The chemical steps involved in the pathway from cellulose to ethanol are similar in principle as starch to ethanol. With the help of enzymes or heating with acid, the cellulose is broken down to simple sugars, which are then fermented by genetically engineered bacteria or yeast to ethanol.
Lignin is a source of many chemicals (e.g. phenolic chemicals for disinfectants, vanillin, wood adhesives and tannin for leather processing). Alternatively, like any other biomass (e.g. food waste, garden waste and farm animal slurry) it can be: (i) used to generate methane gas in anaerobic digesters; combusted in electricity generation; or (iii) gasified to syngas (mixture of CO and H2 sometimes contaminated with CO2). (See article on Coal gasification).
The acid-heat treatment stage required to open up the lignocellulose structure for access to enzymes is energetically and financially costly, but progress is being made in identifying microbes and the particular genes involved that that will both break down the lignocellulose and ferment the sugars from cellulose to ethanol. Through genetic manipulations of these microbes therefore, the efficiency of production should be enhanced.
Although lignocellulose is the most abundant terrestrial biomass form and therefore most research is concentrated on processing it for ethanol production, plant oils have distinct advantages. As referred to earlier, oils have twice the energy density of carbohydrates and can be processed into biodiesel with lower energy inputs, higher net energy output and CO2 emission reduction.
Oil from the seed of the plant Jatophra is poisonous, grows on marginal land unfit for food crops, even in desert environments. Oil yields from Jatophra cultivated in India have been quoted as 3000 kg/hectare, compared with 375 kg/hectare and 1000 kg/hectare for (edible) soybean in US and (edible) rapeseed in Europe respectively. No wonder then that it has been heralded as having enormous potential as a source for biodiesel production. However, this hype has been tempered by studies that indicate that although it will survive on marginal land with minimal inputs, it will not provide the yields referred to above without significant inputs of nutrients and water or it is cultivated on better soils that then brings it into competition with food crops. To overcome these problems much attention is being focused on genetic modification to increase crop yields.
Similar research is focused on soybean and other major oil crops to increase their yields. About 80% of US-produced biodiesel is from soybean, which is mostly GM (>90%). GM strategies have largely been concerned with increasing levels of oil in seeds, but it has been suggested that it may also be possible to increase the oil content of leaves and stems where oil is not normally concentrated. The usefulness of Miscanthus would be enhanced if GM could lead it to accumulate oil in its leaves so that biodiesel as well as bioethanol could be derived from the plant.
A further strategy is being explored to reduce the input of energy for biodiesel production, which normally requires two essential steps: firstly the extraction of the oils and secondly chemical processing (referred to as transesterification – see Figure 2) to convert the extracted oils to the final product. Techniques of GM are being explored so that the plant synthesises novel oils that can be used directly as fuel after extraction. Removing the second step would significantly reduce production costs.
The above discussion indicates there is considerable research activity, but also that are many hurdles that include technical, production costs, determination of environmental impacts and whether there is political will for governments to invest in the substantial research and development costs. These hurdles need to be overcome before second generation biofuels become commercially widespread and can significantly replace petroleum-based fuels.
Third Generation Bio Fuels
Whilst second generation sources for biofuels alleviate to a significant extent the food-energy competition, they still need agricultural land for growth, although some (e.g. Jatophra and Switchgrass) will grow on marginal land. Third generation sources do not take up valuable land, which then can be reserved for food crops. The prime examples of such a source are algae, a diverse group of photosynthetic organisms. The multi-cellular forms may be referred to as macroalgae; a familiar example is seaweed. Unicellular ones are microscopic and referred to as microalgae.
Most research has been focused on microalgae, which can be grown on marine ponds (freshwater or seawater), or in large reactor vessels with a light source for photosynthesis (photobioreactors). Intensive research and development is underway across the world in the use of algae to produce biodiesel.
The Carbon Trust has initiated a research project, the Algal Biofuels Challenge, involving 12 research teams throughout the UK (including the University of Newcastle). The University of Durham has a team looking into the modification of microalgae to produce biofuels and other useful chemicals.
In addition to not using agricultural land, microalgae have further advantages over plants:
1. They have the potential to produce 100-fold more oil per given area than any land plant.
2. They can be genetically engineered and/or grown under different conditions to produce ethanol, lipid or hydrogen. This biological production of hydrogen (biohydrogen) is considered to have enormous potential because it has the advantage of not producing any CO2 when combusted.
3. It may be more convenient to use microalgal growing systems in photobioreactors than plants to couple with CO2 - emitting industries so that the microalgae could capture the CO2, using it for growth. (See page 4 of Royal Society INSIDE SCIENCE Spring 2011, describing an innovation to use algae to capture the CO2 from steel production.
4. There is a multitude of different algal species. It is considered unlikely that even the most thorough search will not find a single species that will provide all the necessary traits (high growth rates on low cost media, high oil contents, making valuable co-products) to enable algal biofuel production to compete economically with petroleum-based fuel. The techniques of genetic engineering (to transfer genes expressing favourable traits to improve a particular strain) and synthetic biology (adding together desirable genes from a variety of species to produce a single efficient strain) are most probably needed. These are the approaches being pursued in research institutions around the world.
Non-photosynthetic autotrophs that synthesise important industrial organic chemicals from CO/CO2
A group of bacteria called Clostridia are able to grow autotrophically or heterotrophically depending on conditions. Strains of the particularly useful Clostridium ljungdahlii have been genetically engineered so that when presented with a feedstock of CO, CO2 and H2, they grow autotrophically, producing butanol and ethanol under the autotrophic conditions. (Butanol (C4H9OH) is an alcohol with a higher calorific value than ethanol.) The feedstock could be syngas (a mixture of H2 and CO sometimes contaminated with CO2) made from the gasification of coal or biomass (see article on Coal Gasification). Equally, certain industrial processes, which emit these same gases as waste could provide the feedstock. Indeed a patent has been awarded in the US on processes that use these bacteria to synthesise a variety of chemicals (including the alcohols butanol and ethanol for biofuel) from industrial waste gases. This could in future have a significant impact on CO2 emissions and reduce dependency on petroleum.
11 September 2011 (revised 8 July 2013)