Processing of waste from pulp and paper plant

Production of sludge as waste in pulp and paper plants can be considered as one of the serious environmental problems that have to be solved. While landfilling is not a suitable solution from the environmental point of view, thermal treatment proved to be the most appropriate one. This paper describes an efficient way of processing sludge including waste-to-energy aspects. A thermal treatment unit with a capacity of more than 100 tons of wet sludge per day had been built in a 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 realised 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 fluidised-bed chamber. Then, the flue gas enters a secondary combustion chamber (afterburner chamber). During the first stage of retrofit, only a part of heat from flue gas was utilised for heat recovery (pre-heating air for combustion and fluidisation) and a contact cooler was involved in the unit. The second stage of retrofit can be characterised as a “waste-to-energy” 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-stage scrubber system. (Several alternatives of retrofit were considered and simulated and the most promising one was selected.)