7.4.5.2 Lime

Generally lime refers both to high-calcium and dolomitic forms containing magnesium. Lime is produced by burning limestone or dolomite in small-scale vertical or large-scale rotary kilns. While in most industrialized countries the industry is concentrated in a small number of larger corporations, in most developing countries lime kilns are small operations using local technology. Even in industrialized countries like Greece there are independent small-scale vertical kilns in operation. Pulp and sugar mills may have captive lime production to internally regenerate lime. Lime is mainly used in a small number of industries (especially steel, but also chemicals, paper and sugar), mining, as well as for flue gas desulphurization. There are no detailed statistics on global lime production, however Miller (2003) estimated global production at 120 Mt, excluding regenerated lime. The largest producers are China, the USA, Russia, Germany, Mexico and Brazil.

Process CO₂ emissions from the calcination of limestone and dolomite are a function of the amounts of calcium carbonate, magnesium carbonate and impurity in the feedstock, and the degree of calcination. Theoretical process emissions are 785 kg CO₂/t (214 kgC/t) calcium oxide and 1092 kg CO₂/t (298 kgC/t) magnesium oxide produced. Energy use emissions are a function of the efficiency of the process, the fuel used, and indirect emissions from the electric power consumed in the process. In efficient lime kilns about 60% of the emissions are due to de-carbonisation of the raw materials. No estimates of global CO₂ emissions due to lime production are available. In Europe process emissions are estimated at 750 kg CO₂/t (205 kgC/t) lime (IPPC, 2001). For some applications, lime is re-carbonated, mitigating part of the emissions generated in the lime industry. Regeneration of lime in pulp and sugar mills does not necessarily lead to additional CO₂ emissions, as the CO₂ is from biomass sources (Miner and Upton, 2002). Emissions from fuel use vary with the kiln type, energy efficiency and fuel mix. Energy use is 3.6 to 7.5 GJ/t lime in the EU (IPPC, 2001), 7.2 GJ/t in Canada (CIEEDAC, 2004) and for lime kilns in US pulp mills (Miner and Upton, 2002), and up to 13.2 GJ/t for small vertical kilns in Thailand (Dankers, 1995). In Europe, fuel-related emissions are estimated at 0.2 to 0.45 tCO₂/t (0.05 to 0.12 tC/t) lime (IPPC, 2001). Electricity use for lime production is 40 to 140 kWh/t lime, depending on the type of kiln and the required fineness of the lime (IPPC, 2001).

Emission reductions are possible by use of more efficient kilns (Dankers, 1995; IPPC, 2001) and through improved management of existing kilns, using similar techniques to the cement industry (see Section 7.4.5.1). Switching to low-fossil carbon fuels can further reduce CO₂ emissions. The use of solar energy has been investigated for small-scale installations (Meier et al., 2004). It may also be possible to reduce lime consumption in some processes, for example the sugar industry (Vaccari et al., 2005).