Natural alternatives - biomass for energy
Following the reports released by Materials UK on materials for energy, John Oakey from the Energy Technology Centre at Cranfield University, UK, discusses the possible use of biomass materials.
Renewable energy technologies, such as the use of biomass as a low CO2 emission fuel, are seen as a key approach to reduce the threat posed by global warming. The use of these technologies is being encouraged by the introduction of legislation in Europe to reduce CO2 emissions from power generation and assist the greater deployment of combined heat and power.
Of these technologies, biomass-firing is a high priority because of the modest technological risk involved and the availability of waste biomass in many countries. The growing of energy crops for liquid biofuel production has, however, been openly criticised because of its impact on food prices.
Tried but not trusted
The combustion of waste and farmed biomass has been practised for many years around the world, mostly for heat production, but more recently for the generation of electricity. Biomass is a difficult fuel compared to those from fossil sources, as it is generally of lower energy density, making it more expensive to transport, problematic to handle, and wet, which diverts some of the heat for drying purposes. It also tends to contain ash species, which may damage power plant components. In addition, biomass fuels often produce higher NOx and NH3 levels in combustion and gasification systems, respectively, than their fossil fuel counterparts.
While the efficiency of energy conversion from biomass to heat has not been a priority where the biomass is a waste material, this is not true for power generated at a larger scale where the bio-energy system has to compete with other fuels, such as coal. In such cases, biomass plant efficiencies have fallen well below those of equivalent fossil-fired systems due to reduced steam conditions enforced by severe fouling and corrosion problems. These result from high levels of aggressive contaminants like sodium, potassium, chlorine and lead in many biomass fuels. While coal plants target 650oC/300bar steam and above, biomass plants operate at under 540ºC/100bar steam, with efficiencies of typically less than 30%.
In biomass-based combustion systems, the key materials challenge is to resist increased levels of corrosion, which lead to poor availability. As the aim is to be ‘fuel flexible’ by taking advantage of cheap, local sources of biomass, effort has been focused on the durability of the heat exchanger materials rather than on biomass pre-treatment to remove aggressive contaminants, although a combination of these approaches may give the optimum result.
Even the best available materials, such as the high nickel alloy (Alloy 625), have problems meeting power plant life requirements. Coatings have been investigated, but these have also struggled to resist the aggressive conditions while being economically viable. Further research in this area for both evaporative and superheating duties is a priority.
Co-firing of biomass with coal in conventional pulverised coal power stations provides a means of using biomass in a high efficiency plant, maximising CO2 reduction. Experience at low levels of co-firing, such as five per cent, has shown few instances of increased fouling and corrosion problems due to dilution of aggressive contaminants. But major problems could occur at the higher levels of co-firing planned for future systems.
Advanced conversion options
In many biomass gasification-based systems, whether for power generation or production of gaseous/liquid fuels, major materials issues can be avoided by careful process and design choices. The need to extract heat energy from hot gas is a lower requirement than in much larger, coal-based integrated gasification combined cycle power plants, although this may prove necessary in the future. However, finding an economic solution for cleaning the gas to meet operational and component life requirements of downstream components, and subsequent emissions, is still a major challenge.
In all biomass gasification power systems, the performance of the gas engine/turbine is vital to overall plant efficiency and economic viability. Where the fuel gas produced is not thoroughly cleaned of ticulates and scrubbed to remove aggressive contaminants, like alkali metals, hot corrosion and erosion are likely to be life limiting for high temperature components in the path of the hot gas, such as gas turbine vanes and blades.
Corrosion can result from the combined effects of gaseous species like sulphur oxides and hydrogen chloride, and deposits formed by condensation from the vapour phase and particle deposition. The mode of corrosion damage is highly dependent on the local component environment. Conventionally, the metal vapour species of most concern was alkalis, mainly sodium, and sulphur in gas turbines fired on clean fuels, either as fuel contaminants or via the combustion of air, or vanadium from heavy fuel oils. In biomass systems, while the level of sulphur oxides may be lower than an equivalent coal-based system, the level of hydrogen chloride can be much higher. Also, the fuel gas may contain significant levels of alkali metals, particularly potassium, and heavy metals such as lead and zinc, depending on the type of biomass.
It may be possible to optimise the materials selection and operating conditions of a gas turbine, in combination with a comprehensive maintenance strategy, to allow it to handle higher than currently specified contaminant levels, instead of bearing the high cost of an elaborate gas cleaning system. A clear research priority is to develop improved protective coatings which will allow biomass systems to take advantage of advanced high efficiency gas engines and turbines, therefore assisting the economic viability of the systems.
The materials challenges from using biomass in energy processes have been evident for many years, but continue to impede the increased deployment of these relatively low risk, renewable energy options. The resulting priorities are – improved alloys and coatings for evaporator and superheater heat exchange duties, life prediction modelling for heat exchangers to optimise maintenance and repair procedures, monitoring of corrosion and contaminants in order to provide early warnings of problems, and improved repair procedures for heat exchanger and gas engine or turbine parts.