Asko Rantee, Jouni Kinni, Kvaerner Power Oy, Finland and Antti Rainio, Kymin Voima Oy, Finland
The pulp industry has traditionally used biofuels to produce a large proportion of its energy demand. The recently-completed Kymin Voima project in Finland is a typical example, and is using the largest biofuel-fired BFB boiler in the world.
In June 2000, Finland's Kymin Voima made the decision to invest in a new biomass-fired power plant. Less than two years later, in summer 2002, the new power plant was in operation, producing 80 MW of electricity and 180 MW of heat.
Kymin Voima is the company that was established for this power plant. Pohjolan Voima, a Finnish forest industries power company, owns 76 per cent of Kymin Voima. Kouvolan Seudun Sähkö, a local utility company, owns the rest. The power plant is located in the UPM-Kymmene's Kymi Paper mill in Kuusankoski. The towns of Kuusankoski and Kouvola, with a combined population of around 50 000, are located within 5 km of the plant. Each of these towns has its own district heating networks.
Construction of the new heat and power plant started in March 2001
The project was spread over ten main contracts and dozens of smaller delivery agreements. The project team was assembled from UPM-Kymmene's project and mill personnel. PVO Engineering and Projekti-insinöörit acted as the main engineering consultants.
Kymi Paper is the third largest pulp and paper mill in Europe with an annual paper production of 920 000 tons and pulp production of 520 000 tons. The mill uses 2.3 million m3 of wood annually as raw material. The bark from this wood accounts for approximately half of the fuel used in the new power plant.
The Hybex boiler is one of the largest biomass-fired BFB boilers in the world
The heat demand from the mill and the towns forms the basis for the operation of the power plant. At nominal load it produces 125 MW of process steam and 15 MW of hot water for Kymi Paper as well as 40 MW of district heat for the area. At this load the power production is 76 MW and fuel input to the boiler is 290 MW. The plant achieves an overall efficiency of over 88 per cent and the power to heat ratio is 0.42.
The Kymin Voima power plant is connected to the Kymi Paper medium and low-pressure steam networks for distributing the process steam to the three paper machines and other heat consumers at the mill. In addition to the new plant, the two existing recovery boilers produce a maximum 94 kg/s of steam for two older turbines with nominal capacities of 28 and 15 MW. As a back-up, the mill has a natural gas fired boiler producing medium pressure steam.
The main fuel for the new boiler will be bark from the pulp mill's debarking plant. It will cover about half of the expected 2100 GWh annual fuel demand. UPM-Kymmene's nearby saw mills produce saw dust and wood residue, which are also combusted in Kymin Voima. The mill's waste water treatment plant produces sludge, which needs to be burned off. After mechanical drying the sludge gets a positive heating value and produces energy in the boiler.
In the first years the main fuel purchased will be peat, covering 28 per cent of the annual fuel energy. However, the target is to reduce the peat consumption and increase the share of forest residue whenever its production in the area increases.
The moisture content and heating value of the bark varies according to the species of wood and the time of year. The moisture is at its highest during winter time, when the boiler load is also at maximum. This situation provides the dimensioning basis for the boiler. Peat and forest residue somewhat level off the variation in heating value. Natural gas is used as start-up fuel. It can be used as an auxiliary fuel with load burners, in the case of any disturbances in the solid fuel handling system.
Bark and water treatment sludge are conveyed from the Kymi Paper mill via belt conveyors to Kymin Voima's 5000 m3 field storage. Wood fuels from the sawmills and forest residues are fed to the same storage. Before passing through to the storage they go though a screening and crushing station. Two screw feeders at the bottom of the wood fuel storage feed the fuel to a belt conveyor taking it to the boiler house.
Peat comes to a separate receiving station and it is also screened, and larger pieces crushed before passing to the 2000 m3 field silo. The peat is conveyed from this field silo to its own feeding silo in the boiler house.
During the last 20 years, bubbling fluidized boiler technology has become a dominant boiler technology in applications where a variety of wet biofuels is combusted and high availability and low emissions are sought. Kvaerner Power Oy was selected as the supplier for the Kymin Voima project. When commissioned in summer 2002, the Hybex boiler was the largest biofuel-fired BFB boiler in the world.
The steam parameters of the boiler are 107 kg/s, 114 bar and 541°C. With these parameters the boiler capacity is 269 MWth. The furnace width is 12 m, depth 11.4 m and height 33 m. The boiler has three-stage superheating, the two last stages being made from high chromium alloy AC66.
The boiler is equipped with a hydrobeam floor. The coarse removal system consists of 12 hoppers, three screw conveyors, one drag conveyor and a sieving station.
The Kymin Voima boiler is equipped with two independent fuel feeding lines. Each line serves three fuel feeding points and has a capacity of up to 70 per cent MCR. In this project special attention was paid to the fuel feeding system in order to achieve accurate and even fuel feeding. Each feeding line has its own bark silo. There is a common peat silo in between the bark silos. The peat is mixed with the other fuels in a mixing screw. The drag conveyor has a special levelling chamber for the mixing and equalisation of fuel flow. Each feeding point is equipped with a feeding screw.
The separate peat silo at the boiler house makes it possible to adjust the heating value without delay when there is a variation in the moisture content of the main fuel.
The air system consists of a primary air fan, a secondary air fan, steam coils and flue gas air preheaters. The boiler has a staged after-air system and the design is based on CFD modelling.
The load burner capacity is designed for 200 MW, and the burners are combined natural gas/oil burners. The boiler is equipped with two ESPs and flue gas fans on the flue gas side.
Boiler commissioning started in spring 2002. The first solid fuel firing was on 10 May 2002 and the boiler was handed over on 1 August 2003, two months ahead the original time schedule. The boiler has been running continuously since the end of July.
The heat demand from the mill and the towns forms the basis for the operation of the power plant. Loads were therefore low during the summer. Towards the end of 2002 load demand had been increasing. The new Hybex boiler also handles the load swings at the mill, while the two recovery boilers are running at constant load. This means that there is a great deal of daily load variation.
The boiler has been operated with different fuel mixtures since start up. On a daily basis the fuel mixture varies a lot, as does the moisture content. The natural gas consumption is exceptional due to the commissioning period.
The official plant performance test was carried out during the last week of January 2003. Preliminary results show that the boiler performance appears to be according to design in terms of aspects such as overall process design, boiler adjustability, controllability and flue gas emissions. The guaranteed emissions for the boiler are 150 mg NO2/MJ, 15 mg N2O/MJ, 250 mg CO/nm3 (at six per cent O2 dry) and dust 30 mg/nm3 (at six per cent O2 dry).
The main fuel for the new boiler will be bark from the pulp mill's debarking plant, which will cover about half of the expected 2100 GWh annual fuel demand
The start up and operation of the Kymin Voima Hybex boiler has proceeded very smoothly. The boiler is the fourth new boiler in this size class (under 300 MW) to be delivered in the last seven years. This signifies that the design for this size of boiler has become very standardized, and the designers were able to utilize all the experiences gained from the earlier deliveries.
It is well known that in fluidized bed boiler operation, the key factor in load and emission control is the fuel feeding system. Special attention was paid to the fuel feeding system in this project and the results so far are very positive.