China bioethanol pump price for five enzyme loading scenarios in (a) 2011 with a 16¢ per liter (60.6¢ per gallon US) subsidy and fuel excise and value-added tax exemptions, and (b) a prospective future scenario with no form of government support measures. Click to enlarge.

In 2011, China contributed 29% of world carbon dioxide emissions and therefore the country has a significant potential to influence the present and future global energy situation. Currently, almost half of China’s oil consumption is imported, and with the projection that demand for fossil fuel oil will reach 250 million tons by 2030, it is crucial for the country to consider biomass alternatives as part of their renewable energy plan. In 2009, the number of private cars owned in China exceeded the United States, resulting in it being the world’s largest auto market. Establishment of a biofuel industry in China is therefore an attractive solution to manage the problems of environmental pollution, energy independence and rural development within the transport sector.

In its development of biofuel policy, China’s 10^th five-year plan (2001–2005) proposed a biofuel industry to utilize surplus grain stocks. Through the government’s support for biofuel production, China has become the third largest bioethanol producer in the world after the US and Brazil, with an overall fuel ethanol production capacity of 1.9 million tons in 2008. Now, approximately 10% of the total liquid fuel supply is accounted for by biofuels, and there has been an increase in pilot plant projects cropping up in Henan, Anhui, Jiangsu and other provinces. However, concerns regarding food security resulted in the government’s order to halt construction of corn-based plants and promote non-food feedstocks which can be grown on marginal and abandoned lands instead. … the lack of a key non-food feedstock that can be grown on such lands is the major constraint on the expansion of fuel ethanol production in China.

… As a member of the Graminae family, the composition of bamboo is highly similar to other grasses utilised for biofuel purposes (e.g. switchgrass, Miscanthus). Its cell wall is comprised of the polymeric constituents cellulose, hemicellulose and lignin. The complex physical and chemical interactions between these components prevent enzymes from readily accessing the microfibrillar cellulose during the saccharification stage of its conversion into biofuel. As a result of this recalcitrance, a pretreatment stage is needed to maximise hydrolysis of cell wall sugars into their monomeric form.

—Littlewood et al.

Schematic diagram of bamboo-to-bioethanol process in AspenPlus. Littlewood et al. Click to enlarge.

In their study, the Imperial College London team used liquid hot water (LHW) pretreatment to enhance sugar release from bamboo lignocellulose while minimizing economic and environmental costs. Pretreatments were performed at temperatures of 170-190°C for 10–30 minutes, followed by enzymatic saccharification with a commercial enzyme cocktail at various loadings. These data were then used as inputs to a techno-economic model using AspenPlus to determine the production cost of bioethanol from bamboo in China.

At LHW pretreatment of 190 °C for 10 minutes, 69% of the initial sugars were released under a standardized enzyme loading. Under this condition a greater proportion of sugar was released during pretreatment compared with saccharification, whereby the predominant sugars were xylose and glucose in pretreatment and saccharification, respectively.

They also found little improvement in total sugar release despite significantly increasing enzyme loading; even at the highest dosage a portion of cellulose (about 20%) remained resistant to enzymatic hydrolysis. Enzymatic saccharification with five loadings (10–140 FPU/g glucan) of Cellic CTec2 led to a total sugar release ranging from 59-76% of the theoretical maximum.

The economic analysis found that the lowest enzyme loading had the most commercially viable scenario (production cost of $0.484 per liter (US$1.83/gallon US) with tax exemption and a $0.16/liter (US$0.606/gallon US subsidy)) even though it produced the least amount of bioethanol and generated the greatest level of co-product electricity.

This economic result was primarily due to the significant enzyme contribution to cost, which at higher loadings was not defrayed adequately by an increase in the amount of sugar released, the team said.

A cost breakdown and sensitivity analysis of the 10 FPU/g glucan scenario demonstrated that the cost of raw materials was the greatest contributor, with bamboo and enzyme purchase accounting for 51% and 17% of the MESP, respectively.

The supply-chain model showed that bamboo would be competitive with gasoline at the pump in scenarios with enzyme loadings of 60 FPU/g glucan and lower. However the prospective scenario, which made the assumption of no tax breaks or subsidy, demonstrated that lower enzyme loadings would still permit bioethanol from bamboo to maintain its economic competitiveness with gasoline under the technical conversion efficiencies modelled.

Alternative approaches to reduce bioethanol production costs are still needed however, to ensure its competitiveness in a possible future scenario where neither tax exemptions nor subsidies are granted to producers. These measures may include improving sugar release with more effective pretreatments and reduced enzyme usage, accessing low cost bamboo feedstock or selecting feedstocks with higher/more accessible cellulose.

—Littlewood et al.

Resources

• Jade Littlewood, Lei Wang, Colin Turnbull and Richard J Murphy (2013) “Techno-economic potential of bioethanol from bamboo in China,” Biotechnology for Biofuels 6:173 doi: 10.1186/1754-6834-6-173