Computer modeling as tools to estimate the impact of fuel type on the capacity rating of lime kilns

In response to the drop in the price of natural gas, the U.S. pulp and paper industry has switched from using fuel oil to natural gas to fire kilns used to regenerate lime in the kraft process. While being financially attractive, replacing fuel oil with natural gas can be challenging. This is particularity true when the capacity rating is constrained by the temperatures of the gas exiting the kiln. In the worst case scenario, the increase in flue gas temperatures associated with switching from fuel oil to natural gas can significantly de-rate the capacity of the kiln. This paper describes a range of computational modeling tools that can be used to estimate the impacts of kiln geometry, fuel type, operating conditions, and burner design on kiln performance. Data taken from operating kilns is presented, which validates the use of these models. A detailed case study is presented showing how small amounts of torrefied wood can be co-fired with natural gas as a replacement for fuel oil without de-rating the capacity of the kiln. The visualization of the flow fields, temperature distributions, and species concentrations provided by computer models are critical to optimizing kiln operations as new fuels are being considered as replacements for more expensive, carbon intensive fuel oil.