furnace exhaust must also be treated for ash recovery. This process is very similar to the combustion of RDF (Refuse Derived Fuel) pellets. Other than exhaust treatment there is little use for filtration equipment in this process.

Liquid fuels are produced from biomass in three ways. The first takes the whole plant material and converts it, by pyrolysis, to a mixture of gaseous and liquid products that can be refined to produce a range of fuel products. This is an old process, called wood distillation, and the one that was the basis for the organic chemical industry, before its replacement by petroleum derivatives. It is now being developed by the use of catalysts to control the range of materials produced. Since the process is quite suited to the conversion of vegetable wastes to oils, then it must be considered as a viable process in any complete survey of liquid biofuel production.

The more active methods for the production of liquid fuels from vegetable matter use two very different processes to produce two quite different fuels. The first process employs the fermentation of sugar-containing materials to produce bioethanol (or biobutanol), in a manner akin to the brewing of beer. The raw materials can be cereals such as wheat, barley or maize, whose starch content must first be hydrolysed to sugars, or the sugar crops, beet or cane. These raw materials are all food crops, and this means that biofuel production is competing with food production.

Most vegetable matter exists not as fruits or seeds, but as stalks and leaves, which also contain cellulose, but in a less available form (lignocelluloses). If this material could be fermented, then a much larger source of raw material, otherwise a waste, becomes available – without the need to compete with food crops. Enzymatic hydrolysis is being developed to convert the lignocelluloses to fermentable sugars.

The same competition with food crops is seen with the other liquid fuel process, which produces biodiesel by the chemical treatment of vegetable oils (including waste vegetable oil and animal fats). The raw oil is expressed from oilseed plants such as soybeen, castor bean, sunflower seed, rape seed, maize, and oil palm (from fruit or kernel). The produced fuel is called biodiesel because, in a vehicle engine, it looks and behaves like petroleum-derived diesel, but it is very different in a chemical sense. Petroleum diesel is a mixture of hydrocarbons, whereas biodiesel is a mixture of (mainly methyl) esters of long-chain fatty acids. As such, biodiesel has a lower energy content per unit volume than petroleum diesel, but it also is less viscous, and burns more cleanly, with production of fewer soot particles.

At the present time, biodiesel is being produced from the above list of oilseed plants, most of which are used also for food manufacture. Work is being done, however, to produce the oils from plants that are not needed for food (such as rapeseed) and especially those that can be grown in poor soil, such as jatropha.

Both liquid fuel processes require large land areas on which to grow their raw material, which is why the development of plants growing well in poor soil is important. Bioethanol can only be used in relatively small proportions in automobile engines, without changing the engine design. Biodiesel, on the other hand, can replace petroleum diesel with no effect on the engine.

The essential feature of the feedstocks for biofuels is that they are renewable, i.e. that they can be replaced, fairly quickly, having been formed by the sun. For many plant-based materials, this means on an annual basis, although two crops a year should be the target wherever possible. (It should be noted that this is a not very efficient way of converting sunlight into an energy source, combining a low capture efficiency with the costs of growing and harvesting.)

Equipment usageThe equipment used in liquid biofuels manufacture is a mix of filters and centrifuges, those for bioethanol production being effectively the same as in brewing. The biodiesel process is more complicated, with centrifuges especially important in the vegetable oil refining stages – as modified for biodiesel production. A typical process might require a separation step:

• For the expression of the oil, and then to clean the crude oil, by filter or centrifuge;

• For degumming the crude oil;

• After bleaching, and/or deodorising, and/or winterising;

• Before transesterification;

• For separation of product oil from glycerine;

• For byproduct separation and methanol recovery; and

• For final product quality.

Future trends

There are obviously no easy paths to a future with limitless sustainable energy availability that creates no environmental impacts. The most likely scenario would seem to be:

a. continued use of fossil fuels, with soaring demand in places, using enhanced recovery where possible;

b. greater emphasis on energy use efficiency;

c. return to growth in investment in nuclear fission power;

d. development of clean coal processes, especially by conversion to gaseous or liquid fuels;

e. extensive deployment of CCS processes, to reduce carbon emissions; and

f. rapid growth in production of liquid biofuels, coming from non-food crops, possibly via a biomass refinery.