Features
Biomass, the Oil Replacement for the Future
The developing economies of China and India are increasing the demand for fossil fuels. The demand for the production of food crops is also increasing. At this time it takes about 1500 litres of petroleum energy to meet the annual food requirements for an adult American. In fact the energy needed to grow crops in the USA is greater than the energy released when the crops are eaten, and that has given rise to the expression 'Eating Fossil Fuels'. That scenario is not, of course, sustainable because fossil fuels are a finite resource, and far more oil is being extracted than is being discovered at this time. The crisis point will occur when it is no longer possible to increase the rate of oil extraction. Oil can only be extracted at a certain rate without damaging the structures that allow its recovery. Most predictions suggest that peak oil production is only a few years away.
Global Warming
It is accepted that the increasing levels of atmospheric carbon dioxide (CO2) is contributing to global warming. The atmospheric levels at this time are of the order of 380 parts per million by volume (ppmv) and these will reach 400 ppmv in about 10 years. However, controlled increased levels may be a boon because the world population of over six billion could not be sustained by food from photosynthesis processes if atmospheric CO2 should be at pre-industrial revolution levels. The probability that world population may reach nine billion by 2030 should focus emphasis on the more efficient recycling CO2 in order to produce the food needed. Levels in the Amazon Region are 320 ppm because of the intense recycling of the gas by the rapidly growing tropical plants.
There is the view that conventional renewable energy technologies (such as wind, solar, tidal, ocean current, hydro, etc.) can fill the fossil-fuel gap. Certainly these technologies can play a role, and probably meet the bulk of the electrical energy needs of Ireland. Nuclear power can also contribute significantly to world primary energy needs, but safety concerns and political risks work against it. It is important to consider, however, that all of these technologies will only directly replace the generation of electricity.
Fossil dependant
We rely on fossil fuels, and on oil in particular, for far more than power generation. The reliance of the transport sector on petroleum-derived fuels is well documented. Not so is the fact that the vast majority of modern products, such as chemicals, plastics, clothes, also come from oil. Crucially, more than 95% of all industrial chemicals are oil-sourced. Thus we should consider how our new-found wealth can be sustained in the era of peak-oil? We could well be among the first to feel the effects because of our lack of indigenous fossil-fuel reserves. We should not worry if we develop of our great national assets, i.e. our climate and soils that offer a huge potential for the production of biomass.
We joke about our abundant rainfall, but it, and our mild winters, allows the mass production of high-yielding, low input energy crops. We also have more good soil per head of population than any other country in the world, and it is sad to see that our appreciation of this is not reflected in the resource inputs needed for advanced research in areas of the soil sciences. The conventional notion of crops for chemicals and fuel is focused on the production of ethanol from crops rich in starch (cereals, potatoes) or sugar crops (sugar beet or sugar cane) and the production of biodiesel from oil-based crops such as rapeseed. These crops have high input requirements in terms of cultivation practices, fertilizer and herbicide/pesticide, and of course energy needs. Thus these are high cost crops requiring high levels of subsidies in order to maintain a viable income for the producers.
Cellulose rich
The ideal crops would provide high utilisable yields with low inputs levels. Lignocellulose biomass crops meet these requirements. Their major components are cellulose (40-45%) and hemicellulose (about 25%) and lignin (about 25%). Cellulose, the most abundant biogenic material above ground, is the major component of the vast majority of grasses and woods. It, like the starch in cereals, including maize, is composed of myriads of glucose units joined together by the elimination of a water when two units are linked by an oxygen atom between the number one carbon in one molecule and carbon 4 in another. Starch, like cellulose is composed of glucose molecules linked through the same carbon atoms in the amylose component. Branching takes place when similar linkages occur between carbon 1 at the end of a chain and carbon 6 in an amylose chain to give the amylopectin branch. Despite the similarities, starch and cellulose react differently. That is because the orientation of the linkage in cellulose (called the beta linkage) gives rise to a linear helix type of structure which allows the strands to come into close contact and are stabilized by hydrogen bonds. That is why cotton (a pure cellulose) gives long spinnable strands. The linkages in starch (called alpha linkages) allow the structures to assume a type of random coil or open structure. It is relatively easy for the hydrolyzing enzymes to break the structures into their component glucose molecules. The glucose can then be readily fermented to ethanol. The relative difficulty of cellulose hydrolysis has meant that, until recently, attention has been focussed on the more expensive starch or sugar crops for ethanol production.
Hemicelluloses are made up of the six carbon sugars (or hexoses) glucose, galactose, and mannose (with glucose as the most abundant of the three) the five carbon sugars (pentoses) xylose and arabinose, with xylose often the most abundant of the sugars. Hemicelluloses have alpha and beta linkages, and are more readily hydrolysed or broken down by the addition of water to their component sugars than is cellulose. However, the pentose sugars when fermented give poor yields of ethanol.
Energy potential
The energy crops with the greatest potential in Ireland include short rotation coppices, such as willow, and miscanthus, a high-yielding grass. Historically, most energy crop schemes have focussed on the combustion of these crops for energy or electricity. That should be considered as very great waste of their valuable chemical constituents. Because the carbohydrates of cellulose and hemicelluloses have relatively high oxygen contents, their heating values are quite low, especially when compared with fossil fuels. Yet oxygen is something that is highly valuable in chemical synthesis and and is often introduced, at high costs, to hydrocarbon-based molecules.
We should consider the concept of biorefining as opposed to combustion. This can be considered to be somewhat analogous to oil refining. In biorefining the large carbohydrate molecules (or polymers) are efficiently hydrolysed (by addition of water to the 'bridges' between adjacent molecules in the sequence) to their sugar (monomer) constituents from which a variety of compounds and products can be made. The lignin fraction is more difficult to break into its component molecules though in theory lignin can be a source of very valuable chemicals. There are various mechanisms for polysaccharide hydrolysis, including uses of acids (concentrated or dilute), enzymes, or steam auto-hydrolysis.
Alternatively, gasification technologies can reduce all lignocellulosic elements to simple gases (hydrogen, carbon monoxide, carbon dioxide) and these can then be recombined through Fischer Tropsch synthesis to chemicals and fuels such as ethanol. There is also potential in pyrolysis/liquefaction technologies that produce a bio-oil through the thermal decomposition of carbonaceous material.
Limerick studies
The Biomass Collaborative Research Group at the University of Limerick has examined numerous technologies. This Group has no vested interest in any of the technologies and its brief is to help in the development of a sustainable biomass-industry in Ireland and beyond. Some of the technologies studied may have potential for the future, but at this time the most commercially viable option is presented by the Biofine Technology. Put simply, the procedure involves the dilute acid hydrolysis of biomass, at high temperatures and pressures, in two reactors. In the first reactor the cellulose and hemicellulose polysaccharides are hydrolysed, and the six-carbon sugars are converted to hydroxymethylfurfural (HMF) and the 5-carbon sugars to furfural (a valuable industrial chemical). In the second reactor HMF is converted to levulinic acid (LA). The lignin fraction of the biomass is converted to a dry char which has a heating value comparable to that of bituminous coal. Char makes an excellent fuel and gives the process a surplus of energy that can be sold as electricity. It may also have potential as a soil ameliorant.
Cheap foodstuffs
LA is an excellent 'platform chemical', and from it a whole variety of products can be made, analogous to those from petrochemicals. The Biofine process has the capability to process any material (crop, residue or waste) with a reasonable carbohydrate content and moisture content of up to 50% means that prices LA can be produced for as little as 20 cents per litre, depending on the feedstock and the size of the biorefinery. Du-Pont Inc. have developed a range of products that can be synthesised from LA and research at the University of Illinois at Champaign-Urbana has shown the promise of the derivative DALA (delta amino levulinic acid) as an insecticide and plant growth promoter as well as an excellent agent in cancer therapy. Perhaps most excitingly, LA-derived additives to diesel and petroleum fuels have been developed. Methyltetrahyrofuran (MTHF) can be obtained in high yields by hydrogenating LA and shows great promise as a petroleum additive (up to 30% with no adverse affects in performance). Ethyl levulinate is produced by esterifying LA with fuel-grade ethanol and can be added up to 20% with diesel with no losses in miles per gallon and with significant reductions in emissions.
Miscanthus grasses would appear to have the potential to be a major biorefinery feedstock. Modelling by John Clifton Brown of Trinty College Dublin has indicated that miscanthus yields will be greatest in the Southwest, and trials at Adare by Dr. J.J. Leahy of UL and his colleagues would suggest yields of up to 30 tonnes per ha in a clay loam soil. The grower should expect an income of €1000 per ha from an average yield of energy crop, and without farm subsudues.
Changes in direction
Economic development and fossil-fuel derived energy have been closely linked in the past century. Before then development depended on the ability of mankind to efficiently utilise land. Perhaps our 20th and early 21st century obsession with hydrocarbon based fuels will become an historical blip in a much longer timeline associated with carbohydrate utilisation. The moves towards a modern carbohydrate-based economy are afoot. Soon we may well and bid farewell to petroleum, oil from rocks, and welcome "carboleum", oil from carbohydrates!
