Premium upgraded Biomass Solid Fuels - Fundamentals of torrefaction and performance of torrefied fuels

Biomass - vegetation such as trees, grasses or straws - is resurging as a source of sustainable, environmentally-friendly fuel for use in power stations. This is because, when grown in a sustainable way, it is almost carbon-neutral - the carbon-doxide emitted when the biomass is burned, is readsorbed from the atmosphere during the photosynthesis of the next crop of biomass. Consequently, there is a great deal of interest in using biomass in coal-fired power stations by substituting a portion of the coal. Today, many power-stations in the UK have adopted this co-firing approach to reduce their carbon (dioxide) emissions. This is a good strategy since the biomass is burned in the very large coal power stations which have a higher efficiency than the small systems needed if the same amount of biomass was to be burned alone. However, in the power stations the coal is crushed to a fine powder in huge mills before being blown into the burners in the boiler. Most biomass does not grind or crush very well because it is springy and fibrous. Consequently, when power generators attempt to powder the biomass in the coal mills it tends to form a mat on the bottom of the mill. This has limited the amount of biomass which can be processed in the mills and hence limited the amount of biomass used in the power-stations, and hence limited the carbon savings from co-firing biomass. Some power stations have invested millions of pounds to install separate, different types of mills for cutting biomass so that they can use more - for example, up to 20% by weight is used in Fiddlers Ferry power station. Another strategy is a process known as torrefaction in which the biomass is pre-treated so that it becomes more brittle and easier to crush. This process involves heating biomass to a moderate temperature (~280 C) in the absence of air. It is similar to the process used to roast coffee beans and so is sometimes refered to as roasting biomass. During torrefaction some material is lost from the biomass - particularly moisture and some gases and volatile substances - but the material which is left, the residue, still contains typically 80% of the heating value of the original biomass, and is transformed into a harder, darker fuel, which is much easier to crush. This process is attracting a great deal of interest from all sectors involved in the bioenergy chain: - growers see this is a way of adding value to the biomass they grow and reducing transportation costs (since the fuel is dry and has a greater energy per unit volume); power-generators see this as a simpler fuel to handle in the power stations; and there is also interest in using torrefied biomass as a fuel in other conversion processes, such as biomass gasification to liquid (transport) fuels (BTL). Furthermore, torrefied biomass does not go mouldy upon storage like raw biomass and so it becomes attractive for extending the supply window for using biomass. In order for torrefaction of biomass to happen on a large scale much information is needed in order to design safe, environmentally-friendly torrefiers. This research is aimed at providing much of this information and answering these questions: What are the explosion risks within torrefiers or mills using torrefied biomass? (Fine dust can result in explosions under certain concentrations, and knowledge of these concentrations is needed in order to incorporate adequate safety design features.) What would the effluents from the process (liquid and gas) be composed of? Can the gas and vapours produced provide the heat to drive the torrefaction? How would torrefied biomass burn in the power station? It also aims to develop a tool which engineers can use to help them design the torrefier itself, so that they know what temperature is needed, and how long the biomass needs to reside within the torrefier so that an optimum fuel is produced.

Torrefaction of biomass is attracting a great deal of interest from industry at the present time, for a number of reasons, which include the improvement in storage options, and in milling properties of biomass. Hence, the industrial beneficiaries from this research will be in the area of biomass co-firing by power generators and the biomass pulverisation equipment suppliers. There is also synergism with the area of biofuels for bioheat on a large scale, as the combustion problems are similar, and torrefaction is already being considered in integrated processes with pellet production. This work is directed at 100% biofuel firing as well as co-firing applications. The work is also relevant in the longer term in terms of the use of torrefied biomass in entrained flow gasifiers, an area that is gaining interest for the production of transport fuels. Because torrefaction is an energy densification process, it could be utilised near to biomass production, and the torrefied biomass transported to large combustion or gasification processes with considerable green-house gas savings, and better revenue for biomass suppliers. The equipment suppliers for the biomass pulverisation and torrefaction equipment will benefit from this work through the provision of explosion protection data for the equipment. Both power generators and equipment suppliers will benefit in regard to their future fuel handling design options. The fundamental combustion data of devolatilisation and char combustion rates, together with nitrogen-partitioning, both during torrefaction and during combustion, will enable appropriate NOx reduction strategies to be selected. The UK Government and society will benefit due to the reduction in non-renewable CO2 emissions from the greater use of co-firing of biomass in power generation using current installations. Torrefaction of biomass should result in a greater proportion of biomass being co-fired in a boiler and hence a greater reduction of CO2 from existing power plants. Many of the UK's coal fired power stations have life extensions through turbine and boiler tube retrofits that will keep them operating for decades to come. Reducing CO2 from these plants is essential if the UK Government's targets for CO2 reduction are to be contributed to by the existing infrastructure for coal based power generation. The explosion safety data for biomass and torrefied biomass that this project will generate is also of benefit to society through the provision of data that will enable any biomass pulverisation plant or other biomass handling plant with a dust hazard to be safely designed. The information will also be relevant to the HSE in issuing guidance on the safe operation of biomass plant. The implementation of biomass into the UK energy mix will be of benefit to society as a whole through the creation of employment and economic benefits in many sectors including agricultural, biomass resources and procurement, power and heat generation, and related industries such as environmental monitoring, health and safety. Two PhD students and one research fellow will be added to the workforce training in this important subject area for CO2 reduction. There is a scarcity of this expertise in co-firing with biomass and in the importance of torrefaction. The dissementation and knowledge transfer plan includes (1) publicity via CampusPR, lecutres, CPD, articles (2) workshops and meetings with stakeholders and user groups, (3) an industrial advisory group consisting of identified partners; (4) development of new partnerships and exploration of joint ventures . Key deliverables from the impact plan have been identified throughout the 42 month project, and will be driven by the research team, led by the PI, with assistance from University teams such as the CPD office, Faculty Enterprise Team, Keyworth Institute and the Leeds Enterprise and Innovation Office.