Emissions Impacts of Alternative Fuels Combustion in the Cement Industry

Authors: Ali Hasanbeigi, Navdeep Bhadbhade

Some of the most commonly used alternative fuels in the cement industry are biomass, industrial and domestic waste materials, scrap tires, and sewage sludge. The high temperatures, long residence times, and alkaline environment in the cement kiln can prevent the formation of hazardous volatile compounds, making it a suitable option for co-processing waste materials as alternative fuels during cement production. Although the substitution of fossil fuels such as coal and pet coke with alternative fuels can potentially reduce total CO2 emissions from the cement industry, the reduction potentials are often marginal (in the range of 1% - 5% for most cases and up to 18% of current CO2 emissions in a few cases) and depend on the source of biogenic emissions. Moreover, due to higher concentrations of sulfur, nitrogen, chlorine, heavy metals, or other volatile matter in some alternative fuels, co-processing can increase emissions of non-CO2 air pollutants of concern in some cases.

In this study, we provide a summary of the required properties, pre-processing methods, typical substitution rates, fuel dosing systems, technical challenges for substituting conventional fossil fuels with alternative fuels, and their impact on CO2 and non-CO2 emissions in the U.S. cement industry. The study focuses on the following alternative fuels: scrap tires, waste plastic, municipal solid waste, waste oil, biomass, and sewage sludge.

About 73% of U.S. cement plants are using some share of alternative fuels in their fuel mix. The potential impact of the co-processing of alternative fuels in the U.S. cement industry on the emissions of CO2 and non-CO2 air pollutants is analyzed for three scenarios: 1) 20% Replacement Scenario: In this scenario, 20% of coal and pet coke is replaced with alternative fuels and natural gas in the total fuel mix of the U.S. cement industry, and 2) 50% Replacement Scenario: In this scenario, 50% of coal and pet coke is replaced with alternative fuels and natural gas in the total fuel mix. 3) 100% Replacement Scenario: In this scenario, the entire share of coal and pet coke is replaced with alternative fuels and natural gas in the total fuel mix (see Appendix 3 for scenario description).

Based on our analysis, the impact of co-processing of alternative fuels on CO2 stack emissions from the U.S. cement industry was marginal for most of the alternative fuels studied, especially if biogenic CO2 emissions are not considered carbon-neutral (Figure ES1). The co-processing of waste oil represents the maximum potential for reducing CO2 emissions from the U.S. cement industry (1% -7% CO2 abatement for the scenarios studied). Co-processing of sewage sludge and scrap tires represents the CO2 abatement potentials in the range of 1% to 5% across three scenarios studied, whereas municipal solid waste and biomass represent the lowest CO2 abatement potential (<2% CO2 abatement across all three scenarios). Replacing coal and pet coke with natural gas in the cement kiln represents higher potential than alternative fuels co-processing for CO2 emissions reduction from the U.S. cement industry (2% - 12% across all the scenarios studied).

Based on the impact analysis of non-CO2 air pollutant emissions, SO2 emissions from the U.S. cement industry marginally increase when coal and pet coke are replaced with scrap tires (1%-4% across all three scenarios) or municipal solid waste (0.5%-1% across all three scenarios). Conversely, SO2 emissions are lower when coal and pet coke are replaced with biomass or waste oil (4%- 48% across all scenarios studied). Co-processing of plastic waste represents the most considerable potential for SO2 emissions reduction (16% -82% across all three scenarios).

Co-processing of waste oil represents the largest NOx reduction potential (17% -87% for all the scenarios studied). Sewage sludge, municipal solid waste, and scrap tires co-processing represent the potential in the range of 14% to 80% across all scenarios studied. The co-processing of biomass represents the lowest potential (5% to 27% across all three scenarios) for NOx emissions reduction from the U.S. cement industry.

Co-processing of all alternative fuels studied in this report can result in a reduction in particulate matter (PM) emissions, ranging from 8% to 75% across all scenarios studied. Similar to CO2 emissions, replacing coal and pet coke with natural gas can potentially have more significant impact on the reduction of non-CO2 emissions from the U.S. cement industry.

According to the U.S. Environmental Protection Agency (EPA) exposure to air pollutants such as SO2, NOx and PM can cause a variety of respiratory health effects, including inflammation of the lining of the lungs, reduced lung function, and respiratory symptoms, increased susceptibility to respiratory infection, premature mortality, aggravation of cardiovascular disease, decreased lung function growth, exacerbation of allergic symptoms, neurodevelopmental effects such as lowered IQ and behavioral problems, reduction in the capacity of the blood to carry oxygen, thereby decreasing the supply of oxygen to tissues and organs such as the heart, and many other negative health effects. Emissions of these air pollutants disproportionately affect the communities in the vicinity of the cement plants, which are often low-income, disadvantaged communities. Hence, the air pollutants originating from cement manufacturing facilities in the U.S. are regulated under the programs such as National Emissions Standards for Hazardous Air Pollutants (NESHAP), Clean Air Act (CAA) and National Air Ambient Air Quality Standards (NAAQS).

To comply with the regulations, emissions of air pollutants can be controlled by various primary and secondary techniques, either by limiting the formation of pollutants in the first place or by capturing the pollutants from the exhaust gas or by a combination of both. NOx emissions can be controlled through techniques such as flame cooling, using low NOx burners, mid-kiln firing, mineralized clinker, staged combustion, and catalytic reduction with reduction efficiencies up to 95%. The emissions of SO2 can be controlled by absorbent addition, wet or dry scrubbers, and activated carbon. These techniques have reduction efficiencies up to 95%. Dioxin and furan emissions can be controlled by minimizing their formation through techniques such as kiln optimization and careful selection of fuels and their dosing points. Electrostatic precipitators and fabric filters are the most widely commercially available technologies to control the emissions of PM.

While there is brief discussion of how substituting conventional fossil fuels with alternative fuels could potentially have other benefits, such as recovery of the energy content of waste, lowering cement production costs in some cases, and reducing waste sent to landfills, it is not a major focus of this report. The main finding of the analysis is that the co-processing of alternative fuels, especially waste-derived fuels, will not result in a meaningful reduction in the CO2 emissions of a cement plant, especially if biogenic CO2 emissions are not considered carbon-neutral, and can potentially be associated with negative environmental and health impacts to the local environment and communities.

To read the full report and see complete results and analysis of this new study, download the full report from the link above.

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