Double Dutch

Projects such as Essent Energie’s Amer plant in the Netherlands and the recently-announced Fiddlers Ferry plant in the UK show that the policy environment in Europe is making co-firing a practical and viable option for coal fired power plant operators.

Sam Saimbi, Alstom Power

Worldwide energy demand is predicted to increase 60 per cent by the year 2030, and most of that demand is expected to be met by fossil fuels. Following the signing of the Kyoto Protocol to limit greenhouse gas emissions in 1997, one of the key European environmental aims is to increase the amount of renewable energy used in the production of power. As a result, governments in Europe and across the world have implemented incentives to increase awareness and to create favourable conditions for investor development in future power generation technologies that reduce or eliminate the production of greenhouse gases, particularly carbon dioxide (CO2). Measures such as improved energy efficiency, carbon sequestration and the use of alternative energy sources such as biomass fuels are among the armoury of weapons available to combat the problem.

Biomass fuels are universally recognised to be a clean, renewable and ‘CO2-neutral’ source of thermal energy. These fuels can be fired alone or co-fired with another fuel, and the decision as to which method to use is typically related to the cost and availability of the biomass fuel and the relative cost of the required fuel handling and firing equipment. The overall efficiencies for utility coal fired plants are greater than smaller scale dedicated biomass plants and therefore co-firing of biomass fuels is an effective way to achieve reductions in CO2, sulphur dioxide (SO2) and nitrous oxide (NOx) emissions in a coal fired plant.

Figure 1. Several types of biomass have been successfully co-fired at the Amer plant in the Netherlands
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Co-firing of biomass can either be done within an existing milling system or with a dedicated system. Using an existing milling system limits the quantity and type of biomass fuels that can be fired, whereas a dedicated system can be optimized for the selected biomass fuels. Furthermore, with a dedicated milling system, there is no influence on the existing coal milling performance and it avoids any load loss due to mixing coal and biomass in the mill.

Alstom uses a dedicated system which involves the introduction of the biomass fuel to the boiler in a separate fuel stream through dedicated burners. This offers a significant increase in the proportion of biomass that can be co-fired, as compared with a non-dedicated system. The dedicated firing system can handle a wide range of biomass materials, thus providing fuel diversity and future fuel flexibility to the operator.

Fuel diversity

Alstom has recently won a contract to build the UK’s first dedicated co-firing plant, at Fiddlers Ferry power plant in Warrington. The UK-based team that will carry out the project is already well versed in the problems of biomass co-firing, having worked on several co-firing projects and having most recently completed one at the Amer power station in the Netherlands.

At Essent Energie’s Amer plant in the Netherlands, Alstom has converted a pulverized coal 640 MWe, tangentially fired boiler to co-fire up to 14 per cent biomass on a heat-input basis. A dedicated biomass fuel feed system was installed, capable of handling up to 50 different types of green fuels.

All of the fired biomass products at Amer are cellulose-based and the calorific value on a weight basis is thus similar. The bulk density range of the biomass, however, is between 360 kg/m3 and 760 kg/m3.

The incoming particle size range of the fuel to the storage silo is such that a potentially explosive dust atmosphere could be present upstream as well as downstream of the mills. To satisfy current European standards and, in particular, the ATEX Directive, the plant was fitted with explosion vents on the silo and inert flue gas was used as the conveying medium in the mill.

The biomass is fired through two elevations of burners to minimize biomass heat input per unit area thereby minimizing the potential for increased local ash deposition and tubewall corrosion rates.

Feed and firing system

At the Amer plant, the to-be-fired biomass blend is prepared outside the boilerhouse to provide a consistent and controllable fuel with less than 17 per cent moisture to the installed biomass feed and firing system. Any tramp material is removed upstream of the storage silo. The flow rate to the silo is controlled by the operation of an upstream conveyor that controls the dispatch of biomass between the Amer 8 silo and one of a neighbouring unit.

Two discharge chutes are fitted to the silo to allow the contents to be discharged to locally parked trucks. This allows the silo to be emptied before a planned shutdown or stoppage of over several days to avoid decomposition during prolonged storage, which can lead to rapid spontaneous combustion. A silo inert gas system is also installed to protect the plant against auto-combustion of the biomass caused by the material’s natural properties as it decomposes.

Figure 2. At the Amer plant, the biomass blend is prepared outside the boilerhouse to provide a consistent and controllable fuel
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The feed system is designed to mill 50 t/h of material, therefore it was necessary to use two milling streams. Thus, two oscillating bar feeders are fitted at the outlet of the biomass storage silo. These are capable of vibrating the product – to improve discharge of any difficult materials – and also to feed a looser product into the screw feeders, which discharge the biomass into the hammer-type mills.

The primary function of the milling system is to reduce the particle size of the incoming biomass material to allow efficient combustion. This produces a particle size similar to that of sawdust. This is necessary to achieve adequate burn-out of the particles when firing wood pellets, which are the design biofuel. As the feed materials have a relatively low moisture content, only a small amount of drying is required to be performed in the mill.

Flue gas from the boiler is used as the inerting medium in the mill. This is taken from downstream of the boiler precipitators and induced draught fan, through an insulated duct to a flue gas booster fan designed to handle the dust-laden gas. The gas is at low temperature to avoid heating the biomass above ignition temperatures. Current dust codes allow a maximum temperature of approximately 265à‚°C. In addition, a recirculating gas system is used around the mill. This requires less inert flue gas from the boiler and minimizes the flue gas supply duct size by avoiding passing all of the mill air/gas flow to the burners. In all cases, the oxygen content of the inert system is maintained below ten per cent.

From the mill, the dried and sized biomass is conveyed to a cyclone, which separates the flue gas from the prepared biofuel material. The milled product is then discharged through the cyclone outlet flange to the pulverized fuel system by a rotary air lock. The air lock has fixed speed operation and effectively discharges all product from the small hopper of the cyclone.

Gas is drawn from the cyclone back to each mill inlet by a gas recirculation fan. The vented air passes to the pulverized fuel transport primary air system from a point downstream of each fan discharge. Biomass output is controlled by the variable speed screw feeders at the bottom of the silo.

For this unit, the biomass is fired through two elevations of burners. This approach was selected to minimize burner heat input per unit area, thereby minimizing the potential for increased local ash deposition and tubewall corrosion rates.

The milled biomass is conveyed to the burner by two pulverized fuel piping systems, one for each burner elevation. A single primary air fan supplies a mixture of ambient air and recycled mill vent flue gas to the two burner systems. A centrifugal fan was chosen to provide a lower maintenance solution than would be required by a higher pressure Roots blower, high solids ratio system. The latter would be more cost-effective to install since the line sizes would be small, but such a system has high pressure losses and it is thus more difficult to split flows without the use of additional motorized feeder devices.

On each stream, fuel from the cyclone falls into a fixed speed motorized rotary valve unit which provides a process ‘air lock’ by isolating the filter system from the higher pressure in the burner primary air system. The fuel is then conveyed to each of four burners per elevation through the piping system where it is admitted to the furnace. A pneumatically actuated isolating valve is provided in each burner line to give quick isolation of the fuel passage in case of emergency.

The burners are situated in existing oil burner compartments. The outer, secondary auxiliary air nozzle was enlarged to maintain the necessary air flow for coal and biomass firing. The burners are ignited from the adjacent coal nozzles and are considered by the burner management system to be auxiliary fuel nozzles. They thus do not require separate ignition equipment. The burners are tilting to match the existing nozzle arrangement.

All the supplied equipment, material and construction conform to the latest AEU standards and ATEX requirements.

Viable investment

Since coming into full operation, the plant has successfully fired several types of biomass. Studies done on other plants show that reductions in NOx and SOx emissions are in direct proportion to the amount of biomass fired. Converting the plant to burn biomass represented a significant investment for Essent, however, the implications of possible penalties under the EU emissions trading scheme, means such an investment is viable for those power producers serious about tackling environmental problems.

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