Metal compounds vaporize and condense during the combustion of coal in boilers and cement kilns, leading to interesting challenges in the design and operation of equipment to recover compounds from the flue gases prior to stack release. In cement kilns, the solids and combustion gases flow counter current to each other, with active exchange between gases and solids. The mixing is promoted by particle entrainment and deposition from and to the moving solid bed. This behavior leads to recycling of metal compounds between the hot and cold kiln ends with a concomitant enrichment of the ash in volatile metals. It is shown that the recycle amount can be varied by the removal of some of the combustion products through a by-pass duct. Flue gas removal through the by-pass provides a degree of freedom that can be utilized to control emissions. The technique used in the cement industry may lead to a possible approach for the use of carbon in the fly ash (through recycle) to regulate mercury emissions from pulverized-coal-fired boilers.