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16 VALMET


Pulp Paper & Logistics


Lime kilns go fossil free


I oil.


During the oil crisis in the 1970s and 1980s, several alternatives to fossil fuel were developed to reduce operational costs. In recent years, there has been an increased focus on environmentally friendly solutions which at the same time can make pulp mills less dependent on fossil fuel. With the latest technologies and expertise, there are now several commercially proven carbon-neutral fuel alternatives. The two main alternatives are wood powder firing and biomass gasification.


November/December 2019


n a modern pulp mill, the lime kiln is the only major consumer of fossil fuel – usually natural gas or fuel


Biomass-based alternatives for moving into 100 per cent renewable fuel for lime kilns have been developed by Valmet. Lotta Forssell reports


At pulp mills, methanol, turpentine and tall oil may also be available for burning in lime kilns. These work well as support fuel, but they cannot normally cover the lime kiln’s fuel requirements. Valmet also has the technology to extract lignin from black liquor, which can be used as fuel.


Safe and proven wood powder firing Typical wood-based biomass fuels used in direct-fired rotary kilns range from wood chips and pellets


to sawdust. Before the wood can be used as fuel in the lime kiln, it needs to go through drying and grinding steps to give suitable characteristics for firing. Designing a lime kiln to


operate with wood powder firing is a balancing act. As in all mill processes, safety is a key factor. To avoid wood unintentionally catching fire, its temperature is monitored, and good housekeeping is important, especially in the grinding system. In addition to safety, the moisture


content and particle size is optimised for energy efficiency and kiln operation. In the drying process wood


moisture content is reduced from a typical 30-50 per cent to between five and eight per cent. The lower the moisture level in the dried wood the more efficient the kiln operation and kiln capacity. A low-temperature belt dryer is often the preferred option for drying, because it can utilise waste heat from other mill processes, is robustly designed


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