Laminar Burning Velocity Measurements Over Wide-Ranging Temperatures and Pressures for Renewable and Conventional Fuels

Laminar burning velocity measurements are needed for models that assist development of clean and efficient combustion in engines, boilers and furnaces, and for the validation of laminar burning velocity models. Our laminar burning velocity measurements are made in a spherical vessel with central ignition, so that a spherical flame front forms and propagates radially. During combustion the pressure rises and the unburned gas ahead of the flame front is compressed isentropically. So, from a single experiment, laminar burning velocity data are obtained for a sequence of linked temperatures and pressures. By varying the initial temperature and pressure the effects of pressure and temperature can be decoupled, and correlations generated for the effect of temperature and pressure on the laminar burning velocity. The schlieren system can detect the onset of cellularity (even when the flame is larger than the 40 mm diameter windows) so that we can avoid using data that violates our smooth flame front assumption. This builds on earlier work at Oxford over 16 years that has led to 3 innovations:+ Free-fall experiments to eliminate the effect of buoyancy.+ A multi-zone combustion model for data analysis, so that the effect of dissociation and the temperature gradient in the burned gas (typically 500 K) is incorporated into the analysis of flame front position and pressure rise.+ The use of 'real residuals' by retaining part of the previous combustion event as residuals, as opposed to the conventional approach of using a fixed composition N2/CO2 mixture to represent the residuals.Our facility has a comprehensive LabView interface for setting-up the experimental conditions and data logging. This system also ensures that condensation of either fuel or water vapour (in the residuals or as a diluent) is avoided. A schlieren system with a high speed video camera records early flame growth and cellularity (the departure from a smooth flame front, if it occurs). The experimental data are analysed by MATLAB routines that incorporate: image processing (of the schlieren system data), a multi-zone combustion model, and experimental pressure data; the code also combines data from multiple experiments in order to generate correlations for the laminar burning velocity. Initial conditions can be up to 450 K and 4 bar (final pressure limit of 35 bar), with combustion data obtained up to 30 bar and an unburned gas temperature of 650 K. Liquid fuels (or diluents such as water) can be added by a Hamilton precision glass syringe which is controlled by a syringe actuator. This facility and software are readily adaptable for testing different fuels.The combustion of fuels from renewable sources and their performance when combined with conventional fuels is very important. In 2008 the UK crude oil consumption was 78.7 Mt (~20% gasoline) - BP Statistical Review of World Energy; June 2009. EU legislation requires bio-fuel to become a minimum 5.75% of the total fuel consumption in 2010. Gasoline vehicles can mostly operate on a 10% ethanol 90% gasoline (E10) blend with no adverse effects. But, to exploit the potentially higher octane rating of E10 and its different combustion in engines, laminar burning velocity data for ethanol and its mixtures are needed. Ethanol is mostly simply made from the fermentation of sugars, but competition with food use means that second generation or cellulosic-ethanol needs to be exploited. Ethanol has been produced from cellulose for over 100 years, but there is now a rapid increase in the commercialisation of the process (http://en.wikipedia.org/wiki/Cellulosic_ethanol). Gaseous fuels from renewable sources depend on the processing route. The anaerobic digestion of waste (by mesophilic bacteria) produces biogas (which is 60-70%CH4, and 40-30%CO2), whilst pyrolysis of waste or biomass produces syngas (a partial oxidation process that gives typically 40% CO, 25% H2, 20% H2O, 15% CO2).

The 'STERN REVIEW: The Economics of Climate Change' concludes that the scientific evidence is now overwhelming: climate change is a serious global threat, and it demands an urgent global response. The overall costs and risks of climate change will be equivalent to losing at least 5% of global GDP each year, now and forever. In contrast, the costs of action - reducing greenhouse gas emissions to avoid the worst impacts of climate change - can be limited to around 1% of global GDP each year. (http://www.hm-treasury.gov.uk/sternreview_index.htm HM Treasury 2006 ISBN number: 0-521-70080-9). Net greenhouse gas emissions can be reduced by the use of fuels from renewable sources, such as cellulosic ethanol, biogas and syngas. However, to ensure their optimum use (high efficiency and low emissions) it is critical to have knowledge of their combustion performance, for which their laminar burning velocity is the most fundamental property. In the case of ethanol, as it will be used in blends with gasoline, it is essential to study the combustion of ethanol mixed with representative gasoline components (iso-octane, n-heptane, toluene), and to investigate mixing rules. Economic, Social and Scientific Benefits - Road transport accounts for 21% of the CO2 emissions in the UK, and the great majority of vehicles are powered by hydrocarbon fuels. Liquid hydrocarbon fuels (including biofuels) are likely to remain the major energy source for transportation for the foreseeable future (i.e. up to 2050). Effort is required to develop new fuels, and reducing NOx, CO2, unburned hydrocarbons and particulates is of increasing importance. The combustion of second generation bio-fuels needs to be understood, so that their use with hydrocarbon fuels is optimised. In addition, new gasoline engine technology is likely to enter the market within the next 5-10 years with considerably different combustion characteristics. It is important for both the fuels and automotive industries to understand the fundamental combustion properties of gasoline components and maximise the energy efficiency of these new engines in order to reduce greenhouse gas emissions. Shell is very active in the area of fuel economy, with global marketing campaigns such as Get the most out of every drop . The correlations for laminar burning velocity and comparisons with modelling will be published in refereed journals (Combustion and Flame, Progress in Combustion Science and Technology) and international conferences (International Combustion Symposium, SAE Congress). A WWW based database will be created, and this will be particularly important for the data on cellularity, as modelling the onset of cellularity (the departure from a smooth flame front) is beyond the scope of this project. Informal contact will be maintained with Andrew Clarke (a former research student who is now a Senior Lecturer University of Loughborough, who uses similar techniques) and University of Leeds (Malcolm Lawes and colleagues) who use complimentary techniques. We will maintain contact with Northeastern University, Eindhoven, and the University of British Columbia. The Research Student will be trained in advanced methods of research, and gain transferable skills in writing reports, giving presentations, writing papers, and programming (LabView and MATLAB). The student will spend 3 months at Shell Thornton to learn the use of CHEMKIN supported by Shell's own experts and Marie Curie Fellows in the second year.