A life cycle comparison of disposal and beneficial use of coal combustion products in Florida

Beneficial use of coal combustion products (CCPs) in industrial or construction operations has the potential to minimize environmental and human health impacts that would otherwise be associated with disposal of CCPs in the life cycle of coal used for electricity generation. To assess opportunities for reducing impacts associated with four CCP materials considered in this study, fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) material, this paper reports results of expanding a life cycle inventory of raw material and emissions (part 1 of this series of papers) by performing life cycle impact assessment on five scenarios of CCP management.

SimaPro 5.1 software (PRé Consultants) was used to calculate comparative environmental impacts of all scenarios using CML2001 and Environmental Design of Industrial Products 1997 midpoint impact assessment methods and Heirarchist and Individualist levels of the Eco-indicator 99 end point method. Trends were compared for global and local environmental and human health impact categories of global warming, acidification, smog formation, human toxicity, and ecotoxicity.

In each impact category, beneficial use of fly ash, bottom ash, and FGD material resulted in a reduced impact compared to disposal of these materials. The extent to which beneficial use reduced impacts depended on several factors, including the impact category in consideration, the magnitude of potentially avoided impacts associated with producing raw materials that CCPs replace, and the potential impact of CCP disposal methods. Global warming impacts were reduced by the substitution of fly ash for Portland cement in concrete production, as production of Portland cement generates large quantities of CO₂. However, for categories of global warming, smog formation, and acidification, impact reductions from CCP beneficial use are small, less than 6%, as these impacts were attributable, in greater part, to upstream processes of coal mining, transportation, and combustion. Human toxicity and ecotoxicity categories showed larger but more varied reductions, from 0% to 50%, caused by diverting CCPs from landfills and surface impoundments.

When comparing beneficial use scenarios, the four impact assessment methods used showed similar trends in categories of global warming, acidification, and smog formation. However, results diverged for human toxicity and ecotoxicity categories due to the lack of consensus among methods in classification and characterization of impacts from heavy metal release. Similarly, when assessing sensitivity of these results to changes in assumptions or system boundaries, human toxicity and ecotoxicity categories were most susceptible to change, while other impact categories had more robust results.

Impact assessment results showed that beneficial use of CCPs presented opportunities for reduced environmental impacts in the life cycle of coal combusted for electricity generation, as compared to the baseline scenario of 100% CCP disposal, although the impact reductions varied depending on the CCPs used, the ultimate beneficial use, and the impact category in consideration.

As regulators and electric utilities increasingly consider viability and economics of the use of CCPs in various applications, this study provides a first-basis study of selected beneficial use alternatives. With these initial results, future studies should be directed towards beneficial uses that promise significant economic and environmental savings, such as use of fly ash in concrete, to quantify the currently unknown risk of these applications.