Pulp Paper & Logistics


and is suitable for heterogeneous particle sizes. 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 has also a significant impact on the kiln operation as the moisture in the wood powder will evaporate, which reduces the flame temperature in the burning zone. Table 1 shows the impact on the kiln temperature profile, exhaust gas flow and fuel consumption when increasing the moisture in wood powder. The values are compared to the main fossil fuels oil and natural gas. Controlling the flow of wood

into the kiln is an important factor in stable kiln operation. Unstable flow will cause a variation in heat input to the burning zone, impacting product quality. The lime kiln burner needs to be flexible and able to safely and efficiently burn several different types of fuel.

Suitable solution for both old and new mills Valmet has delivered several wood powder firing systems utilising sawdust, pellets or wood chips.

Typical wood-base biomass fuels used for direct fired rotary kilns range from wood chips and pellets to saw dust

SCA’s Munksund paper mill in Sweden utilises pellets as a wood powder source.

Claus Jensen-Holm, director for Exhaust gas

Oil – reference, typical values Natural gas

Wood powder, 0% water Wood powder, 3% water Wood powder, 5% water Wood powder, 10% water Wood powder, 15% water

temperature, °C ~285 ref

+50 +3

+12 +18 +35 +56

lime kiln technology at Valmet, says: “We have shown during the last decade that wood powder firing has become a fully

Exhaust gas flow, Actual cubic metres/ sec (wet)

~52 ref 114 99

102 104 110 117


consumption, % 100 ref

105 101 101 102 104 107

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 level. The values in the calculations are typical values for a 700tpd modern lime kiln with cooler and flash dryer

commercial, proven and – most importantly – safe solution for lime kilns. Today it is possible to convert a pulp mill into a fossil- free mill and thus reduce the mill’s carbon footprint and fuel costs.” Fredrik Lind, production engineer at the SCA Munksund mill, adds: “Our lime kiln was 50 years old and had seen better days. To match our strategy of being fossil fuel free, we decided to build a new kiln utilising biofuel. The new kiln was ready and in operation in January 2015. At first, we had major problems with, for example, ring formation and variations of fuel flow. “After many hours of

4 November/December 2019

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