Cellulose: An Opportunity For New Zealand Biofuel

Cellulose: an opportunity for New Zealand biofuel

Although biofuels have become a welcome answer to the shortening supply of fossil fuels, they remain shrouded in controversy. First generation biofuels include; ethanol made from sugarcane, and maize or biodiesel made from vegetable oils. One of the disadvantages of these types of fuels is that they utilise crops which would normally be grown for food purposes. As a result there is a growing concern that global food shortages will result if too much land is converted for this purpose (Marchbanks, 2008). Second generation biofuels are currently being developed utilising feedstocks that are non food crops, such as plant biomass (cellulose) and algae. It is generally accepted that these types of processes will start becoming commercially viable from 2016 (Hale & Twomey, 2006). This essay focuses on the technical issues in converting cellulose to ethanol and its application to the New Zealand biofuel industry.

Lignocellulose makes up the cell walls of plants and is comprised of cellulose micro-fibres embedded in a matrix of lignin and hemicellulose. The crystalline nature of cellulose makes it much more resistant to chemical or enzymatic breakdown than the starch derived from maize. There are several methods for converting lignocellulose into alcohol products, which are well explained by Granda and Holtzapple, 2007. In most cases the plant matter undergoes a pre-treatment step involving milling/grinding to decrease biomass size and chemical treatment with dilute acid to separate the cellulose, lignin and hemicellulose. At this stage the biomass can be either fully or partially hydrolysed with various combinations of acid, heat and pressure. Full hydrolysis breaks down both the hemicellulose and cellulose into pentose and hexose sugars. Partial hydrolysis only affects the hemicellulose, and enzymes are used to break down the remaining cellulose (DemÐ"ЇrbaÐ*ÑŸ, 2005). It takes a minimum of four types of cellulase enzymes to carry out the saccharification reaction, which is similar to that which goes on in the stomachs of cows and sheep (Chang, 2007). The advantage of using enzymes over full hydrolysis is that it avoids producing toxic chemicals such as hydroxymehtyl furfural which are inhibitory to fermentation organisms.

Fermentation occurs in two stages, one for the pentose and the other for the hexose sugar. Currently no organisms can efficiently convert both sugars, which would be an opportunity for genetically designed yeast. The dilute ethanol solution is then distilled and dried in a similar fashion to the corn-ethanol industry. The resulting ethanol can then be directly blended at low levels with fossil fuels. The unfermented lignin based residue can be burnt to provide energy for the process. Since 2004, Iogen, a Canadian based company has operated a demonstration plant that produces 50 tons of ethanol a day from plant biomass using the enzyme based method (Granda and Holtzapple, 2007). Although the process is technically more difficult than first generation biofuels, the net energy output is much greater than standard biofuels, as the feedstock requires little fertilisation or labour. Mitchell, Perrin, Schmer & Vogel (2008) conducted a field trial with switchgrass (Panicum virgatum L.) and showed that it could be cultivated and then converted to ethanol with an energy output of over 500% compared to the energy input.

The New Zealand government has recently committed to the use of biofuels, making it mandatory for fuel companies to incorporate biofuels as 3.4% of sales from 2012 onwards (NZ Cabinet, 2007). Currently, tallow, and to a much lesser extent, whey and maize are the only realistic feedstocks currently in the country. Tallow could account for up to 150 million litres of biodiesel, or in other words 6 of the targeted 7.25 petajoules (1015J) that will be needed by 2012, the remainder would have to be imported (M.o.T, 2008; Hale & Twomey, 2006). Fortunately, New Zealand is in a unique situation where it has ideal land and infrastructure to use plant biomass as an ethanol feedstock. Cellulose based ethanol would easily be able to cover the upcoming biofuel deficit and even allow for exports. Taking the radiata pine industry in 2004, if all the residual wood from harvesting was collected then approximately 250 million litres of ethanol could have been produced using the new technology (Hale & Twomey, 2006). Other viable options are to plant short rotation tree crops such as eucalyptus, or grasses such as miscanthus. These can be grown on low fertility land and more rugged terrain than would be suitable food crops. Hale & Twomey (2006) estimated that 20,000 Ha of land would need