Emerging Worker Safety Concerns Associated with the Recycling of Fly Ash from Coal-Fired Power Plant in Concrete Mixtures

Focus Area

Construction and Maintenance Practices


Environmental Process, Natural Resources







Research Idea Scope


In recent years, coal-fired power plants are more frequently utilizing air pollution control technologies that result in fly ash with elevated concentrations of ammonia (referred to herein as ammoniated fly ash, AFA).  Uncertainty exists regarding the potential implications of AFA when used in transportation construction projects.  Concerns of AFA use have been raised and scientifically-based recommendations for appropriate limits need to be developed based on considerations of acute and chronic construction worker exposure concerns as well as  nuisance odor problems.

To meet regulatory mandates for nitrogen oxide (NOx) control, coal-fired power plants often employ a technology known as selective catalytic reduction (SCR) to mitigate and reduce NOx emissions.  In the SCR process, ammonia is injected to the combustion exhaust prior to a catalytic reactor where the ammonia reacts with the NOx to form nitrogen gas.  Usually, when the SCR unit is staged upstream of the particulate control devices (e.g., baghouses), excess unreacted ammonia will be captured with the fly ash in the baghouse. 

Ammonia is considered a high health hazard because it is corrosive to the skin, eyes, and lungs. Exposure to 300 ppm is immediately dangerous to life and health.  It can explode if released in an enclosed space with a source of ignition present.  OSHA’s TWA-PEL (time-weighted average-permissible exposure limit) is 50 ppm, while NIOSH’s REL (recommended exposure limit) is 25 ppm for 8-hr TWA and 35 ppm for 15-min.  Ammonia has a low odor threshold of 20 ppm, so most people will seek relief from the pungent odor at low concentrations. 

Since coal fly ash is routinely used as a fractional cement substitute in concrete mixes, the impact of ammonia presence in AFA must be considered.  Several concerns related to AFA have been raised recently, including:  (1) the off-gassing of ammonia during various phases of handling and use of the AFA and the resulting AFA concrete; (2) worker health and safety due to the exposure to released ammonia; and (3) nuisance odor.  To ensure the health and safety of workers working on transportation construction projects, research needs to be conducted to identify safe thresholds of ammonia loading in fly ash.  Exposure routes and factors that affect ammonia discharge patterns and rates also need to be identified.

The Hinkley Center for Solid and Hazardous Waste Management and the Department of Environmental Engineering Sciences at the University of Florida were contracted to assist FDOT to identify potential concerns of AFA use and to provide recommendations for appropriate limits based on considerations including worker health and safety (acute and chronic exposure concerns), and nuisance odor problems.  This project is underway and is being conducted in two phases.  In Phase I, existing knowledge on the subject from the scientific literature, the regulatory community, and stakeholders was gathered and synthesized.  Phase I also included conducting a basic mass balance assessment to determine whether further research was warranted to better quantify potential ammonia emissions and worker exposure, and development of methodology for such research.  The results of Phase I prompted initiation of Phase II of the project, the focus of which was laboratory testing in support of developing recommendations for ammonia concentration restrictions for fly ash used in FDOT construction projects.

As part of Phase II, the researchers gathered information from published material and consider policies and practices in other states and conducted a survey of current or past experience of other transportation agencies regarding AFA issues. The current regulatory limits for worker safety and environmental health with respect to ammonia exposure or release are being reviewed and summarized. The current use of ammonia injection at coal-fired units is being reviewed and summarized, and the coal ash and cement industries in Florida have been queried for their current knowledge and experience with AFA.  This information is being used to estimate the range of potential ammonia exposure to workers as a function of the amount of ammoniated fly ash used and its respective ammonia concentration.

The information gathered in Phase I suggested that exposure to ammonia could be an issue under plausible use scenarios, and thus experimental studies have been initiated to determine an appropriate threshold(s) (i.e., ammonia concentrations) that FDOT can establish for use of ammoniated fly ash used under different conditions (Phase II).  A series of laboratory-scale experiments are being conducted using ammoniated fly ash at different cement replacement ratios in concrete mixes and the ambient ammonia concentrations were measured in different media, i.e. air and water.

LITERATURE SEARCH SUMMARY – There is very little literature available on this topic.  A project conducted in Tennessee and a project conducted in the UK were reviewed.


The objective of this research is to develop data and information regarding the use of ammoniated fly ash in concrete and to conduct further investigations regarding the off-gassing of ammonia from concrete containing ammoniated fly ash.  Specific research objectives are discussed in the following paragraphs.  Specific to AFA use in concrete,  field studies should be conducted with this material. The University of Florida project team is in the process of developing a maximum recommended concentration of Ammonia in fly ash to be used at Florida DOT jobsites.  It would be critical to also perform field experiments using the proposed maximum ammonia content in fly ash using various concrete mix designs to gauge the relationship to ammonia off-gassing. Additional field studies are also necessary to address the impact of admixtures, (e.g. air-entraining agents) and water reducers to see how the use of admixtures and water reducing agents affect the ammonia loss rates. 

The current scope of the University of Florida research regarding the ammonia emission rates was undertaken in the lab.  The scope does not include field studies, therefore robust field studies are a necessary phase of research as work conditions vary a lot in practice.    These field tests are necessary to provide more relevant and accurate diffusion coefficients.  In addition, research is needed to develop  “adjustment factors” for  use  in developing emission rates of ammonia from concrete poured in the field.

According to our study and a study on AFA in the United Kingdom, water/cement ratio, concrete thickness, fly ash/cement ratio, initial ammonia concentration in concrete and concrete mixing speed of the mixing truck (or use in a Shot-Crete application) all affect the ammonia release rate. However, due to the time limit of the University of Florida project, these aspects were not examined.  Research is needed to thoroughly examine these issues and see how these various conditions impact the diffusion model that is being developed by the University of Florida.  This examination should support the development of a more complete and accurate model to use in predicting actual ammonia behavior.

Other related questions should also be addressed by the research, including:

(1)  How does the use of granular blast furnace slag and steel-belted tires as a source of iron in manufacturing Portland cement affect the long-term loss of the ammonia?

(2) What are the diffusion characteristic of AFA when used incorporated into other construction materials such as flowable fill. 

(3) What is the best method for testing ammonia concentration in fly ash. This should be investigated, with respect to, simplicity of the test, speed of testing, generation of hazardous waste, accuracy and precision and detection limits.  The University of Florida did not find any standard test method that is used by the manufacturers and users for testing for ammonia in fly ash.

(4)  How many coal-fired power plants are in the process of installing SNCR/SCR air pollution control equipment to reduce airborne NOx emissions?  We don’t have a good understanding of how many power plants are currently generating AFA and how many power plants are planning on installing this technology. We need to have a better understanding of the future production rate of AFA as more power plants install SNCR/SCR technology to reduce NOx emissions. To address this issue, a survey of facilities could be completed along with projections of future production rates based on the number of coal-fired power plants that are required to install SNCR/SCR units or the use of ammonia injection for flue-gas conditioning.

(5)  Determine the most accurate means for measuring the exposure level of workers (to ammonia gas) who are “pouring” and handling concrete that has AFA in it.  The NIOSH 6016 sampler (used to determine time-weighted ammonia concentrations) is not able to accurately measure ammonia at high concentrations.  According to our research, for a concentration of 50 ppm ammonia (the OSHA standard), the sampler’s capacity is only about 50% of what was reported in the NIOSH document. That means, the measured (time-weighted average) exposure of ammonia is most likely underestimated.  

Urgency and Payoff

This is a very time-sensitive and important research topic. Very little is known about the impacts of the use of ammoniated fly ash in concrete but these materials are beginning to enter stream of materials that are recycled into construction materials.  Further research needs to be done as outlined in V. above. 


The development of a simple and reliable test for measuring ammonia in ammoniated fly ash is a very important task that needs to be done quickly to ensure adequate worker protection.  As more coal-fired power plants install pollution control equipment to control for the air emissions of nitrogen oxides, more and more ammoniated fly ash is going to “come into the supply system” for the use in concrete in pavements and structures.  As the power plants install the pollution control equipment necessary to be in compliance with emission standards, there is going to be less and less non-ammoniated fly ash available for use in concrete.

Suggested By

John Schert, University of Florida College of Engineering

[email protected]