Bark drying in a large integrated Finnish pulp mill

Abstract

In the pulp and paper industry, bark and sludge are commonly burnt in biomass boilers. Often, bark water content accounts for half its mass when it is fired in biomass boilers and sludge moisture is generally over to two thirds of its mass. To maintain combustion temperatures, boilers need a supporting fuel. In the biomass boiler presented in this thesis, flue gas flow from combustion of wet biomass and combustion air demands are limiting its operational capacity. Drying biomass could potentially diminish these flows and the consumption of natural gas. The first will allow biomass boiler to operate at higher loads and the second will reduce the mill´s natural gas costs as well as fossil fuel emission allowances. Several drying technologies such as rotary, flash, superheated steam and conveyor dryers are presented in this work.