From coal to natural gas: Its impact on kiln production, Clinker quality and emissions

In the North American energy market, natural gas prices have been gradually decreasing during the past several years, primarily due to advances in shale gas extraction techniques. The availability of cheaper natural gas (NG), while seen as an attractive short-term fuel switching option, is viewed with caution by most cement plants due to long-term procurement concerns. Also, due to traditionally higher NG prices, cement plants have invested heavily into solid fuels, including storage, grinding, handling, and dosing systems - often achieving high thermal substitution rates of solid AFRs (Alternative Fuels & Raw Materials). As a result, a wealth of knowledge has been acquired on firing solid fuels, including some of the more difficult ones, e.g. higher sulfur petcoke and bigger size AFRs, where operational issues such as build-ups, emissions and production losses have been and are being minimized. Switching to gas firing, however, requires re-adaptation of combustion and process guidelines for a fuel which, although in principal is easier to burn, but has relatively lower radiative heat transfer and sharper burning characteristics than coal. As such, the plants, which have switched to NG firing, have observed inconsistent trends in production, energy and emissions performance, mainly due to the lack of sufficient information on combustion/process interactions of the two fuel types required for cost effective optimization. A NG flame ignites earlier, releases intense heat but lacks dissipation of heat as compared with a solid fuel flame, thereby requiring plant specific adjustments. This paper presents actual results of NG firing trials at selected cement plants along with MI-CFD predictions, subsequent to validation from the plant data, on four kiln and four calciners. Recommendations are also made to improve and optimize NG firing by taking into consideration the combustion and mineral interactions.