This paper describes an industrial project in the field of thermal treatment of sludge from pulp production showing an original way of substituting fuel (natural gas) with an alternative one (mining gas). A thermal treatment unit with a capacity more than 100 tons of wet sludge per day had been built in one large pulp and paper plant some years ago. This pulp and paper plant is located in a mining area. When built, this unit was quite modern. However, because of more and more sweeping environmental laws affecting the process industry, the unit needed a complete retrofit. The retrofit has been realized in three stages. A brief description of the unit for the thermal treatment of sludge is as follows: The waste sludge (after transport from a sludge storage system) is burnt in a multiple hearth incinerator with a fluidized-bed chamber. Then flue gas enters a secondary combustion chamber (afterburner chamber). During the first stage of retrofit, only a part of heat from flue gas was utilized for heat recovery (pre-heating air for combustion and fluidization) and a contact cooler was added into the unit. The second stage of retrofit can be characterized as “waste-toenergy” one. The contact cooler was replaced by a system for preheating water for steam generation. Off-gas cleaning system consists of a filter for particulate removal and a three stages scrubber system. (Several alternatives of retrofit were considered and simulated and the most promising one was selected.) The third stage of retrofit was based on interesting and original approach - substituting the currently used fuel with mining gas containing approximately 50 to 60% of CH4. Utilizing this waste gas represents an important contribution from both environmental and economic points of view. Removing the gas from mines decreases also risk of creating an explosive mixture with air and thus it is important from safety reasons as well. At this stage of retrofit some particular problems had to be solved. First it was a transport of mining gas from the closed mines including related economic analysis. Further, a special burner designed specifically to enable co-current combustion of two different gaseous fuels (dual burner) with both fuel and air staging has been developed and tested. Tests were performed in order to confirm functional performance specification of the burner and to create a data base for a new semi-empirical Nox formation model. This way of modelling of the Nox formation rate was based on careful consideration of all possible approaches including predictions by CFD (Computational Fluid Dynamics). The presented dual burner points to a possible way, how to solve several problems together (by use of mining gas, decreasing of fuel cost and lowering formation of nitrogen oxides). The simple semi-empirical mathematical model achieves a good agreement with the measured data and thus may contribute to decreasing the number of measured operational regimes during future testing.
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