Abstract:In industrialized countries, it is expected that the future generation of bioenergy will be from the direct combustion of residues and wastes obtained from biomass. Bioenergy production using woody biomass is a fast developing application since this fuel source is considered to be carbon neutral. The harnessing of bioenergy from these sources produces residue in the form of ash. As the demand for bioenergy production increases, ash and residue volumes will increase. Major challenges will arise relating to the efficient management of these byproducts. The primary concerns for ash are its storage, disposal, use and the presence of unburned carbon. The continual increase in ash volume will result in decreased ash storage facilities (in cases of limited room for landfill expansion), as well as increased handling, transporting and spreading costs. The utilization of ash has been the focus of many studies, hence this review investigates the likely environmental and technological challenges that increased ash generation may cause. The presence of alkali metals, alkaline earth metals, chlorine, sulphur and silicon influences the reactivity and leaching to the inorganic phases which may have significant impacts on soils and the recycling of soil nutrient. Discussed are some of the existing technologies for the processing of ash. Unburned carbon present in ash allows for the exploration of using ash as a fuel. The paper proposes sieve fractionation as a suitable method for the separation of unburnt carbon present in bottom ash obtained from a fixed-bed combustion system, followed by the application of the gasification technology to particle sizes of energy importance. It is hoped that this process will significantly reduce the volume of ash disposed at landfills. OPEN ACCESSEnergies 2012, 5 3857

It is expected that increasing amounts of energy will be generated from the direct combustion of biomass residues. However, biomass combustion processes are known to produce large amounts of bottom ash, resulting in ash storage and disposal problems. The presence of unburned carbon in some bottom ash suggests its potential for beneficial uses, such as an energy source. This comparative study characterizes two bottom ash samples obtained from an industrial scale fixed-bed boiler. The physical and chemical properties of each bottom ash, as well as their respective particle fractions obtained by sieving, are analyzed and discussed. Analyses included proximate and ultimate analysis, Brunauer–Emmett–Teller (BET) surface area, thermogravimetric analysis (TGA) and bulk density. The percent fixed carbon in the samples was 30% and 50%. The higher heating value (HHV) ranged from 5 - 25 MJ/kg for the ash samples when characterized within fractions. The boiler ash showed that 68 % or more of the energy could be recovered in fractions ? 425 µm for high carbon ash. Low carbon fractions of ash have four times the bulk density compared to the high carbon fractions. By reburning the larger fractions, ash volumes can be decreased by over 50%.

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 H 2 /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.

The direct combustion of biomass residues produces large quantities of bottom ash. Environmental sustainable management requires that ash recycling should be carried out whenever possible. Suitable applications of bottom ash are based predominantly on its chemical properties. The presence of major ash forming and trace elements along with other intrinsic properties unique to bottom ash, suggest its potential as a soil additive. But, ash quality must be of a high standard to prevent environmental pollution. This comparative study characterizes bottom ash obtained from three types of bioenergy systems -a fixed-bed boiler, a downdraft gasifier and a wood pellet burner. The chemical properties were analyzed and discussed for each bottom ash, together with their respective particle fractions that were obtained by sieving. The pH of the starting ash samples for the gasifier, boiler and pellet burner were 10.36, 12.49 and 13.46, respectively. Ni with a concentration of 229 mg/kg in the pellet burner ash, exceeded the maximum limit for soil amendments (in British Columbia, Canada) within the particle size fraction ≥ 850 µm but < 2000. All samples were significantly enriched in both Ca (50-61%) and K (10-26%). The elements Mg, Al, Mn, Fe, P and Na each contributed 10% or less to the inorganic portion of the ash. Concentrations of inorganic contents varied with particle size. Water soluble phosphates were very low in the samples. The results suggest that size fraction separation can be a useful method to isolate fractions containing higher (or lower) amounts of some metals. This method may be a useful technique for managing ash that contains elements exceeding environmental limits.

In recent years, there has been an increased shift towards using renewable biomass as a source of energy generation. Wood pellets are widely used for energy production and are manufactured by densifying wood into pellets for increased energy efficiency. The manufacturing process of these pellets typically generates a tar-like byproduct resulting in increased production costs associated with waste disposal, equipment clean-up and handling operations. The current study focuses on characterization of this wood-based tar, which can create significant technological problems and environmental hazards. The tar was characterized by gas chromatography-mass spectrometry (GC-MS), thermogravimetric analysis (TGA), 1 H nuclear magnetic resonance spectroscopy ( 1 H-NMR), infrared spectroscopy (IR), solubility and moisture content. A total of 29 compounds were identified.