Vaskiluodon Voima Oy, a traditionally coal-fired power plant on the Baltic Sea coast of Finland, has gone green. It successfully completed a project to substitute wood biomass for a sizable portion of its coal fuel source in December of last year (the official ceremony took place in March). In partnership with supplier Metso Power Oy, the plant installed a 140 MW CFB (circulating fluidized bed) wood gasifier, the largest biomass gasification plant in the world according to Metso. The unit feeds an existing 560 MW boiler.
Established in the late ’60s, Vaskiluodon Voima, owned in equal shares by EPV Energia Oy and Pohjolan Voima Oy, has two power plants—one in Seinajoki and this one in Vaasa. The plants generate a combined total of 2 TWh/a of electricity and most of the district heat required by the nearby towns. Prior to this gasification installation, Vaasa (designated Vasiluoto 2) fired only coal in a Benson type PC boiler.
Company principals opted for wood gasification because they determined it to be more cost effective than building a new biofuel power facility and about half the cost of wood pellet production. While the gasification plant runs at full capacity, the coal-fired plant has reduced its contribution to the boiler loads. The plant reports that it now uses 3-6kg less coal per second than before; that amounts to one less shipment of coal per month. Vaskiluodon Voima’s total investment was under 40 MEUR (million euros).
The project was driven by the decision of plant executives to achieve the flexibility to access multiple fuel sources and optimize costs by utilizing forestry residues in addition to coal. With the gasification process they found it possible to replace from 25-40% of coal. With environmental legislation tightening, the renewable, CO2-neutral fuel and the reduction in emissions is an important step for the company. Plant executives also hoped to extend the lifetime of the existing power plant unit and to guarantee a cost-effective district heating solution for the surrounding area.
Installation
Metso was involved in the full scope of the project, supplying not only the gasifier but the fuel receiving and handling lines, the drying line, and Metso DNA automation system (including electrification and instrumentation for the entire system), as well as making modifications to the existing boiler to receive wood gas. All in all Metso’s contract accounted for 80% of the total investment cost; the rest came from the plant’s direct costs for site work, foundations, piping, fire protection systems and electricity, via its main consultant Novox Oy.
“Usually my challenge is how to plan the installation so production is not interrupted,” according to Timo Honkola, product manager for Metso. “ With gasification it is pretty easy or possible to put it next to an existing power plant during its normal operation, and we were able to make the modification of the boiler during the normal power plant outages so they didn’t lose any production.”
Although the total project costs stayed within budget, installation time schedule was very tight, according to Honkola. That pressed final installation work to late autumn 2012, making the test running period quite challenging. Even so, the boiler modifications were made as planned during the five weeks outage. During the test run there were only minor effects on PC-boiler operation, and no boiler trips were necessary—which was important for the plant managers.
Management at Vaskiluodon Voima has been quite pleased with the system and is confident it made the right decision. The company reports that, in the seven months since installation, there have been no instabilities in the boiler and that NOx emission levels have been between 350-400mg/m3(n) (milligrams per normal cubic meter), well under the limits of the environmental permit, which is 500mg/m3(n). The dryer was quite a good investment, they add. In fact it proved particularly critical this year. Wood biofuels have come in with a higher than normal moisture content, up to 60%, due to excessive rain in Finland last summer.
Process
Wet biomass enters through receiving and pre-handling station to the belt dryer, at the end of which dried biomass enters the storage silo and then the CFB gasifier. Product gas from the gasifier unit exits to the existing PC boiler, which then generates electricity and thermal energy for the city of Vaasa. The process can also use bark, peat and straw as fuel.
The wood supply comes from primarily within 100 km of the plant, but some percentage comes in from abroad via the adjacent harbor, as the plant property sits directly on the Baltic Sea coast. Ships in the harbor also deliver coal to the plant. Notably, the largest windmill in Finland is also located in sight of the property.
The rest of the fuel comes in via truck. The plant consumes about 30 truckloads daily. A weigh station features four bays, all of which can receive wood, though two are adapted to also take peat. Truck loads are weighed, unloaded and analyzed—suppliers are paid based on weight and quality. After unloading and sampling, fuel proceeds to the screen and crushing station.
Normally, wood is already chipped upon delivery to the site. “As a fuel specification more than 90% of the fuel should pass through a 63 mm sieve and no particles longer then 200 mm are accepted,” according to Claes Breitholtz, a service and technology specialist with Metso. “In the fuel supply chain we have two sieves, one for 200 mm where the oversized particles are rejected, and one for 80 mm where oversized particles are sent to a crusher before being sent to the silos.” Oversized particles are removed from the fuel stream to be reduced in a Bandit chipper. A magnet removes all ferrous metals. The material then moves to one of the storage silos—one for wood and one for peat. Each silo has the capacity to contain 2,500 cubic meters. All of this was supplied by Metso as part of the package.
Fuel is loaded from silos via conveyor onto the top of the Swiss Combi belt dryer (licensed to Metso). The fuel is distributed in an even layer about 15-20 cm thick. The low-temperature dryer (under 110° C) runs on heat from the plant’s own processes and reduces gaseous emissions and VOC (volatile organic compound) release during drying. Ambient air is drawn through heat exchanger coils by an axial fan as the product layer moves slowly through the dryer tunnel. Moisture content is gradually reduced from an average of 55% to 20% as the material moves through the dryer. The percentages can be adjusted depending on the operating parameters of the system, and the speed of the dryer is adjusted according to moisture content. The belt also functions as a filter, minimizing dust in humidified air exhaust.
A conveyor belt moves the dried fuel from the dryer to the gasification chamber, which is surrounded by walls with open slats to allow good air flow. The CFB gasifier itself has an outside diameter of six meters and stands at a height of 36 meters. Two fuel storage silos and feeding lines, adjacent the gasifier, lead into the CFB reactor.
The reactor burns biomass by partial oxidation into product gas. It uses silica sand as its bed material—other similar systems often use limestone. The sand is fed to the system pneumatically from a silo. It serves as a heat conductor, circulating throughout the reactor, which has an internal temperature of 850° C (1,562° F). “It is important to have a good conductor for fuel and then to have a good heat transfer to the fuel, which needs to be hot particles,” Honkola explains. The heat converts biomass to a combustible gas. As it moves upward through the reactor it enters the cyclone at the top, which separates the sand from the product gas or syn gas. The cyclone returns sand to the bottom to be recirculated, while the ash removal systems dispose of charred material. The main unburned gases are CO and H2—around 10% each—with some N2, H20, CO2 and tar.
After the cyclone, the product gas flows down through a heat exchanger where it is cooled by gasification air, which is pushed in from top. This process allows the gasification air to heat up while cooling down the product gas. As the product gas exits through a pipeline to the boiler, the air is fed into the gasification chamber. In the boiler, gasified biomass and coal are fed with separate burners to combust together and generate electricity and heat.