GCI TECH NOTES ©

Introduction

It has been postulated that cement kiln dust (CKD) insufflation impacts metal emissions and metal equilibrium in a cement kiln. Cement kilns that utilize hazardous waste fuel must demonstrate compliance with metal emission limits. To demonstrate this compliance, kilns conducted emissions tests where selected toxic metals were spiked into the system. This spiking is independent of process conditions. The regulations require that the metals emissions tests be conducted with the system in equilibrium, i.e. the metals content in the waste CKD and stack emissions consistent with a steady-state input of the targeted metals. EPA has expressed a concern that CKD insufflation would materially increase metals concentrations in the CKD and metal emission rates.

During the 1992 COC testing, Continental Cement Company monitored CKD insufflation rates on a continuous basis. In addition, the CKD insufflation rate was materially changed (~ 35%) during stack testing. This was required as a control mechanism to reach the worst case operating conditions required in the BIF regulations. The data from this test will be examined for evidence that changes in CKD insufflation impacts emissions and/or equilibrium. This examination should address and satisfy EPA's concerns regarding this issue.

Discussion

Because the volatile heavy metals concentrate in CKD, it is worth examining whether the CKD insufflation rate might affect the metal balance in the kiln. A 35% increase in CKD insufflation results in only a 12% and 14% increase in cadmium and lead input/output rates from insufflated CKD respectively. The 35% change reflects the increase in the dust insufflation rate from the first run to the third run of emission testing.

Table 1 provides dust insufflation rates and emission rates for each of the three runs.

Prior to the start of Run 1, the kiln had undergone approximately 12 hours of metal spiking to achieve equilibrium. Dust insufflation remained relatively constant at 9-10 tons/hr for most of this period except for one 2-3 hour period of increased insufflation about half way through the equilibration period. Clearly if increased insufflation materially impacts equilibration, the 35% increase during testing should have resulted in a corresponding increase in metal emissions, particularly volatile metal emissions.

Figure 1 is a graph of the data from Table 1 and demonstrates that this did not occur. It should be noted that upon the start of metal spiking between one-third and one-half of the increase in metals in wasted CKD occurs within the first hour. This demonstrates that should a shift in equilibrium occur, the impact on emissions would be seen within one hour. Had there been an impact on emissions during this test, it would have easily been seen within the series of three, two hour runs which were performed over a nine hour period.

Figure 1

Conclusions

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

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

- The Continental Cement data has demonstrated that changes in the insufflation rate of CKD have little if any impact on cement kiln metal emissions or equilibrium so long as a sufficient portion of CKD is wasted.

- The impact of insufflating CKD on non-volatile metals which do not partition to CKD should be negligible since this would increase the proportion of metal output in the clinker.

- 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.

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