Cement waste heat recovery
The renaissance of waste heat recovery in clinker manufacturing
With the introduction of rotary kilns in cement clinker manufacturing at the end of the 19th century, clinker was burned in wet and dry rotary kilns without preheater technologies as we know them today. The kiln exhaust gas left the furnace with temperatures between 600 and 1000 °C according to the kiln type and kiln length. Lost heat was recovered for the drying of the raw materials as well as for the preheating of the combustion air. Surprisingly, from today’s point of view, even steam boilers were used for waste heat utilisation in the early days of cement production.
History of waste heat recovery
In the last couple of years, waste heat recovery systems have been experiencing a renaissance. Today, in the Middle and Far East more than 100 applications in the cement industry are known, in the beginning initiated especially in cases with high energy prices or with a low service security of the public electric power grid. State-of-the-art technology for waste heat recovery systems, the development of energy prices, and the growing need to improve energy efficiency all provide new opportunities for implementing waste heat recovery systems in the already highly efficient kiln systems of the cement industry. To date, three applications are currently in operation in European cement plants while a further two are under construction.
Heat losses in the clinker process
Specific thermal energy consumption in the cement industry has declined significantly over the past 60 years. This is mainly attributable to improvements in plant and process technology. In the pyroprocess of clinker manufacturing, thermal energy is required for the drying and calcination of the raw material as well as for the clinker burning process. In addition, heat losses increase the energy demand of the kiln line.
Heat losses occur in the flue gas and the bypass gas from the rotary kiln and cooler exhaust air. They also appear as kiln shell heat losses and heat remaining in the clinker granules leaving the cooler. Potential heat sources for recovery are therefore the flue gas, the kiln shell, the freshly-produced still-warm clinker and excess cooler exhaust air. In clinker manufacturing there is no need for an advanced process-integrated use of heat as the process does not require any further heat.
Waste heat for power generation
A maximum possible heat transfer in the cooler from still-hot clinker granules to the combustion air should be envisaged as the combustion process benefits very significantly from the heat recuperation in the cooler. A high secondary air temperature reduces the required fuel energy input into the firing.
Waste heat can be recovered either for other processes requiring thermal energy, heating or drying purposes, or used for electric power co-generation. The subsequent use as heat for thermal processes or heating, which is usually more energy efficient than power generation, requires either a heat consumer on site or close to the cement plant, or a district heating network nearby. This however is usually not the case, particularly since seasonal heat supply and demand differ from each other. If there is no use for the heat as such, generating electricity from waste heat can be an option. Electricity can either partly cover the cement works’ energy demand or be given to the public power grid.
Electric power generation requires a heat recovery boiler and a turbine system. Power co-generation can be based on a conventional steam process, the Organic Rankine Cycle (ORC) process or the Kalina process. Fig. 2 shows a recovery boiler using ORC technology.
Gas impurities and the need to uncouple the clinker manufacturing from the waste heat utilisation unit require a heat exchanger to transfer the captured heat. Such heat exchangers are placed downstream of the preheater or the cooler to capture heat from the preheater exit gas or from the cooler exhaust air. If the raw material moisture is low, the adaptation of cyclone stages can optimise the overall efficiency of the heat economy of the pyroprocess, the drying purposes and the power co-generation. The steam turbine is the technology best known from power plants. If the available temperature level is too low to run a steam cycle with water as a working medium, other mediums like organic fluids (ORC) or ammonia (Kalina cycles) can replace water. Both working fluids are used to drive a steam turbine. The best economic benefit can be achieved by implementing waste heat recovery applications in a greenfield cement plant. On the other hand, existing cement plants can also benefit from power co-generation. However, the subsequent retrofit is challenging with regard to building and construction sites as well as the cross-linking of the thermal and electric energy flows. In any case the economic feasibility depends on the overall situation on site and might in many cases not be given.
Depending on the waste heat sources used and the applied technology, between 8 and 22 kWh/t clinker of electricity can be achieved by power cogeneration without modifying the process and with the same energy input. Values of up to 45 kWh/t clinker have been reported in cases of considerable modification of the kiln line or if additional fuels are co-fired into the boiler.
Despite the progress that has been made in waste heat recovery in the cement industry it is important to keep in mind that the overall efficiency of waste heat recovery and the economic situation is very plant-specific. The experiences of one plant might not be easily transferred to another and a detailed individual analysis is required in each single case.