INTRODUCTION In a modern pulp mill, the lime kiln is the only major consumer of fossil fuel – usually natural gas or fuel 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 CO2 neutral fuel alternatives. The two main alternatives are wood powder firing and biomass gasification. 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 for 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 obtain suitable characteristics for firing. Designing a lime kiln to operate with By Lotta Forssell, Vice President, Marketing and External Communications, Pulp and Energy, Valmet Lime kilns go fossil free Designing a lime kiln to operate with wood powder firing is a balancing act . wood powder firing is a balancing act. As with all mill processes, safety is the natural starting point. To avoid wood unintentionally catching fire, the temperature is monitored, and good housekeeping is important especially around 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 typical 30 – 55% to 5 – 8%. 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 and is suitable for heterogenous particle size. Moisture content also impacts the grinding process – the higher the moisture content, the higher the power consumption. The particle size needs to be optimised as fine and moist wood powder starts to block the hammer mill screens. Moisture also has a significant impact on the kiln operation as the moisture in the wood powder will evaporate which, in turn, reduces the flame temperature in the burning zone. Table 1 shows the impact on the kiln temperature profile, exhaust gas flow WORLD PULP&PAPER 101 Table 1. Estimated temperature profile, exhaust gas volume and fuel consumption relative to heavy fuel oil reference case for natural gas and wood powder with various moisture levels. The values in the calculations are typical values for a 700tpd modern lime kiln with cooler and flash dryer. Valmet has developed biomass-based alternatives for moving into 100% renewable fuel also for lime kilns. Exhaust gas temperature, °C Exhaust gas flow, Actual m3/s (wet) Fuel consumption, % Oil – reference, typical values ~285 ~52 100% Natural gas +50 114% 105% Wood powder, 0% H2O +3 99% 101% Wood powder, 3% H2O +12 102% 101% Wood powder, 5% H2O +18 104% 102% Wood powder, 10% H2O +35 110% 104% Wood powder, 15% H2O +56 117% 107% BIOMASS FUELS