GCI TECH NOTES ©
In 1990, I authored and presented at the AWMA Specialty Conference, a paper entitled HCl Emissions from Portland Cement Manufacturing. In this paper I stated, “this (data) validates previous proposals that HCl emissions from cement kilns are impossible”. On December 15, 2000 the United States Court of Appeals, in a case brought by the National Lime Association and the Sierra Club vs. the USEPA, handed down a decision stating “HCl is emitted in sufficient quantity from most cement kilns”. This appears to be in direct conflict with my statement therefore, an update is needed.
First off, the paper is 11 years old and there has been a great deal of research since then. The EPA has used data from the Method 26 trains to determine that HCl emissions come from a cement kiln despite the prior research and even in the face of EPA's on air group that developed the method indicating that it was not appropriate for use on cement kilns. This was largely a politically motivated issue. EPA felt the need to impose an HCl testing and control program for those plants that burn hazardous waste as fuel back about the time the paper was written. For that reason Method 26 was used even though they knew it would not work and provided biased high results based on ammonium chloride. The industry did not protest too loudly because the limit used was so high that no one would have problems. Then, under the MACT rules, arising from the updated Clean Air Act, EPA used that data to "demonstrate" that cement kilns were a major source of HAPs. Quite frankly they were desperate because there was no other HAP that they could use to pull cement kilns in as a major source. Since then the air group in EPA that acknowledged that Method 26 would not work came up with Method 321 - FTIR. They stated “Accurate measurements of HCl in the kiln exhaust gases are necessary for major source determination. The EPA agrees with commenters that Method 26 may have positive biases attributable to chloride salts rather than to HCl…Therefore, the Agency has decided that Method 26 and 26A use without concurrent validation with Method 321 or Method 322 will only be acceptable for measuring HCl from NHW kilns to confirm that the Portland cement plant is a major source.” The problem is that it provides misleading results for another set of reasons. Now we have MACT rules for hazardous waste burning cement kilns that still require the use of Method 26 and MACT rules for the kilns that do not burn hazardous waste require the use of Method 321. Ironically, under MACT they did not determine that there was any appropriate control technology for this "HCl" and there is no limit for the kilns that do not burn hazardous waste but it allowed them to go after other HAPs such as metals.
Now to the science . What we have found is that under unusual circumstances a cement kiln can emit real HCl. These circumstances include over chlorinating a kiln, that is exceeding the kiln's ability to absorb the Cl with Na and K, and pushing the recycling of cement kiln dust too high for some types of kilns. Neither of these are good scenarios for the cement kiln operation but we have seen both done. What we do know now is that ammonia released in the cool end of the kiln system reacts with a portion of the alkali chlorides to form ammonium chloride. This probably occurs in the air pollution control equipment. If the environment were static this reaction would quickly reverse itself but because it is dynamic and the ammonium chloride is a gas it gets swept away in the gas stream. This looks like it is possible even in the sampling trains for both Methods 26 and 321 if ammonia and moisture are present. Further, at temperatures in the stack and sampling trains, there is a dissociation constant that is high enough so that a portion of the ammonium chloride dissociates into HCl and ammonia gas. Further downstream as the gasses cool they recombine but not before a method like 321 detects the actual HCl molecule. The chemistry of the cement manufacturing process along with the alkali nature of the particulate being emitted from a cement kiln would seem to make actual HCl emissions which impact the environment nearly impossible. However, the complexity of the situation may prevent final resolution of the issue.