Energy from biomass is inevitably important since it is renewable and widely available. There are three categories of biomass used for energy: solid, liquid, and gas. Solid biomass includes wood and wood waste, sulphite lyes (black liquor), other primary solid biomass such as plant matter, vegetal waste, and animal materials/wastes, and charcoal. Liquid biomass includes biogasoline, biodiesel, and other liquid biofuels; and gas biomass covers landfill biogas, sludge biogas, and other biogas. In terms of applications, solid biomass can be used to generate heat and power through combustion of the solid fuel, liquid biofuels can be used in transport applications because of the high energy density, while gaseous biofuel or biogas can be used for power generation. In general, the combustion of gaseous biofuel is cleaner than that of the solid biofuel; accordingly biogas combustion is expected to be important in our future energy utilisation which has to be cleaner.
Biomass plays a significant part in the EU’s ability to meet its renewable targets. If biogas and biofuels for transport are included, bioenergy is projected to account for more than half of the target of 20 per cent of renewable energy by 2020 – or 12 per cent of total European energy consumption. Biomass will account for 18 per cent of total electricity production (source: B. Atanasiu, The Role of Bioenergy in the National Renewable Energy Action Plans: A first identification of issues and uncertainties. Institute for European Environmental Policy, 2010). Globally, the trend is similar. The following diagram shows the world renewable energy consumption by application and sector, comparing that of 2010 and roadmap of 2030 (source: International Renewable Energy Agency (IRENA), Global Bioenergy: Supply and Demand Projections. A Working Paper for Remap 2030. September 2014). There is clearly a mode shift from heat generation to power generation, with a significant change in the ways we utilise biomass.
Of the three categories of biomass used for energy (solid, liquid, and gas), biogas plays a relatively small part in renewable energy utilisation, although there is a huge potential because of the wide range of availability. Biogas is normally produced by the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material, and agriculture crops. Despite being a renewable and cleaner fuel (than solid biomass) for combustion, biogas has not been broadly used in power plants such as gas turbine combustors. The main reason is that the complex fuel composition leads to unpredictable combustion performance. Hazardous combustion phenomena such as combustion instability can happen. Different fuel compositions may lead to very different flame distributions in a combustor. The wrong distribution may generate flashback or hot spots which might cause expensive damage to the combustor hardware. Typical biogas comprises primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of nitrogen (N2), hydrogen sulphide (H2S), hydrogen (H2), moisture and siloxanes etc. The fundamental issue with biogas combustion is associated with the significant variation in the fuel composition that changes the flame speed, heat release rate, local fuel consumption rate, pollutant formations, and more importantly flame stability mechanisms such as local extinction and re-ignition. The responses of turbulent flames to fuel variability are also not well characterised or understood. For biogas, its composition varies depending upon the origin of the anaerobic digestion process. Its main constituent CH4 concentration can vary from around 50% for landfill gases to around 75% if advanced waste treatment technologies are employed. The combustion performance of biogas can be very different when CH4 concentration changes from case to case, causing serious uncertainties in its utilisation.
Because of the wide range of possible compositions of the fuel mixtures, the effects on the combustion and emission characteristics can be substantial. For biogas to be widely used in practical combustors there is a need to elucidate the underlying chemical processes and fluid mechanics responsible for fuel variability both fundamentally and practically. Unfortunately, a comprehensive study of the effects of the fuel variability of biogas on its combustion performance that is essential to the deployment of energy conversion systems is still not available. In order to meet the challenges posed by fuel variability of biogas, a detailed parametric study by systematically varying the percentage of the constituents that could map the fuel composition with the combustion performance is highly desired. High-fidelity numerical simulations are a natural choice for such a parametric study. Well-validated numerical simulation and optimisation can play a significant role in investigating the issues related to fuel variability. In order to obtain a thorough understanding of the effects of fuel variability on energy utilisation of biogas combustion, the Work Package no. 5 of HPC4E project https://hpc4e.eu/ employs a coupled approach which covers three important distinct areas:
- developing the chemical kinetic mechanisms,
- analysing the physical characteristics of the flame,
- and finally providing an optimised industrial guideline for biogas compositions.
The industrial guideline will particularly address the applicability of biogas for gas turbine combustion and the combustor operability. Using advanced modelling and simulation, this project aims to develop science-based knowledge for biogas utilisation using the cutting-edge modelling and simulation techniques. The modelling approaches to be developed and numerical simulations will be validated against experimental combustion diagnostic data. The project aims to develop a hierarchical approach to utilise biogas for combustion applications.
Xi Jiang - Lancaster University
Daniel Mira - Barcelona Supercomputing Center
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