They m,ay be cleaner that real dirty coal or they may be worse than some cleaner types of coal. One also has to be skeptical about self-reported metal levels in coal, such as mercury, because coal users want to show less mercury emissions than is the case. This is a serious public health issue near cement kilns. Cement kilns certainly do have combustion upsets and smoke may be emitted during such events. The afterburner is required because of the potential for flame outs and total combustion failure in the primary burn chamber, which is all that cement kilns possess.
Cement kilns have no fail safe combustion devices which is unthinkable today in all incinerators.
E. Other kinds of upsets. Kiln meltdowns are not rare events and have happened here in Texas at several plants in the last ten years. Cement companies do not like to talk about this problem. These are just a few of the technical issues surrounding combustion problems observed in cement kilns.