Abstract
The pulp and paper industry in an effort to offset fossil fuel demand uses woody biomass combustion as a renewable energy source to meet their ever-growing energy demands. Boiler combustion systems are often used to provide this energy. However, large amounts of high carbon ash are produced from some boilers resulting in technological, economic and environmental challenge. This high carbon ash is considered to be of very little economic and environmental value and is typically sent to landfills. Reuse of this ash in some boilers requires upgrading and is not economically feasible. Therefore, this study investigates the feasibility of gasifying high carbon wood ash of particle sizes smaller than 3 mm, while comparing its behaviour to that of unburned wood. Gasification was conducted in a stainless steel bubbling fluidized bed reactor 3-inch diameter and height of approximately 800 mm using air and air-steam as gasifying agents. Parameters of interest included equivalence ratio (ER), gas calorific value, carbon conversion efficiency and produced gas yield. High carbon ash was successfully gasified at low temperatures and atmospheric pressure and showed similar trends as woody biomass. The higher heating value (HHV) and carbon conversion efficiency increased with increasing temperature. The H2/CO molar ratio was higher for the air-steam process. Future areas of research could include investigating the viability of producing a gas of even higher heating value.
Highlights
The pulp and paper industry produces large volumes of high carbon ash from boilers
The higher heating value (HHV) and carbon conversion efficiency increased with increasing temperature
1) High carbon ash was successfully gasified in a bubbling fluidized bed reactor at low temperatures and atmospheric pressure
Summary
The pulp and paper industry produces large volumes of high carbon ash from boilers. While the carbon content is relatively high in this residue, the energy content of all of the ash produced is approximately 1% of the energy content of the wood. Combustion of the high carbon ash presents a number of operational problems such as corrosion and scouring. Due to the design of some boilers the ash may be carried by the flue gas through the boiler tubes creating problems. The variation in particle sizes causes inherent problems in a fixed bed system. Fixed bed systems usually require a uniform feedstock to avoid channelling (Warnecke, 2000; Ryu et al, 2006). Exorbitant costs are associated with design and system alterations
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