Alholmens: the world's largest biofuelled plant (part 1)

1 January 2002

The Alholmens Kraft biomass-fired CHP plant, with a boiler steam capacity of 550 MWt, has just entered commercial operation. Located at UPM-Kymmene's huge Wisaforest pulp, paper and saw mill complex, close to Pietarsaari on the west coast of Finland, fuels include bark, sawdust, wood chips and peat, with coal as reserve.

Many Finnish pulp and paper mills have built new fluidised bed boilers to meet their steam requirements and to cogenerate electricity in back pressure turbines. At the same time, the mills strive to make maximum use of logging byproducts like bark in the most economic way. Steam is also typically produced in pulping-related black liquor boilers feeding smaller back pressure turbines.

In recent years, however, mechanical pulping and paper production technologies have increased electricity demands substantially, requiring the purchase of additional electricity from the power utilities.

This was very much the situation at UPM-Kymmene's Wisaforest facility at Pietarsaari. Here, the power utility companies initiated co-operation with the pulp and paper mill side to find a way of meeting power needs that maximised the utilisation of logging byproducts and other local biomass fuels to produce steam and heat in a utility-sized power plant - culminating in the Alholmens project.

Originally, the steam and heat required by the Pietarsaari pulp and paper mill was provided by two black liquor boilers, and one bark boiler with a 25 MWe steam turbine (the old AK1 power plant). In 1996 this bark boiler was converted from a grate-fired boiler to a bubbling fluidised bed (BFB) type boiler. It burns bark, wood waste, sludge and a small amount of REF produced from industrial plastic and paper waste. The power plant also supplies district heating to the city of Pietarsaari.

Over the years, the production capacity of the mill has increased steadily, to the point where the quantity of bark and wood waste could not be fully utilised, while additional electricity was being bought in via the grid.

To develop power generation for the site, a new company was established, Oy Alholmens Kraft Ab. Set up in 1997, Alholmens Kraft has a unique set of shareholders, from both Finland and Sweden. The Finnish owners are Pohjolan Voima Oy (PVO), the second biggest energy company in Finland, which has a 49.90 per cent holding, and Oulun Seudun Sähkö. In turn, the Pohjolan Voima shares within Alholmens Kraft are owned by UPM-Kymmene Oyj, Perhonjoki Oy (Katternö), Kokkola town and Päijät Hämeen Voima. The Swedish ownership amounts to about 45 per cent and the Swedish shareholders are Graningeverkens Abp, Skellefteå Kraft, and Revon Sähkö Oy.

Studies for the project go back to 1996. Involved at an early stage in project development was Electrowatt-Ekono Oy, which was selected by PVO and other shareholders as principal external consultant. Electrowatt-Ekono's scope in the project development phase included: energy balance analysis; feasibility studies; value analysis; preparation of contractual packages; environmental impact assessment; and pre-engineering.

After several years of work on development and optimisation, as well as on securing environmental permits, the board of Alholmens Kraft decided in February 1999 that the company should construct a large biomass-fired power plant at Pietarsaari - indeed, the world's largest.

The board of Alholmens Kraft empowered PVO to handle project execution and management. Project responsibilities were split between PVO-Engineering and Electrowatt-Ekono, who carried out project co-ordination and management in their respective areas. Engineering contracting, automation, electrical and turbine and boiler project management services were carried out by PVO-Engineering. During the implementation phase, Electrowatt-Ekono's scope included: general and process engineering; HVAC, piping and layout engineering; and procurement and supervision services.

The main contracts for the boiler, with Kvaerner Pulping, and turbine supply, with Energico-LMZ, were signed at the same time and the construction work at site started immediately.

The power plant construction was a multi-stage project consisting of 17 separate construction contracts. The work started with the excavation of a one-and-a-half-kilometre cooling water intake and exhaust water tunnel. PVO-Engineering was responsible for construction, with its main subcontractor being Construction Consortium YIT-Forsström Oy. PVO-Engineering also supervised implementation of electrical and automation systems. Equipment for the power plant section was supplied by ABB Industry Oy, while the planning and building development of a 110kV substation associated with the project was the responsibility of PVO-Engineering.

The low- and high-pressure piping was delivered, prefabricated and installed by YIT Power Oy. YIT Power also completed the 3D modelling of the boiler plant pipework and maintained the 3D model of the turbine plant. The total amount of piping delivered by YIT Power was approximately 1100 tons, in a wide range of materials, from carbon and stainless steel up to X10CrMoVNb91. Effective use of prefabrication enabled the tight time schedule to be met, with improved project management and site safety. Modern information technology at site also helped - not just transferring information between parties but also printing axonometric drawings for welders and fitters.

Boiler house erection started in January 2000 and turbine plant erection in September of that year. Commissioning of the equipment started in February 2001 and boiler blow-outs were performed in June and the first turbine start-up and generator synchronisation in early July. The plant entered commercial operation at the very beginning of 2002.

The total investment cost for the plant was about 1 billion Finnish marks (E170 million), with a Finnish content of about 70 per cent. The project was supported by the EU´s Thermie programme.

About 50 people are employed at the plant on operation and maintenance, while it gives work to more than 400 people in fuel production and transport.

Project objectives

When the planning of the new Alholmens Kraft power plant (AK2) was started, the most important objectives for the proposed high capacity unit were:

• Electricity production at a competitive price for sale in the

open market.

• Utilisation of the process steam and heat in the paper mill and for the city of Pietarsaari.

• Maximum use of combustible byproducts from the adjacent pulp, paper and saw mill with high efficiency.

• Optimisation of the unit size and process values.

• Ensuring sufficient additional fuel resources within economic transport distance.

• Balanced and equitable share of costs between the owners.

The Alholmens Kraft plant has the capacity to produce 240 MWe of electricity (condensing mode) or 205MWe plus 100 MWt of process steam to be supplied to the UPM-Kymmene pulp and paper mills, along with 60 MWt of district heating for the mills and for the town of Pietarsaari.

The new plant is located next to the existing 25 MWe bark boiler unit, between the pulp and saw mills. The location allows for 30 per cent of the fuels (bark, wood waste from saw mill) to be transported via conveyors from the adjacent mill area.

The new unit is connected to the existing steam and heat network together with the existing 25 MWe unit. In late 2001 ownership of the existing 25 MWe unit (AK1) was transferred to Alholmens Kraft and will be operated by the same personnel as the new plant. The operational concept is that the new 240 MWe unit will operate as baseload, producing heat, steam and electricity with high efficiency. During periods of high electricity demand the 25 MWe unit will operate to produce steam and heat, enabling the bigger unit to produce the condensate electricity. The smaller, AK1, unit can also take care of the summer consumption periods.

A major objective of the Alholmens project has been to demonstrate what is effectively a new multifuel power plant concept, with novel technology for solid multifuel and low emission cogeneration at a new commercial size, and co-firing of biomass with fossil fuel. One of the starting points of the power plant design was the maximum exploitation of bark and other wood based fuels, peat resources from neighbouring areas and the existing commercial port and the possibility of importing coal. The share of a particular fuel may vary seasonally.

Alholmens Kraft will use about 3500 GWh fuel annually, with some 35-50 per cent in the form of wood-based biomass, 45-55 per cent as peat and about 10 per cent coal. It is envisaged that the coal, as a reserve fuel, will only be used for start-up and support purposes, along with a small amount of heavy fuel oil.

Peat has an important role in balancing the irregular moisture content of wood harvesting residues. Using wood only it is difficult to get maximum output from the boiler, with seasonally high water content low heat valve limiting the capacity of the boiler. Compared with coal, peat is domestic and CO2-neutral, as it binds CO2 from power production.

In the future the target is to increase the utilisation of forest harvesting residues. There are several development projects in progress to lower the procurement costs. Currently the issue under investigation is the risk of chloride and alkalines causing harmful layers on heating surfaces and increased risk of corrosion in the boiler. The power plant's fuel handling system is designed to accept multiform fuel and the target is to balance the different fuel characteristics to ensure as stable a combustion process as possible to control emissions and also generate electricity and steam at required levels.

The plant aims to use about 150 000-200 000 m3 of logging residue annually. The logging residue is hauled to the plant as loose material or as bundles (also known as bales), which are pictured below right. This wood fuel procurement system, based on bundling (baling) of forest residues, is innovative. Alholmens Kraft expects to use about 300 000 bales of forest residue per year.

The plant is participating in a project which aims to develop a new logging residue harvesting method, where the logging residues are bundled (baled) in the field. The main idea is to use traditional timber harvesting equipment. Four baling (bundling) machines are already in use, each producing some 20 bundles per hour. The volume of one bundle is about 0.5 m3 (450-500 kg, 3.3 m long). An ordinary timber truck is used and it can take 60 to 70 bundles in one load. Eventually there will be 5-6 baling (bundling) chains in action.

Overall, the new plant can be seen as introducing a "best-practice" biomass/fossil fuel co-fired power plant concept with extremely diverse fuel range, and is believed to be suitable for replication almost anywhere in Europe and North America.

Alholmens Kraft is competitive as a CHP-plant because coal and peat are taxed when used for heat production. But the plant has also additional capacity to produce electricity where it cannot use the heat. In that sense the plant is competing with coal power.

The plant's electricity is likely to become more competitive in the future. Coal capacity is decreasing steadily because no new investments are being made. Coal's future price and taxation level is highly uncertain. At the same time electricity consumption and the demand for green energy are increasing.

Alholmens will certainly run at full capacity in winter when the Nordic annual precipitation is lower than average. Precipitation is a major price setter in the Nordic markets, because hydro accounts for such a large share of power production.

Plant outline

The key process issues were that steam and heat had to be supplied as economically as possible, and with high reliability in a wide range of operational situations. Electricity production also had to be able to respond economically to the base and variable load demands. A great deal of process analysis was carried out with the turbine supplier, in particular during the project planning phase, to optimise turbine extractions and process reduction valves.

To achieve the best overall efficiency, high steam parameters were studied along with supercritical steam values. The following conclusions were arrived at:

• A reheater was feasible because the plant unit size is sufficiently big that increased efficiency compensates for the increased investment.

• The reheater increases the availability of the steam supply to the paper mill in the case of a turbine shut down with the boiler continuing in operation.

• A maximum steam temperature of 545°C for the HP and RH side was desirable to minimise the risk of high temperature corrosion and to enable the use of conventional materials on the heating surfaces.

• A sub-critical pressure, 165 bar, was best. Moving to a supercritical process was estimated to be too expensive in relation to the benefits.

The turbine selected consisted of an HP cylinder, IP cylinder and LP cylinder. The main process connections are from the IP turbine and they are arranged so that steam extraction slides according to the turbine load. The LP cylinder is extremely large, benefiting from the cold cooling water, having 1.2 m titanium blades in the last stage. The turbine is capable of operating from extraction mode to full condensate operation mode within the minimum and maximum boiler loads.

Boiler. At the heart of the plant is a CFB (circulating fluidised bed) boiler supplied by Kvaerner Pulping. CFB technology can cope with the wide range of fuels envisaged, biofuels, coal and peat, while at the same time achieving low emissions. The individual design features of the CFB are proven and conventional, but the way they have been put together at Alholmens is innovative and using biofuels as the main fuel for this size of boiler is believed to be unique. The operational environment at Alholmens means that the boiler is effectively half a utility boiler and half an industrial boiler. The utility side requires higher steam data and the industrial side requires better load flexibility. The boiler design requirements combine both these features, which is a challenge.

The wide range of fuels envisaged, makes the challenge even tougher. The range of planned fuels is from high moisture content peat, bark or wood residue - moisture content 50-58 per cent, heating value 7.5 MJ/kg - to bituminous coal (25 MJ/kg). The huge difference in flue gas amounts is compensated with recirculation gas in coal combustion mode.

The output parameters for the boiler are 194 kg/s, 165 bar and 545°C on the high pressure side and 179 kg/s, 40 bar and 545°C on the intermediate pressure side. This achieves a boiler steam capacity of 550 MWt. Both the superheater and reheater are spray water controlled. Parts of both surfaces are located inside the furnace to give a wider control range for superheating (from 50 per cent) and reheating. The turn-down ratio of the boiler is 35 per cent with solid fuels.

The boiler furnace cross section is large due to the high fuel input capacity and the planned use of biofuels (peat and bark). The dimensions of the furnace are 8.5x24x40m giving a free-board cross-section of over 200 m2.

There are three steam cooled cyclones in the boiler. These cyclones are of conventional type, with a diameter of 9 m. The cyclones are the first superheater surface in steam circulation to keep the temperature difference between the furnace and cyclones minimal. The reason for making the cyclones steam cooled instead of water-cooled was to ensure the circulation in all conditions with a high operational pressure (165 bar). The expansion joints of the cyclone are formed of pressure parts and the advantage of this is that it is service free.

The boiler is equipped with four independent fuel feeding lines. Each line serves three fuel feeding openings. The feeding points are staggered so that the stoppage of one feeding line only causes a small disturbance in the combustion process. Three feeding lines can achieve full boiler capacity, one of several design features incorporated in the plant to increase the reliability of the whole process.

Rotary air preheater. One utility boiler feature in the Alholmens boiler is the rotary regenerative air preheater, which transfers heat energy from the flue gas coming from the boiler to the incoming combustion air required by the boiler. The rotary air preheater, supplied by Howden Power, has a 37 768 m2 of heating surface, a total weight of 260 tonnes and rotates at 1rpm.

The major benefit in using a rotary air preheater is to achieve higher boiler efficiency with moderate pressure loss on both the air and flue gas sides. With a tubular air preheater the pressure drop would be double and the same outlet temperature (130°C) could not have been achieved.

However, a rotary air preheater can give rise to leakage between the air and gas streams and in the boiler. At Alhomens, leakage has been minimised by adopting a quad-sector design in which the high pressure primary air is located between the secondary air sections, together with triple seals and sensor activated actuators on the sector sealing plates. This limits the leakage to a one percentage point rise of O2 in the flue gas.

The potential for corrosion at the "cold end" of the airheater is minimised by having a tier of enamel coated elements configured to be easily cleanable using the combined sootblower/water washers.

Air fans. The combustion air system consists of two axial type secondary air fans and two radial type primary air fans. The two axial fans are single stage variable pitch axial flow fans with an impeller diameter of 1938 mm and a hub diameter of 1250 mm. Both have a 1500 kW motor at 1490 rpm. The primary air fans are connected in series with the secondary air fans. This was done to minimise power consumption and to allow use of electrical motors of roughly the same size in all fans. The primary air windbox below the bed is divided into several sections. The secondary air is divided into two feeding levels and these levels are controlled by several dampers. These controls provide the capability to correct deviations in the right-left and front-rear directions independently. The ID fans placed after the boiler are also variable pitch axial flow fans, however, due to the relatively high pressure drop in the boiler they are of a two stage design, ie two hubs on each fan. The fan impeller diameter is 2438 mm and with a 1250 mm hub. The motors are 1600 kW each at 990 rpm. The choice of variable pitch axial flow fans assures a high fan efficiency at lower boiler loads and a quick response time with precise regulation and low hysteresis when changing boiler part load.

Emission control. Good combustion and emission control requires even fuel and air distribution and combustion air staging. In the boiler the SO2 emission limit is so tight (100 mg/MJ) that limestone feeding is needed not only with coal combustion but also with peat combustion. When the share of wood based fuels is increased, the mass flow of limestone is reduced since the alkalis in the ash react with fuel sulphur resulting in lower sulphur dioxide emissions. The limestone is injected pneumatically, close to the fuel feeding points.

Nitrogen oxide emission is controlled by using low-NOx technologies. An SNCR (selective non-catalytic reduction) system is used to fulfil the NOx emission requirement with all loads. If NOx emissions reach the limit of 50 mg/MJ, ammonia spraying is started in the cyclones. Ammonia can be injected either to the cyclone inlets or into the furnace with lower loads.

Dust control is handled with a four-field ESP (electrostatic precipitator). It is intended to recycle fly ash and the gypsum from the sulphur retention reaction.

Materials handling. Equipment for fuel feeding, sand handling, bottom ash handling, fly ash handling, and lime feeding was supplied by Finnish materials handling and conveyor specialist, Raumaster Oy. The Raumaster scope covered feeding equipment inside the boiler plant for peat, wood chips, bark, and coal, including the four biofuel-feeding boiler silos equipped with screw reclaimers, with four feeding lines, one from each silo, leading to the boiler, and eleven feeding points to the boiler in total. Two coal lines feed coal to the biofuel lines, where coal is mixed with the biofuel and then fed to the boiler. Sand feeding equipment supplied by Raumaster included a sand silo and a screw feeder, while, for the two lime feeding lines, Raumaster supplied a storage silo, two metering silos, and two screw feeders.

Raumaster also supplied the two ash removing lines for bottom ash handling. Bottom ash is removed from the boiler from twelve points through four cooling screws. Bottom ash is screened with rotating screens. After screening, fine grained bottom ash is returned to the boiler through the sand silo. Coarse grained ash is removed to the ash containers.

For fly ash handling, Raumaster delivered equipment for discharging both dry ash and wet ash from the silo. The equipment included the silo as well as the ash humidifier and dry outloading device.

Steam turbine. The three-cylinder (HP, IP, LP) steam turbine was supplied by the Russian/Finnish LMZ-Energico consortium (St Petersburg/Helsinki). The intermediate - and low-pressure chambers, as well as the condenser, were supplied by the Russian company LMZ. The high-pressure chamber was supplied by Siemens, as a subcontractor to LMZ.

The main steam is supplied to the high pressure cylinder through two blocks of valves consisting of one stop valve and two covering valves in each block. Steam after the reheat is supplied via two hot reheat pipelines in two steam chests arranged on both sides of the IP casing. The IP casing has two automatic stop valves and four control valves. Steam leaving them enters the IP casing common nozzle chamber.

Energico Oy was in charge of co-ordinating the deliveries and supplying condensation/seawater pumps, the pipe system, four low-pressure preheaters, two high-pressure preheaters, the supply water tank, and three heat exchangers for district heating that were part of the total turbine delivery.

The intermediate outputs of the IP chamber supply the process steam required at the paper mill; these are conveyed through a pipe bridge of about 400 m to the paper mill. The LP turbine also makes it possible to operate in condensing mode only. The high efficiency of electricity production in condensing-mode operation can be put to good use where the heating load is low but electricity is required, or when the process steam is produced at AK1.

The condenser, with 270 km of titanium tube, operates at 0.02 bar and is cooled by seawater, with around 7.5 m3 passing through the condenser per second.

The generator was manufactured by VA Tech Hydro of Austria.

Fuel supply

It is envisaged that wood-based biomass will eventually account for 50 per cent of the plant's fuel. Although no such bio fuel market has ever existed in this area before, the goal is not unrealistic and takes into account the fact that the fuel must not be more expensive than coal.

The pulp mill and timber activities of the neighbouring UPM-Kymmene facility will supply bark, sawdust and other wood residue at a steady pace to the storage areas of Alholmens Kraft. This will make up about 30 per cent of the total fuel consumption. A long-term contract has been signed with UPM-Kymmene and the amount supplied by its mills will increase in step with expected rises in production levels.

The logging residue bundles/bales, already referred to, are transported to the plant by lorry. The power plant has already invested in an electrically powered stationary crusher. When the bundles are brought from within a radius of 100 km of the power plant, this fuel is competitive with coal. The land owners benefit from being able to dispose of this reside and it is assumed they will not claim any remuneration.

UPM-Kymmene is bound to supply about 10 per cent of the power plant's fuel requirements in the form of the residue bundles (bales). The remainder of the wood-based fuel, ie bark, sawdust and wood chips, will be supplied by local private sawmills. Long-term contracts have also been drawn up for this.

Before the power plant investment decision was taken, a study was carried out on the accessibility to peat within 100 km of the station. It was found to be abundant and delivery agreements were drawn up with various suppliers. They own the peat areas, cut the peat and are responsible for its transportation.

Alholmens Kraft has also purchased about 1500 hectares of peat land for its own peat production. The harvest cutting will be outsourced to private contractors and this source could account for about 15 per cent of the plant's fuel requirement. Alholmens Kraft took this step to make use of its expertise in peat production and transportation, environmental issues, etc. It will also give the plant access to a reserve supply which can be regulated when required. To meet the projected 45 per cent of plant fuel consumption about 4500 hectares of peat land is needed.

Fuel handling

The large size of the Alholmens Kraft plant and the diversity of fuels to be dealt meant that the biofuel handling system, supplied by Roxon Oy, posed a number of challenges. Some totally new components had to be developed, for example, the automated sampling system.

The totally unprecedented dimensions and the new approaches being adopted required a good deal of broad-mindedness when developing the equipment and carrying out tests during the design phase.

Fuel is handled in two treatment zones, western and eastern. The fuels handled in the western area are peat, wood based materials and waste pellets. The eastern zone receives wood -based fuel conveyed directly from UPM-Kymmene's debarking facility, and also coal. Also on the east side is the crusher station for the logging residue bundles/bales.

The western zone. The bulk density of material received in the western zone varies from 80 to 400 kg/m3, maximum moisture content being 63 per cent. The fuel materials are transported to the plant by trucks with trailers with a capacity of up to 140 m3 per vehicle. The fuel receiving system is designed for all the truck types used - side tippers, rear tippers and those with a live bottom. Every vehicle is registered on arrival and weighed before and after unloading, with all the data recorded in a fuel database.

The fuel receiving system consists of two separate receiving stations, from which the fuel is conveyed on two separate lines through iron separation, screening and crushing to intermediate storage. The conveying capacity from the receiving station to the fuel storage is 1200 m3/h for each line.

Waste pellets are received through a screw feeder, which feeds the pellets directly onto the belt conveyor ascending to the CFB boiler silos.

In biomass fired power plants, samples of the received fuels are traditionally taken manually or semimanually. Automatic sampling systems which would be able to handle all the variety of biofuels have not been available on the market.

For Alholmens Kraft, the target was to develop a sampling system that would automatically take two representative samples from each truck and from each trailer, and then automatically collect the samples per each fuel supplier so that the sample corresponds to the average of all the fuel deliveries made by this fuel supplier during 8 hours. The system was dimensioned for eight fuel suppliers.

The sampling system is constructed so that the samples are taken directly from the falling material flow when the trucks unload. After crushing and screening the samples are divided into smaller samples and led to a collecting vessel. The vessel is divided into eight sections, each fuel supplier having its own section. Each section is provided with a mixer for mixing the single samples into one homogenous total supplier sample. At the end of each eight hour shift, a laboratory sample is separated manually from the total sample of each supplier.

The sampling system was part of Roxon's total delivery for the fuel handling system.

The fuels themselves are conveyed from the receiving station on two parallel lines through iron separation to screening and crushing and then to the three intermediate storage silos. Screening is done with disc screens. Smaller particles (underflow) go directly to intermediate storage, while larger particles (overflow) are conveyed to crushing and then to the intermediate storage silos. On both lines, fuel materials are crushed with Roxon heavy-duty, slow-rotation shredders (MNR2-3048H110).

The intermediate storage consists of three round storage silos, 3500 m3 each. The bins are furnished with continuous surface level indicators and fire safety equipment. The silos are emptied with rotating screw reclaimers, with a maximum capacity of 800 m3/h each. From the intermediate storage silos, the fuels are transported on belt and flight conveyors with a capacity of 1600 m3/h to the boiler house bins.

The eastern zone. In the eastern area, bark is transferred from the existing stock area with screw reclaimers and belt conveyors to iron separation and screening and then on belt and flight conveyors to the four boiler biofuel silos.

Coal is received through an existing receiving hopper and conveyed along its own line to the boiler coal silo.

BMH Wood Technology Oy implemented the crusher station for logging residue bundles/bales, which is also located in the eastern zone. The crusher, with a capacity of 400 m3/h, was manufactured by Saalasti Oy. It is operated by an outside contractor.

Fuel consumption calculation. The quantity of electricity and heating is set every day before 11.00 am for the following day. Fuel consumption for the following day is then calculated. Notification of this will be sent daily before 14.00 by e-mail to the outside biomass fuel suppliers. Wood residue is delivered directly from UPM-Kymmene's production process on a daily basis and stored in intermediary warehouses as required. The major peat suppliers will make daily deliveries of peat while the smaller suppliers will deliver only during the winter period (of 6 months) when fuel consumption is greatest. When loading is at its peak around 100 fuel loaders of cut peat and tree waste will be required every 24 hours, in addition to the fuel supplied directly from the UPM-Kymmene mills.


Fuel price (r/MWh), showing tax effect

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