Olmsted / USA
Olmsted / USA
Flexible Use of Energy through Sustainable Waste Management
Hitachi Zosen Inova’s expansion of the Waste to Energy plant in the small American city of Olmsted, Minnesota, not only created additional thermal processing capacity, but also brought the city a reliable and safe step closer to its ultimate objective: to divert 90 % of its waste from landfills. Moreover, now 37 public and private buildings can be supplied with heat and electricity from the plant’s waste heat recovery.
Olmsted has been sharing its thermal waste treatment facility with Dodge County since 1987. The community pursues an all-encompassing waste management concept. Its short term objective is to reuse approximately 37% of the 150,000 tonnes of waste produced each year, to treat another 50% thermally, and to only landfill the remaining 13%. Long term, 90% of the waste produced would be recycled or thermally treated. The two original process trains implemented in 1987 would soon no longer accommodate the needs of a growing population. It was decided to expand the facility by one additional train. Hitachi Zosen Inova won the bid to engineer, procure, and commission the complete combustion and flue gas cleaning train. The project began in 2007 and was completed three years later at the beginning of 2010 when the facility started its
operation. It now has the capacity to process an additional 200 tons per day (180Mg per day).
Proven Grate Technology
A crane thoroughly mixes the heterogeneous waste upon arrival at the plant. The waste is then pushed onto the Hitachi Zosen Inova five zone grate and becomes completely burnt out. A wet expeller discharges the resulting bottom ash. The ash is de-watered and compacted before a vibrating conveyor transports it downstream for further processing. A fully integrated combustion control system allows quick and simple adjustments so the combustion conditions can adapt to the continuously changing waste composition. This ensures the steady and efficient operation of the system.
Optimum Combustion and Flexible Use of Energy
The thermal energy released during the combustion process is used in a two pass horizontal boiler which produces approximate 61,000lb/hr (27.7Mg/hr) of superheated steam. The chosen parameters of 635 psia (44bara) / 655°F (350°C) ensure high energy efficiency. The superheated steam is expanded in a condensing turbine. The new and the two existing trains were combined to form an efficient heat and power coupling. The thermal energy can either be converted to up to 5.5MW of electrical power in a turbine generator or a slip stream can deliver heat into the district heating grid. This concept allows the plant the required flexibility to supply electricity or heat to 37 public and private buildings depending on the season.
Efficient Flue Gas Cleaning
The flue gas cleaning system uses a Selective Non-Catalytic Reduction (SNCR) process to lower the nitrogen oxides (NOx) formed during combustion. This process takes place at flue gas temperatures of 1,560°F to 1,830°F (850– 1,000°C) and uses aqueous ammonia as reduction medium. Lime slurry and activated carbon injected into a spray dryer serve to remove acid gases, such as hydrogen chloride and sulfur dioxide, and heavy metals that are entrained in the flue gas. Heavy metals adsorb to the activated carbon and are removed in a downstream baghouse (existing). The cleansed flue gas is then released into the atmosphere via a stack. Continuous gas measurements and monitoring ensure that all values fall below the mandatory emission limits at all times.