In a number of scenarios of the global food and agriculture system in 2030, we examine to what extent increases in livestock and crop productivity, and changes in human diets, may expand the bioenergy potential. The results from the scenarios indicate that if the recent projections of global agriculture made by the FAO come true, the prospects for bioenergy plantations will be less favorable. In our scenario depicting the FAO projections, it is estimated that total agricultural land area globally will expand from current 5.1 billion ha to approximately 5.4 billion ha in 2030, leaving little room for a major expansion of bioenergy plantations(Wirsenius 2003).

Biofuels hold out the promise of a win-win-win solution

 Biofuels will reduce greenhouse gas emissions, promote energy independence, and encourage rural development.

 This enthusiasm translates into significant government support. Annual global subsidies for biofuel production were $11 billion in 2006 and could rise to $50 billion by 2020.

 Many governments have enacted new pro-biofuel policies in recent years.

Developed country governments like the UK and EU have set consumption targets for biofuels

Next generation biofuels can reduce negative impacts.

The first to arrive will be lignocellulosic biofuels made from the lignin and cellulose in the cell walls of plants.

Cellulose :

Plants produce about 180 billion tons of cellulose per year globally, making this polysaccharide the largest organic carbon reservoir on earth. Cellulose makes up 15–30% of the dry mass of primary and up to 40% of secondary cell walls, where it is found in the form of 30 nm diameter microfibrils.

Hemicellulose

Cellulose microfibrils are coated with other polysaccharides such as hemicellulose or

xyloglucans. All dicotyledonous cell walls and about half of monocotyledonous ones consist mainly of xyloglucans. Like cellulose, hemicellulose could be converted into fermentable sugars by enzymatic hydrolysis for the production of cellulosic ethanol.

Pectin

About 35% of dicotyledonous plant dry matter is made up of pectin, a mixed group of

various branched, hydrated polysaccharides that are abundant in galacturonic acid.

Lignin

Lignin is a major constituent of secondary cell walls, and accounts for about 10–25% of

total plant dry matter. Lignin is composed of a complex of phenylpropanoids (aromatic

compounds) linked in a network to cellulose and xylose with ester, phenyl and covalent

bonds.

The feedstocks for these biofuels – trees, grasses, or leftover plant materials – have several potential advantages.

They require less intensive agriculture and may be grown on “marginal” land, reducing competition for resources.

Lignocellulosic biofuels could be made from agricultural or forestry residues such as rice husks and corn stover.

A 2007 UN report estimated that these biofuels would be commercialized by 2015 and become competitive with petroleum-based fuels in the next 10-15 years.

Bioethanol and Metabolic engineering

Bio-ethanol can then be used as a cooking, lighting and automotive fuel. As for low-income countries, local production of bio-ethanol from locally grown crops can cut dependence and cash expenditure on imported fuels, increase community self-reliance, and provide a spur for local job creation and growth. It can also cut dependence on fuel wood, which is often scarce and causes serious health problems through indoor air-pollution. For example, 2.5 t of bio-ethanol (corresponding to approx. 1.6 toe) used as a cooking fuel in adequate stoves (40% efficiency) could potentially displace 9 tons of wood-fuel (considering for the latter an energy content of 0.36 toe/t, used in traditional stoves with a 20% efficiency).

Sweet sorghum produces diversified products: starchrich grains and sugar-rich stalks. While grain typically yields 2-4 t/ha, the stalks (53-68 t/ha) can provide liquid sugar (5 t/ha) and bagasse (25-30 t/ha).The sugar-containing juice (28-32 t/ha) is removed from the plant with mechanical presses or by means of extraction with water or possibly by centrifugation. After extraction, sugar juice is converted into a “beer” which is distilled to low-grade ethanol (95%).On the basis of a total above-ground fresh biomass yield of 70-90 t/ha, a sugar yield of 6-8 t/ha can be expected, which in turn can result in approximately 2.5 t/ha of bio-ethanol. The combustion of the leftover bagasse, which amounts 25-32 t/ha, could theoretically provide enough heat for the ethanol distillation. However, in the present case, bagasse is preferably used for compost production as soil fertility recovery is the main objective. The thermal energy source for distillation can vary, and some simple solar distillation units could be used in specific situations. In India, a pilot solar distillation was built, consisting of 38 m2 of plate solar collectors coupled with a hot water storage tank of 2.15 m3. The unit operated at 50-70°C, and on a 4,000 hours period, it produced 30-40 L/day of 95% ethanol

Recent advances in synthetic biology and metabolic engineering suggest that, rather than limiting ourselves to fuel molecules provided by nature, we should engineer microorganisms to produce new fossil-fuel replacements (Keasling 2008). Such products, which might include short-chain, branched-chain and cyclic alcohols, as well as alkanes, alkenes, esters and aromatics. To produce longer-chain alcohols and alkanes, it should be possible to tap into the fatty acid pools of nearly any organism. Sequential reduction, decarboxylation or decarbonylation followed by reduction of fatty acids to alcohols and alkanes could yield valuable fuel candidates. It is also possible to esterify fatty acids with alcohols from any number of sources to produce candidate biodiesels. Isoprenoid biosynthesis offers an even richer source of next-generation biofuels. With the ability to produce branched-chain and cyclic alkanes, alkenes and alcohols of different sizes with diverse structural and chemical properties, this pathway could produce fuels or precursors to gasoline, diesel and jet fuel additives or substitutes. Efficient production of isoprenoid precursors has been engineered in E. coli and Saccharomyces cerevisiae, and many different isoprenoids have been produced using these engineered hosts.

Although production of next-generation biofuels is an important endpoint, it will be critical to first tap into sugars, the most inexpensive starting materials at our disposal, to make these new fuels economically viable.

In the long term, these fuels will certainly be produced from less expensive sugars, such as those released from cellulose depolymerization.