Hiring and firing: the joy of fuel flexibility

Finland’s Alholmens Kraft CHP plant is unique in that it utilizes different fuels from biofuels to coal to fire its large-scale boiler. By using this wide range of fuels, the plant is able to be both environmentally friendly and highly competitive. PEi finds out how.

Stig Nickull, Oy Alholmens Kraft & Pertti Petàƒ¤nen, Metso Power, Finland

The overriding objectives of the Alholmens Kraft power plant, which was commissioned in early 2002 in Pietarsaari, Finland, was to demonstrate for the first time novel technology for multi-fuel and low emission cogeneration at a new commercial size and co-firing of biomass with fossil fuel.

The Alhomens power plant in Pietarsaari, Finland utilizes a range of fuels, chiefly bark, peat and coal
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Another crucial objective was that the steam and heat had to be supplied as economically as possible, and with high reliability under different kinds of operational situations. The electricity production also needed the capability to respond to the base and variable load demands in an economical way, so that it could be sold at a competitive price for sale in the open market.

This is easier said than done, but is has been achieved by Alholmens using a wide fuel mix, enabling the Finnish operator to acquire its fuels according to supply, which makes the power plant not dependent upon a single supplier. The ratio of fuels used is varied according to availability, quality and price. In addition, Alholmens can operate either in combined heat and power (CHP) mode or in condensate mode depending on the electricity demand and market price.

Fuelling the fire

The main fuels used at Alholmens are bark, wood residue, stumps and peat, with coal as an alternative, back-up fuel. Bark and wood residue are low calorific, high moisture fuels with varying energy content and very heterogeneous physical properties. Peat is also a low calorific fuel, with high moisture content but a much more stable quality. The maximized use of carbon dioxide (CO2) neutral fuels is favoured, and additional fuels can be used for increased capacity. The fuel mixture varies constantly and is optimized to fulfil operational, environmental and economical requirements.

Barking up the right tree

The first solid fuel firing was carried out with a mixture of main design fuels – peat and biomass, bark and harvesting residue. The tuning of the four independent fuel-feeding lines went very smoothly despite the large amount of equipment that had to be synchronized. All the load levels were tested with the biomass mixture and the parameters for the controllers were fine-tuned. The coal-biomass mixture was then tested followed by pure coal combustion tests. All the load levels were investigated with all fuel combinations in order to optimize the control loops built into the plant’s DCS system.

At Alholmen the proportion of biomass in all fuels varies from 35 to 45 per cent annually. The most important biomass fuel is bark, which represents two thirds of the biomass. The biomass is bought-in based on long-term contracts and some spot-lots. There is a constant flow of wood residues and bark from the saw and pulp mills throughout the year via the plant’s conveyors. The ratio of soft wood to hard wood used in the mill is 50:50, and there are seasonal moisture variations. The summer moisture content is 45-50 per cent and 55-60 per cent in winter, depending on wood quality. Incoming snow causes additional moisture variations and decreases the heating value of harvesting residues, which has to be compensated by adding some higher heating value fuel into the fuel mixture.

Next in priority is peat, which is the main fuel for the plant, representing roughly 45-55 per cent of the annual energy input. Peat has an important role in balancing the irregular moisture content of harvesting residues. With wood only it is difficult to get maximum output of the boiler, since high water content is limiting the capacity.

The availability of peat, however, depends on weather conditions. Harvesting is easy and quality is good in good summer weather.

Coal as the back-up fuel presents only 5-15 per cent of the annual fuel consumption. It is purchased on the international market to balance the fuel input and in order to meet the energy demand at all times around the year. The boiler’s fuel flexibility can be fully utilized because it can be used as a main fuel during wintertime when there are temporary restrictions in biomass or peat availability.

On the boil

A circulating fluidized bed (CFB) boiler was chosen to enable biofuel combustion with coal and peat, and to meet the strict environmental requirements concerning emissions. The boiler capacity is 550 MWth. The large fuel flow and fuel variation make even mixing of the fuel components essential in order to secure optimal combustion, which ensures the lowest emissions for the circulating fluidized bed process.

Emissions in the guarantee tests, April, 2002
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Fluidized bed technology has one important advantage: it can simultaneously burn several fuel types that have very different properties. A multi-fuel boiler provides a good deal of flexibility for the operation. It is always possible to select the fuels with the best availability. The capability to select the fuel combination also gives the owner a significant opportunity to keep fuel costs at a reasonable level when negotiating fuel contracts. Flue gas emissions can be minimized through fuel mix choice, and the high operating efficiency of the boiler plant enables the management of the challenges related to corrosion. Utilizing biofuels as the main fuel in a boiler of this size is unique.

Boiler accommodates load changes

The boiler has now been in operation for approximately five years. At the start-up phase it was following a regular weekly operation mode: weekdays at full load, and night times and over the whole weekend at a 40 per cent load. This meant two load changes per day typically at 2-3 MW/min (max. 10 MW/min). These load changes came from the power production demand instructed by the owner’s central control room.

Due to radical changes in the electricity market the plant has practically been running at full load throughout the past few years. Load changes caused by the paper mill are small in relation to boiler capacity. These are typically immediate stepwise changes, ten per cent at one time, which the boiler can easily manage without any process fluctuations.

A breakdown of the availability of Alhomens Kraft
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Apart from some very short stops the boiler has been in continuous operation since the start-up. It is operating according to design, and steam production has met the required steam parameters. Mechanical strength and durability is good. These are carefully followed-up, since continuous good availability is very important because of the plant’s operational environment.

Beside power production, the boiler produces process steam for the adjacent paper mill, which operates through the year. The boiler was in operation for a total of 7700-7900 hours in 2006, with an average electricity output of 217 MW.

Keeping the boiler under control

Controllability of the boiler is very important. The load changes due to one paper machine start-up or shutdown are reasonably small (à‚±ten per cent), but the change in process-steam demand happens at once and directly affects the boiler load. Furthermore, another load change requirement comes from power markets. The load changes for Alholmen can be from 40 per cent to 100 per cent or vice versa according to the hourly rates in the Nord Pool (The Nordic Power Exchange). There are big load changes between day and night times.

The Alholmen boiler features a membrane furnace and steam-cooled cyclones
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The Alholmen boiler features a membrane furnace and steam-cooled cyclones, the lifecycle costs of which are lower than those of an uncooled construction despite higher capital cost. Considerable savings also stem from uninterrupted operation.

A rotary type air preheater helps achieve higher boiler efficiency with moderate pressure loss in the air and flue gas sides. The challenges are leakages and possible corrosion problems. The leakage has been minimized by adopting a four-sector design in which the high-pressure primary air is located between the secondary air sections, which limit the leakage to a one per cent unit O2 (oxygen) rise in flue gas. The corrosion problem has been solved by using an enamelled cold-end in the preheater.

Co-combustion enables the burning of demanding fuels with high efficiency, without corrosion
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Chlorine and alkalis in biomass together create a potential for high temperature corrosion on heating surfaces with a metal temperature in excess of 500 à‚°C. In co-combustion even a small amount of sulphur containing peat or coal significantly decreases the risk. Co-combustion enables the utilization of demanding fuels à‚— biomass or even waste à‚— maximally and with high efficiency.

The boiler is equipped with four biomass silos. Each silo has its own independent fuel feeding line, which serves three fuel feeding openings. The feeding system is designed so that stopping one feeding line causes minimal disturbance in the combustion. Full boiler capacity can be achieved with the three feeding lines. Coal is fed via one coal silo having two hoppers and feeders. After the feeders, coal is distributed into the same feeding lines as the biomass. The feeding lines can operate with any proportion of biomass and coal. The boiler 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 boiler features three steam-cooled cyclones. These cyclones are the first superheater surface in steam circulation to minimize the temperature difference between the furnace and cyclones. The reason for making the cyclones steam-cooled instead of water-cooled was to ensure circulation with a high operational pressure (165 bars) in all conditions. The cyclone is service free due to expansion joints formed of pressure parts. The construction also eliminates the need for heavy refractories and expansion joints. There are also other verified advantages, such as the capability of handling fast load changes, tolerance of high gas temperature, low heat losses and reduction of the combustor size. All these features improve reliability, which was clearly the most important factor, with a demand for increased operating hours and shorter maintenance breaks.

Dealing with erosion

The total maintenance costs of the boiler plant are approximately g2.5 million a year. Erosion is strongest in the three 30 m-high cyclones, especially on the gas target surfaces. It covers an area of approximately 15 mà‚².

Deep crates have formed on the walls above the masonry. A boiler shutdown in 2007 included modernization a total of 2500 primary air nozzles and the cover plates in the central tube of the cyclones.

The new primary air nozzles feature new material and manufacturing technology which have been tested for three years in very demanding conditions. Standard nozzles were replaced by new, more robust design because of extremely wearing operating conditions. The cover plates in the central tube of the cyclones also feature a new construction, specially designed and tested for the conditions prevailing at Alholmen.

Burning biomass either alone or together with other fuels lowers net CO2 emissions. This is an important advantage with regard to emission trading and the Kyoto Protocol. Emission control requires even fuel and air distribution and combustion air staging.

The sulphur dioxide (SO2) emission limit is tight at Alholmen. The plant features a limestone feeding system and a selective-non-catalytic-reduction system. Cyclones separate unburned fuel particles and return them to the furnace, as well as give the limestone particles used for sulphur removal more time to react.

Dust control is handled with a four field electrostatic precipitator. Power demand of the electrostatic precipitator in only 100 kW, and dust emission is very low, only 3-5 mg/nm3.

Despite the fact that the Alholmen plant is equipped with this equipment, the need for additives can be eliminated through optimization of biofuels and other fuels. For example, in 2005 there was no requirement for the addition of lime or ammonia.

Future challenges

The demand for biofuels within the power generation sector looks likely to grow as we move away from fossil fuel based generation. Thus, the combustion of biomass, and other new alternative fuels are likely to gain favour, with biofuels and peat being treated favourably in future energy taxation.

The Alholmens Kraft co-fired biofuel project is clearly a success with the boiler operating at maximum efficiency. It has provided the opportunity to produce ‘green energy’ according to the principles of sustainable development and confirms the rationality of this investment decision made five years ago.

Considering the future outlook of the power production field boiler manufacturers should be encouraged to further enhance the multi-fuel boiler efficiency and availability, and to continue to develop solutions that require minimal maintenance.

This may be a good time for the authorities to study the possibilities to increase co-combustion. With its flexibility and wide variety of opportunities regarding different fuels and fuel mixes it can be considered an operating solution for many years to come.

This article is based on a paper entitled ‘Co-firing experiences of biomass with fossil fuel in the world’s largest biofuel power boiler’, which was presented at POWER-GEN Europe 2007, and won a ‘Best Paper’ award.

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