Utilization of Fly Ash from Biomass and Biomass-Coal
Biomass fuel is being incorporated by some coal-based power plants as an alternative to cocombustion or cogasification with coal. This cofiring strategy has demonstrated reduced sulfur and nitrogen emissions and could be used as a strategy to reduce the net carbon dioxide (CO2) emission impact of a power plant. Some consider the combustion of biomass to be essentially CO2-neutral because although the biomass produces CO2 on combustion, CO2 is taken up by the plant during its growth.
One question that we hear a lot at the Energy & Environmental Research Center regarding cocombustion of biomass with coal is in regard to the utilization of the fly ash. Many power plants burning coal sell their fly ash particularly to the cement market. The fly ash resulting from the combustion or gasification of biomass fuels, either alone or with coal, has the potential for different characteristics than fly ash from coal alone. This can affect the salability of the ash. The elevated alkaline content in fly ash from biomass cocombustion, most notably sodium and potassium, tends to make these ashes undesirable for use in applications identified as beneficial use applications for conventional coal combustion fly ash.
According to the American Coal Ash Association, the largest utilization application of coal combustion fly ash is in concrete, concrete products and grout. ASTM International C618 “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete” is the standard used to determine whether a specific fly ash is suitable for use in concrete. Some coal-biomass fly ashes have been shown to meet the ASTM C618 standard specifications. However, currently ASTM C618 does not specifically address cocombustion fly ash. Alternatively, the European standard EN 450 “Fly Ash for Concrete— Definition, Specifications, and Quality control” allows for the use of fly ash derived from burning of pulverized coal and cocombustion materials at high enough temperature to facilitate glass formation in the fly ash. Further details are provided within the standard.
Both laboratory data and experience with fly ash from full-scale systems burning low percentages of biomass (less than 10 percent) indicate that fly ash from the coal-biomass blends has qualities similar to that produced with the coal alone. However, if the combustion fuel feed contains greater than approximately 10 percent biomass, the fly ash quality impact can be significant enough to affect utilization in cementitious applications. It is not anticipated that coal-based facilities that currently produce fly ash for the concrete market will use more than a small percentage of biomass fuels in the future.
Many biomass and cocombustion fly ashes may not meet the ASTM C618 standards or be allowed for use in concrete. This does not preclude the materials from use in other applications. These fly ashes may be suitable for use in markets such as structural fill, soil stabilization for construction, and other high-volume applications. A potential application of fly ash from biomass or coal-biomass is as a fertilizer substitute since the ash often retains the nutrients of the biomass such as potassium and phosphorus.
As with other coal combustion products, getting a good chemical, physical and/or mineralogical analysis of the biomass or coal-biomass fly ash can help determine the suitability of the specific material in various utilization applications. The EERC and other laboratories routinely provide these types of analyses.
In summary, the suitability of biomass-derived fly ash for use in cement or other salable products is a question often asked by utilities or power providers. Since there is no standard for coal-biomass fly ash yet, most combustion systems are relegated to burning low percentages of biomass. A few states have developed rules, regulations, standards, policies or guidelines regarding coal fly ash use, and perhaps if biomass becomes more prevalent as a cocombustion fuel, similar rules will be developed for biomass. For now, applications for permission to use biomass-derived ash are handled on a case-by-case basis.
Author: Loreal V. Heebink
Research Chemist, Energy & Environmental Research Center