Two of the largest biomass gasification systems currently in operation in the world have been built by Finland-based Metso Power. One of the systems processes 250,000 tons per year (TPY) of sorted municipal waste as the feedstock to produce 50 megawatts (MW) of electricity and 90 MW of thermal energy for the city of Lahti, Finland. The second system is cofiring syngas with coal at a 230 MW electric utility and is displacing up to 40 percent of the coal and 230,000 TPY of carbon dioxide in the city of Vaasa, Finland.

Metso’s first circulating fluidized bed (CFB) gasification system was installed on a papermill lime kiln in Varo, Sweden, in 1987 and is still operating today.

Today, Metso’s gasification technology has the ability to deal with a broad range of biomass materials inclusive of sorted municipal solid waste (MSW) that would normally be disposed of in conventional landfills.

Metso’s gasification technology uses atmospheric air fed through nozzles in the bed of the gasifier to suspend solid feedstock and bed material in an upward motion during the gasification process. The resulting turbulence mixes the air, feedstock and bed material in a substoichiometric environment to provide for uniform gasification.

This gasification technology makes it possible to use low-grade feedstocks and still be able to meet more stringent regulatory air emission limits. The power generation segment’s demand for less expensive emission controls and lower lifecycle costs are all incorporated into this new line of industrial gasification equipment.
 

Technology Up-close

The Metso gasification technology is based on the principals learned from more than 70 industrial and utility CFB applications currently in operation around the world and from Metso’s first commercial gasifier installed on a lime kiln in 1987.

The self-supporting refractory-lined gasifier converts solid fuel into a syngas in a substoichiometric environment. This syngas, including all of the hydrocarbons, contains 90 to 98 percent of the energy that was in the original feedstock and transfers all of that energy directly to the boiler.

The gasifier’s versatility lies in its range of commercially available sizes and fuels that can be utilized as feedstock. Retrofitting an existing boiler is also a significant advantage since extended outages are not required. The entire gasification system can be constructed at the host site while the existing facility remains in operation. Once the gasification system installation is complete, the necessary plant tie-ins can be completed, usually during a normal planned facility outage.

One issue associated with syngas is low heat density. This relates to the fact that this is an atmospheric gasifier which uses ambient air for fluidization which is made up of up to 61 percent nitrogen gas (N2).

Moisture also has a significant effect on the quality of the syngas. Fuel moisture also has an ultimate effect on the heating value of the syngas. At 40 percent moisture, the higher heating value (HHV) of the feedstock will be less than half of what it would be at 10 percent moisture. In a utility application where very moist fuel was available, a drier was installed in front of the gasifier which used heat from the returning district hot water system as the energy for the dryer. Using this waste heat did not decrease the overall system efficiency which is more than 85 percent. Drying the fuel also reduces the mass flow through the boiler and can eliminate any boiler derating.
 

Commercial Applications

The two gasification systems currently in successful commercial operation include the Lahti, Finland, gasifying sorted municipal waste and the facility at the Vaskiluodon Utility gasifying waste wood and peat and cofiring the syngas with coal.

In Lahti, Finland, the gasifier at Lahti Energia began commercial operation on sorted MSW in December 2011 and is currently operating at a plant availability of 80 percent. Some minor modifications have been made to the startup cycle which will shorten the startup time and are expected to increase overall plant availability to 90 percent. The system provides 50 MW of electricity and 90 MW of thermal energy to the city and replaces an old coal-fired boiler that consumed 170,000 metric tons of coal.
 

Metso supplied a complete turnkey installation for the city of Lahti. This included feedstock handling; feedstock feed system; gasifier; gas cooling; gas filter; gas boiler and flue gas cleaning.

The syngas leaves the gasifier between 1,560 and 1,650 F and passes through the gas cooler. This reduces the syngas temperature down to approximately 750 F prior to entering the gas filters. This temperature is high enough to prevent any hydrocarbons from condensing onto the filters. As the gas is cooled down, most of the metal chloride will condensate out and form particles. The gas filter is made up of a number of ceramic cones which filter out some of these chloride particles and other particulate matter such as lead and zinc. This reduces the corrosive elements and particulate entering the boiler and results in a gas similar in cleanliness to natural gas.

Because the corrosive elements in the syngas have been removed, this clean syngas can be fired in a boiler at a higher steam temperature and pressure which is not possible in a conventional grate-firing waste boiler due to corrosion. The increased steam temperature and pressure going to the turbine improves the electrical efficiency of the plant. A typical grate system burning MSW would have a heat/power ratio of 42 percent while a gasifier would have a ratio of 56 percent. This results in a 6 percent increase in power output. A grate-fired boiler with a fuel input of 100 MW would typically produce 25 MW of electricity. With a gasifier with gas cleaning and a gas boiler, this same fuel input would produce 31 MW of electricity or 24 percent more electricity.

Because the corrosive elements have been removed from the syngas, Metso technology is not limiting steam parameters. This system has the highest power production efficiency in waste firing. It is possible to build a plant with even higher power efficiency than Lahti.
 

Cofiring Syngas with Coal

The Vaskiluodon Voima utility has a pulverized bituminous coal-fired boiler that can fire peat in addition to pulverized coal. The boiler has a capacity of 230 megawatts of electrical output (MWE) and 175 megawatts of thermal output (MWT) producing steam at 2,700 pounds per square inch gage (psig) at 1,000 F and supplies district heating and electricity for the city of Vaasa, Finland.

Biomass is one of the easiest and least expensive feedstocks to cofire with coal in a stoker boiler by blending the biomass with the coal. As the percentage of biomass feedstock increases, there is a significantly higher mass flow of total feedstock to the boiler resulting from the lower heating value of the combination of feedstock. Based on the Btu-per-pound differences, it could take more than two times as much wood to provide the same energy content as Central Appalachian coal, depending on the moisture in the wood feedstock. Wood waste cannot be directly fired in a pulverized coal (PC) boiler.

Syngas does not present any fuel-handling issues since the energy going to the boiler is in the form of a gas. The syngas still contains 90 to 98 percent of the energy that was in the original feedstock. If the available fuel has a high moisture content, adding a dryer in front of the gasifier removes moisture thus reducing the mass flow through the PC boiler. This can result in no boiler derating as is typically the case when firing wet wood directly into a stoker boiler. The coal-fired boiler is always available and can fire 100 percent coal if the gasifier were to go down for unscheduled maintenance.

The article was submitted by Metso Power. More information is available at www.metso.com.