Cutting OPEX & Emissions

During cement the manufacturing process large quantities of carbon dioxide (CO₂) are emitted in the clinkering phase. Only one third of these CO₂ emissions result from the combustion of fossil fuels, such as coal or pet coke, with the largest portion being unavoidable and released by the decomposition of limestone.

The cement industry today is responsible for approximately 7% of the world´s total carbon footprint. It already makes use of numerous possibilities for reducing CO₂ emissions released in the manufacturing process, either by reducing the clinker factor in cement or in the near future by carbon capture projects.

Aside from tail-end capture solutions such as amine scrubbing, the so-called ‘Oxyfuel’ combustion technology also plays an important role. In this process, combustion air is substituted with pure oxygen so that the exhaust gas consists mainly of CO₂ and water vapour, i.e. it is not diluted by nitrogenous combustion exhaust gas.

thyssenkrupp Polysius GmbH developed an enhanced process for Oxyfuel operation. This process requires adjustments to be made in the kiln line, particularly in the calciner, preheater, clinker cooler, and the chlorine bypass, if any. The adjustments include modified or new equipment, which particularly relates to any sealing used in the process, the gas separation in the cooler, material modification in any part of the kiln exposed to high temperatures, and a high oxygen containing atmosphere.

The polysius^® pure Oxyfuel process is based on pure oxygen as a combustion gas and does not require a costly or complex exhaust gas recirculation. Here pure oxygen is fed to the foremost zone of the clinker cooler and routed to the clinkering zone burner. In contrast to today’s clinker production plants, all combustion oxygen is routed to the sintering zone burner. A combustion air bypass for the calciner firing system, the so-called tertiary air duct, is waived. This approach ensures that the sintering zone burner is supplied with an adequate gas volume and that overheating of, for example, the refractory and sintering zone burner is avoided.

Thus, the combustion gas routed from the sintering zone to the calciner firing system has an oxygen content of approximately 60 – 75%. In the calciner burning zone this oxygen containing gas is consumed by the rest of the fuel. In spite of the high oxygen content, no excessive temperature increase occurs in this area as the decomposition of limestone results in a considerable heat sink, limiting the temperature in this area to approximately 930°C.

In today’s cement plants the exhaust gas downstream of the calciner is used for preheating the raw material. The new polysius pure Oxyfuel process concept reduces the portion of heat from the cooler conducted directly to the kiln. Instead, hot air from the rear section of the cooler can be routed to the preheater bypassing the rotary kiln. Here the heat can be utilised for material preheating; separate gas strings prevent mixing of the CO₂ rich Oxyfuel exhaust gas and the hot air from the cooler. Downstream of the additional preheater cyclone, the hot cooler exhaust air still contains sufficient thermal energy to dry the raw material in the raw mill. Thus a thermal integration is achieved which is at least equivalent to the state-of-the-art, and this is achieved without additional heat exchangers or similar equipment. Alternatively, the very hot cooler off-gases could be efficiently used in a WHR system or other processes, requiring heat.

Due to the lower gas volume, many systems in such a plant can be designed to be considerably smaller; in particular, the preheater cyclones can be designed for approximately half the gas volume. Consequently, the preheater tower is substantially smaller, a fact which is notably mirrored in the investment costs.

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Read the article online at: https://www.worldcement.com/special-reports/11092023/cutting-opex-emissions/