GCI TECH NOTES ©

Introduction

In the same way that it has been postulated that metal emissions may be impacted by CKD recycling (see GCI Tech Notes, September, 1995) the potential impact of CKD recycling on "HCl" emissions has been raised as a concern. In order to examine this question, data from the 1992 Certificate of Compliance (COC) tests will be examined.

First, however, it should be noted that "HCl" emissions from cement kilns are not necessarily real HCl. Ammonium chloride is a known interference⁽¹⁾. Ammonia concentrations in stack gases may be a significant controlling factor in both run to run and plant to plant variations of measured "HCl" emissions. Recent data also suggests that "HCl" from ammonium chloride may be manufactured in the Method 26 sampling train itself. For these reasons, both small and large differences in "HCl" emissions may be attributed to ammonia concentrations. In addition, there is also data to suggest that excess alkali (K+Na) levels in kiln feed may play a significant role in "HCl" formation. Plants with high alkali levels (>0.5%) appear to have lower measured "HCl".

Discussion

Table 1 provides a comparison of wet process kilns that performed a BIF COC test in 1992. Six kilns did not recycle CKD; eight others did recycle CKD. Of those that did not recycle CKD, "HCl" emissions ranged from 0.68 lbs/hr to 72.7 lbs/hr. For those that did recycle CKD the range was 0.49 lbs/hr to 48.8 lbs/hr. There would appear to be no significant differences between the two subsets of data.

This is a reasonable observation since chlorine that re-enters the kiln system with recycled CKD is chemically different than chlorine entering the kiln with waste fuel. Waste fuel chlorine, when combusted, is momentarily free to combine with any other reactive species present. The majority reacts with potassium and/or sodium and becomes simple chloride salts. Chlorine entering with the dust is already in the form of potassium (or sodium) chloride. These molecules contain tight ionic bonds. It is extremely unlikely that these salts could, in the highly caustic environment in a cement kiln decompose to become HCl.

Conclusions

The following conclusions are drawn from the data and the facts regarding how a cement kiln operates.

- Plants with ESPs naturally concentrate volatile species in the latter stages of the ESP. This material from the latter stages is wasted, and therefore optimizes chloride removal and reduces any impact of CKD recycling. This allows for efficient chloride removal and is the reason why CKD recycling has little impact on chloride emissions.

- The industry-wide data has demon-strated that recycling of CKD has little, if any, impact on cement kiln "HCl" emissions so long as a sufficient portion of CKD is wasted.

- Plants that utilize hazardous waste fuels have a significant dis-incentive to waste too little dust. Such an approach could allow alkali chlorides to build up in the kiln causing process problems. In addition, wasting too little CKD would cause a build up of metals that could jeopardize the Bevill status of that CKD which is wasted. There are, therefore, operational reasons that prevent this potential route to excessive stack emissions of "HCl".

There is little potential for a cement kiln utilizing hazardous waste fuel to change CKD recycling rates so drastically as to materially impact stack "HCl" emissions.

References

(1) Von Seebach, Dr. Michael and David Gossman, Cement Kilns, Sources of Chlorides not HCl Emissions, Proceedings from the Waste Combustion in Boilers and Industrial Furnaces Conference, SP-72, Air & Waste Mamagment Association, Kansas Cty, MO, April, 1990.

(2) Gossman, David, Commercial BIF Compliance Test Results - 1992, Hampshire, IL, Gossman Consulting, Inc., 1992.