Life cycle assessment of converting lime kilns from fossil fuels to wood powder

Abstract

Carbon footprint refers to the amount of greenhouse gas emissions associated with a product or system. This is expressed as CO2 equivalents, which take into account the global warming potential of each greenhouse gas (GHG). Achieving current climate targets requires significant reductions in GHG emissions across all sectors. This study examined the use of wood powder as an alternative to fossil fuels in lime kilns through a life cycle assessment (LCA). A lime kiln is an essential part of the chemical recovery cycle in pulp mills. Many modern pulp mills generate sufficient energy for their operations, utilizing wood as the primary raw material, which results in most of their CO2 emissions being biogenic. The lime kiln has a significant role in GHG emissions of a pulp mill, as it is typically the only unit operation where fossil fuels are used in normal operations. Natural gas or heavy fuel oil are typically used as fuel. Therefore, the selection of the fuel can have a significant impact on the carbon footprint of a pulp mill. To reduce the environmental impacts of lime kilns, various alternative fuels, such as biomass gasification, wood powder, lignin, tall oil, methanol, and turpentine, have been considered. Among these, wood powder firing has been identified as a promising alternative to replace fossil fuels in lime kilns. Its implementation requires additional equipment for fuel storage, preparation, transport, and dosing. These systems can be retrofitted to an existing plant. Essential requirements for the fuel include suitable moisture content and particle size, stable fuel flow, and consistent feedstock quality. The LCA aimed to quantify the potential reduction in GHG emissions when transitioning from fossil fuels to wood powder in lime kilns, taking into account the required equipment modifications to enable the use of wood powder and the fuel change. The existing equipment was excluded from the scope of the study as it remained the same regardless of the fuel. The study was conducted in accordance with ISO 14067 standard and was modelled using SimaPro software. The studied impact category was climate change, and the functional unit was one ton of burnt lime. The results indicated that an 88.8% reduction in GHG emissions could be achieved by replacing HFO with wood powder. When replacing natural gas with wood powder, the study indicated an 85.7% reduction in GHG emissions. Additionally, GHG emissions from natural gas use were 21.4% lower than those from HFO. The study also identified the primary contributors to the carbon footprint of the wood powder case study. The majority of GHG emissions occurred from the use phase, which accounted for 99.3% of the total life cycle emissions. Contributions of other life cycle stages were minor. The primary sources of GHG emissions were the production of wood pellets (80.7%) and electricity generation (17.8%). Within the wood pellet production process, the two most significant single contributors were heat production and electricity generation. The results indicate that this type of system modification and fuel change can lead to a substantial reduction in GHG emissions, offering great potential to support broader industry efforts to reduce dependency on fossil fuels and mitigate climate change. However, a more comprehensive evaluation of the environmental impacts of the system would require the inclusion of additional impact categories.