Mills use close to 200 litres of furnace oil to produce one ton of lime, one of the single largest expenses in its operations. Moisture content in the lime can significantly increase the furnace oil requirement and also disrupt smooth operations of the lime kiln. Consistent quality of reburned lime from the lime kiln is critical to the entire pulping process.
Large amounts of furnace oil are consumed in the caustic recovery process. Reducing the moisture content in the lime mud is an effective way to reduce heat loss in the lime kiln and save significantly on furnace oil consumption. Additionally, the yield of the lime kiln increases and scaling problems decrease improving the overall efficiency of the operations.
HABER’s lime mud dewatering program was established at one of India’s largest paper mills, resulting in significant reduction in total cost of operations for the customer as well as improved plant operations. The mill uses the kraft pulping process, which is the most widely used pulping process in the paper-making industry due to its advantages over other pulping processes. The process involves digestion of wood chips at high temperature and pressure, with black liquor being the by-product of the process. The black liquor is then processed in the chemical recovery plant where components (inorgranics) are recovered for reuse in pulping and other components (organics) are resued as fuel in the boiler. This process is highly efficient and ensures significant reuse (>95%) of chemicals used in the process.
Green liquor from the recovery boiler is sent to the causticizing plant where it is causticized using lime. The causticized liquor (white liquor) is returned to the digester. The lime mud generated in this process is sent to the lime kiln for reprocessing. The lime kiln converts the lime mud generated in the causticizing plant into reburned lime which is again reused in the causticizing plant resulting in full recovery and reuse of the lime. The conversion is accomplished through calcination reaction at temperatures greater than 800 O C requiring large quantities of furnace oil. Hence, dryer lime mud going into the lime kiln would result in decrease of furnace oil requirement in the lime kiln. Moreover variations in quality of the reburned lime can cause problems in the causticizing plant and inefficient processing in the lime kiln may even produce emission of undesired noxious TRS (total reduced sulfur) gases.
Reducing the moisture content in the lime mud has a direct impact on the fuel economy of the lime kiln since heat is required to remove the excess moisture. There is thus a very strong correlation between lime mud moisture content and lime kiln fuel efficiency – the fuel consumption of the lime kiln improves by 2% for every 1% reduction in moisture in lime mud. High moisture content can also result in other kiln operational issues like decrease in efficiency of calcination, ring and ball formation and scaling on the surfaces. On the other hand, lime mud that is too dry can cause excessive dusting and overheating resulting in failures of critical parts. Therefore, an optimum moisture content needs to be maintained for both economical and operational reasons.
Lime mud dewatering efficiency depends not just on the mud and liquor properties but also on the equipment design and operational parameters. And these properties change continuously resulting in variations in performance and variations in fuel consumption. A program that monitors and reacts in real-time to these variations is thus highly beneficial to reduce variability, optimize operations and lower costs. After implementing HABER”s lime mud dewatering program, the mill saw >10% increase in lime mud solids resulting in a significant reduction of fuel consumption in the lime kiln. The decreased use of fuel resulted in an overall reduction of the customer’s carbon footprint by nearly 1,000 tonnes per year. Lime production also increased by >10% improving the plant’s throughput significantly. Moreover, the operation is now experiencing fewer scaling and operational issues. HABER’s approach to lime mud dewatering ensures optimum moisture levels are maintained in the lime mud resulting in both economic gains as well as smooth operations. HABER’s solution is highly standardized and requires little to no customization resulting in quicker implementation and value realization.