Beneficial management of biomass combustion ashes

Abstract

Use of biomass for energy production is increasing, so management of the resultant ash is important. This review compares current and future production, chemical composition, and reuse options for ash from common feedstocks (agricultural residues, energy crops, woody biomass, forest residues, recovered wood, paper sludge, sewage sludge and municipal solid waste). Global production is ~170 Mt/yr, but could increase to ~1000 Mt/yr if all available biomass were exploited. Current production is dominated wood and waste derived ashes, but there is capacity to greatly increase use of agricultural residues. Combustion of virgin biomass in modern furnaces can produce ash with negligible persistent organic pollutants and low contaminant metals concentrations, so application to land is possible. Agricultural residue ashes contain abundant potassium and useful phosphate, so could potentially be used as fertiliser. Forestry ashes are rich in CaO, but slightly higher contaminant metals levels may restrict their use to forestry soils. Recovery of potassium from these ashes, and their use in cementitious materials have also been demonstrated. Biomass containing waste ashes potentially contain more persistent organic pollutants and contaminant metals. However, municipal solid waste bottom ash is routinely used as a construction aggregate for prescribed applications. Paper sludge ash is suitable for restricted use as a soil conditioner and possibly as a secondary pozzolan. However, controlled disposal may be required for recovered wood ash and sewage sludge incineration ash. As persistent organic pollutants tend to partition to the flue gases, fly ash and air-pollution control residues are likely to require controlled disposal.