Sixty-six delegates from 13 countries enjoyed great Yorkshire hospitality at Drax power station on 16-17 September, venue for the 5th IEA CCC Workshop on Cofiring Biomass with Coal. Steve Tosney, of Drax Power Limited opened the workshop with a detailed description of the largest user of biomass in Europe. Three of the 6 units have been converted, and there are some discussions about converting a fourth. When the original boilers were built they were over-specified and so are extremely large, which has been an advantage when converting to biomass. Drax Power now emits 12mt/y CO2 less than when it burned just coal. Some of the biomass is locally sourced, but most of the 7mt used annually is imported from North America.
The potential of a variety of fuels for cofiring were discussed, as well as progress on torrefaction.
An energy cane plant has been bred in Brazil from various sugar cane varieties which has a reduced sugar content and increased fibre content, giving it great potential as a biomass resource. Compared to regular sugar cane production, productivity is increased by 3.8 times and there is 83% more fibre/harvested tonne, leading to 7 times the average amount of fibre/ha/y (compared to regular sugar cane). IKOS claim to be able to produce 700 kt by 2018 and possibly up to 10 mt/y by 2024-25. Development of the energy canse is focusing around the Port of Açu in Brazil which already operates at international standards so biomass pellets could be shipped from there.
Progress on the development of ‘Coalgae’ in South Africa was presented by Vitus Ejesieme. Coalgae is a single fuel made of microalgae and discard coal. Live microalgae irreversibly absorbs onto the surface of the coal. With low temperature heat activation (drying at 100 – 130 oC) it appears to form a single substance. Work is underway to produce 6 – 10t of Coalgae® in fully integrated system.
Subcoal is a pelletised waste derived fuel made of paper and plastic waste. Ground Subcoal typically has a particle size of < 5 mm, which results in excellent burnout times. It is hydrophobic and has a decent calorific value, but has a chlorine content of about <0.8 W.-% DW.
Straw as a fuel has the disadvantages of a low bulk density, poor flowability, an exothermic heat of reaction and it is hard to pelletise. But it may be suitable for torrefaction as it has a low moisture content and a high reactivity, so releases 10-50% of the chlorine in the torrefaction gas. The process and product is improved by pelletising the straw prior to torrefaction.
Torrefied biomass has some advantages. For example, it can be stored for longer outside before there is any real damage, and it has a lower minimum ignition energy. Michiel Carbo (ECN) believes that torrefaction technology is ready for commercial market introduction and that the basic drivers for torrefaction still hold although several factors have slowed down its introduction.
An ISO standard for torrefied material may be ready by mid-2016. According to Michael Wild (IBTC) a number of successful test burns of torrefied biomass at European power plants have been carried out, at cofiring rates of 85% (mass) and more. The performance of the biocoal in the mill and at the burners was excellent, and the emissions performance was satisfactory.
Several industrial size projects are in the planning or construction phase (Europe, America, Asia) and additional orientation towards heat markets is underway. This may be a better way to grow the technology and market for the product.
Impacts on the power plant
The main problems of biomass as a fuel are the dust when handling, its hydrophilic nature, and slagging, fouling and corrosion on combustion. Biomass dust can be very dangerous and has caused fires at a number of power plants. The best ways to control the dust are through the fuel spec and the plant design.
Slagging and fouling were covered in a few of the presentations. Biomass fuels tend to be high in chlorides and alkalis. Various additives are being trialled to limit slagging and fouling. For example, coal fly ash reduces slagging and corrosion as it traps KCl. SNCR is an efficient and cost-effective technology for NOx control in large power stations, especially, for those cofiring biomass.
Birte Everts found that in the boiler, biomass cofiring caused an absolute change of less than 1% on various parameters such as: live steam mass flow, reheat steam temperature and flue gas temperature downstream. The influence of cofiring dried wood on the investigated coal-fired boiler was small, and was even less when using torrefied wood. The biomass drying and torrefaction processes reduced net efficiencies by 4%pts compared to the coal only case.
Fuel flow in the power plant is important, and should be optimised to improve efficiency, save fuel costs, control emissions, enable the increased sale of fly ash, reduce safety hazards, and improve performance of emission control equipment. Flow measurement systems can be used: to control the fuel mass flow to each burner and to optimize transport conditions depending on fuel structure and sizing. Then the balance between biomass and coal input to the combustion chamber can be adjusted. This will help to adjust the secondary air flow to the coal/biomass burners.
Boiler design, modifying operation and the right fuel and fuel mix improves biomass combustion.
In Denmark, cofiring for CHP is now slightly cheaper than coal combustion due to a green energy tax. This highlights the need for financial support for biomass if it is to be used on a serious scale, as coal is virtually always cheaper than biomass.
GDZ Suez (Engie) have constructed a new 800 MW ultra supercritical coal fired power plant in the Port of Rotterdam which is designed to operate at 46% efficiency (LHV). The permit for the plant includes 60% of biomass co-firing and the Dutch government has introduced a new renewables subsidy scheme (SDE+) which is based on a contract for difference system. GDF Suez are thus investigating cofiring at this power plant. It is important to have the right combination of biomass quality and coal quality. For example, particle size distribution of biomass has a major effect on the flue gas exit temeprature and therefore the fouling and corrosion characteristics. The configuration of the biomass and coal burners also matters. It seems that cofiring in ultra supercritical boilers is feasible, but there are severe risks if the biomass/coal quality do not meet the specifications, and the right combustion parameters are not followed.
Looking further to the future for cofiring, and the possibility of negative emissions from a combination of biomass combustion and carbon capture there were two presentations about oxyfuel combustion of biomass. Juan Riaza found that replacing N2 with CO2 in the combustion atmosphere with 21% of O2 caused a decrease in the burnout values. When the O2 concentration was increased to 30 and 35%, the burnout value was higher than in air conditions. An increase in the burnout value was observed after the addition of biomass, this trend became more noticeable as the biomass concentration was increased. The emissions of NO during oxyfuel combustion were lower than under air-firing. Emissions of NO were significantly reduced by the addition of biomass to the bituminous coal, although this effect was less noticeable in the case of the semi-anthracite.
Thomas Ekvall looked at the fate of KCl during oxyfuel combustion of biomass as it has a higher alkali and higher chlorine content than coal. Potassium chloride can deposit and cause corrosion. It is preferable if sulphates, rather than chlorides are formed from a HTC point of view. A higher degree of sulphation is reached during oxyfuel combustion compared to air combustion. He concluded that it seems to be possible to use a higher biomass to coal ratio with fewer alkali-related problems if the two fuel categories are cofired in an oxyfuel environment, than is the case for air combustion.
Finally, the feasibility and sustainability of cofiring biomass in coal power plants in Vietnam was presented. Vietnam has a fast growing demand for electricity and a large amount of biomass, resulting from agricultural waste after the rice harvest. It is often just burnt in the fields. Currently, cofiring in Vietnam is not yet economically feasible due to coal subsidies and a low electricity tariff. But it could provide various benefits including local air quality improvement, additional income for local farmers and employment.
Tour of Drax
Drax Power Ltd laid on an excellent tour of the power plant which everyone enjoyed. We are very grateful to them for hosting the workshop and to sharing so much of their information and expertise.