Influence of fuel mix on mass balance of Cl, S, and K in a long rotary kiln

Abstract: Lime is a material with a wide range of applications including land improvement, water treatment as well as the paper-, steel-, and cement industry. Annual worldwide production was estimated at 350 Mt/year for 2016 and 2017 and the production is responsible for a large contribution of the greenhouse gas CO2 to the atmosphere, contributing to global warming. Since there is a need for high temperatures in the production process it is common that fossil fuels are used due to their high energy density and favourable combustion characteristics. Therefore a net emission of CO2 comes from the fuel, but a large amount is also released from the calcination of the raw material, limestone, approximately 1 ton of CO2 is released for every ton of quicklime (CaO) produced. Efforts are being made toward reducing emissions from both the fuel and the limestone. Nordkalk AB, for whom this work has been performed, are aiming to partly replace their use of fossil fuel in their lime kiln in Köping with renewable biomass alternatives. The purpose of this study was to establish a mass balance over the lime kiln in Köping for different limestone and fuel mixes and study the effects they had on mass balance for sulphur, potassium, and chlorine. Five different periods were studied where four of them had fuel mixes consisting of fossil fuels (coal and oil) and in the fifth period olive pomace, a renewable fuel, was added to the fuel mix at 11 % of the net calorific input. It was found that no significant change in product quality occurred and no increase in sulphur and chlorine emissions through the flue gas was detected with this fuel addition. However there were indications from throughput calculations that buildup of corrosive deposits, such as KCl, could be occurring with the addition of olive pomace due to comparatively large input of potassium and chlorine. The increased sulphur input form olive pomace appeared to accumulate in the lime kiln dust mass flow while flue gas emissions remained unchanged. This method of establishing a mass balance is considered to be a good way to understand how different inputs affect the elemental balances over a lime kiln. However the quality of the mass balance is highly dependent on how representative the composition of the mass flows are and how they are acquired.