Rice straw to Biogas (R2B)

The research at Southampton seeks to maximise biogas from rice straw through the use of innovative pre-treatment techniques, evaluation of nutritional requirements of the anaerobic consortium, and optimisation of the bioreactor in terms of its configuration and physical environment. The work will build on the rich but fragmented knowledge which represents the state of the art in rice straw digestion. This suggests that mechanical pre-treatment may be effective, although reliable quantitative data are not available. Batch methane potential tests will be carried out to assess the reaction kinetics of the straw after the application of mechanical treatments. The flail mill is targeted as a preferred technology because of its simple and robust construction, lack of reliance on cutting blades that are rapidly blunted in rice straw applications, and history of successful use in developing countries e.g. in the tea industry. Its performance will be compared with that of other systems such as hammer and ball mills. The batch tests will be followed by more extensive trials using semi-continuous fed digesters to establish specific and volumetric gas productivities and digester operational stability. Compositional analysis of the straw will indicate not only of its theoretical methane potential but also its nutritional value and buffering capacity, as a basis for selective additions of essential elements to improve digester performance. The semi-continuous trials will be conducted at both mesophilic and thermophilic temperatures over a range of loading conditions, with monitoring of key operating parameters to establish baseline kinetic data against which pilot plant performance can be compared and optimised. Prior art also suggests that rice straw may be deficient in natural pH buffering and performance can be enhanced through co-digestion with animal slurries. To date, work has mainly focused on cattle and swine manures. The planned work will additionally consider duck and poultry manure, as these are readily available in the Philippines and other parts of the world where rice is extensively cultivated. Both shredding and manure addition will significantly alter the rheology of the substrate, which will impact on the operating mode of a dry digestion system. Permeability tests will therefore be conducted at various stages through the batch dry digestion process. This will involve running small-scale 'dry' digesters that simulate the pilot plant and provide data for direct comparison and assessment of factors of scale. The propensity for acidification will also be assessed, and the potential requirement for second-stage methane recovery from the leachate will be considered: if necessary this could be achieved by coupling to a low-tech second stage reactor (e.g. anaerobic filter) with recycling of acid-stripped liquid to overcome buffering issues. Other control measures, such as batch sequencing of the dry digesters and optimisation of the substrate-to-inoculum ratio at start-up, will also be evaluated as part of an operational strategy to match harvesting schedules and minimise storage requirements. Where permeability is an issue the use of inert bulking material will be tested to improve hydraulic distribution and reduce short circuiting. Previous work has shown that agricultural residues with a high solids content require long operating periods to reach the pseudo steady state conditions needed to determine the nutritional needs of the anaerobic consortia and the digestate properties. This provides the opportunity to monitor changes in biomass characteristics and population structure using advanced microscopy, Raman spectroscopy and gene techniques. Of key importance is the overall sustainability of the rice straw energy production system, and the laboratory and pilot plant data will feed into previously developed energy balance models to provide processed data output for the work of the Manchester team.

Biomass is a key component in all policy thinking on long-term solutions to sustainable energy production, and in particular to meeting the demand for substitution of biofuels for mineral oils and natural gas. Anaerobic digestion has already demonstrated that it can play a valuable role in meeting these policy objectives: in the UK for example, food waste digestion already accounts for over 1GW of generation capacity. One of the major stumbling blocks to the introduction of new substrates, as was seen in the case of food waste digestion, is process optimisation to meet the unique characteristics of different biomass types. Considerable effort has already focussed on the use of chemical, enzymic and thermal pretreatments of cereal straws for the production of fermentable sugars for ethanol production, and these 'high tech' solutions are undoubtedly necessary for this scenario. It is not proven that these approaches are necessary or even desirable for anaerobic digestion, and in some cases they may even have negative effects (c.f. autoclaving of food waste). There is, however, ample evidence to suggest that particle size reduction can improve digestability of fibrous substrates and there are reports that this has been successfully used as a pre-treatment of rice straw in a small-scale demonstration biogas plant in India. If this improvement can be quantified and optimised to produce a favourable overall energy balance it opens up the potential for energy production from a huge, as yet untapped, biomass resource with a worldwide impact.

To achieve this goal in an environmentally benign and sustainable manner will have additional beneficial impacts. It would reduce or eliminate the burning of straw stubble in rain-fed dry harvested production systems and the decomposition of organic sediments to fugitive methane emissions in irrigated bunded field production systems, both of which have serious environmental consequences on a regional and global scale. Recent reports indicate that rice straw burning contributes significantly to air pollutant levels in the region, while the IPCC has estimated methane from rice paddies could account for 5-20% of total anthropogenic emissions.

Worldwide over 150 million hectares of land are used for rice production, giving 700 million tonnes of product and an estimated waste straw yield of between 560-840 million tonnes per year. The methane potential of this material could be between 200-350 m3 per dry tonne which at the lower end estimate is equivalent to 96 million Tonnes Oil Equivalent, or around 2.5% of current production capacity. The impact is particularly significant as this resource is available to energy-poor communities and offers empowerment through provision of decentralised supplies.

A further major local impact will be the potential for health improvements in communities which lack access to clean fuels for cooking. Where wood and straws are burnt as cooking fuel the smoke produced is one of the biggest factors in reducing life expectancy and quality of life. A number of projects in developing countries have shown that biogas can be cleaned and bottled using relatively unsophisticated equipment and substituted for wood as a cooking fuel. This also has the environmental benefit of reducing one of the main drivers of deforestation.

A key element in the proposed work is to develop local solutions that can provide energy at a community level, using advanced, but appropriate, technology that can be prefabricated from proven designs. This may have further impacts in creating a global market for the technology providers, as rice is produced in over 81 countries.

The expected impacts thus address all three aspects of the energy trilemma by reducing emissions, improving security of supply and reducing costs through providing an affordable technology, while specifically addressing the needs of developing countries.