Cement is the most widely used building material in modern human society. The invention of cement has completely changed the history of human architecture. Today’s cities and towns are forests of reinforced concrete, which has changed the progress of mankind and the face of the earth. However, under the background of increasingly severe global climate problems, the extensive production mode of cement began to be criticized, and more and more countries began to introduce policies to limit the carbon emissions of the cement industry. Therefore, how to reduce the pollution and carbon emissions in the process of cement production has become an urgent problem for major cement manufacturers to overcome.
Why Cement Production Generates Huge CO2 Emissions
Limestone (CaCO3) is the main raw material for producing portland cement. But to make cement clinker, the limestone needs to be crushed and calcined together with other materials at extremely high temperatures. In this process, CaCO3 becomes useful CaO and useless CO2. This process itself produces more than two-thirds of the carbon emissions in cement production. If we did not do so, this carbon dioxide would have been safely locked up for hundreds of millions of years, but as more and more limestone was dug up and calcined into clinkers for cement production, it instead entered the atmosphere in large quantities. Research shows that the production of cement produces a large amount of carbon dioxide emissions up to 8% of the total global carbon dioxide emissions, which is still an underestimated value.
In the current new dry cement production process, there are two sources of carbon dioxide emissions: one is from the combustion of fossil fuels such as coal (accounting for 32%); The second is from the chemical process of limestone burning into clinker in the rotary kiln (emission accounts for 63%).
How to Lower Carbon Emissions in Cement Industry
In order to realize the production of low-carbon cement, we need to start with raw materials, fuels, technological processes, and other links, and make breakthroughs in new technologies. Carbon emission reduction paths of the cement industry mainly include:
1. Fuel substitution
Use combustible wastes such as garbage and biomass and new energy such as green hydrogen and photovoltaic to reduce the combustion of traditional fossil fuels such as coal. The data shows that if 40% of alternative fuels are used in cement production, about 100,000 tons of carbon dioxide will be reduced for every 1 million tons of clinker produced. At present, more than 2/3 of cement plants in Europe have used alternative fuels, and the replacement proportion of combustible waste in the cement industry is an average of 20%.
2. Raw material substitution
Using calcium-rich wastes such as calcium carbide slag, steel slag and slag to replace limestone as raw materials for cement production can save a lot of natural mineral resources. Therefore, using industrial solid waste as alternative raw materials is one of the important means for the cement industry to co dispose industrial solid waste, reduce natural mineral consumption, and reduce carbon dioxide emissions. However, at present, the available amount of calcium-rich waste is difficult to meet the needs of cement production.
3. Revolutionary cement clinker production process
At present, the fatal weakness of the rotary kiln, which is most widely used in clinker calcination, is its low heat transfer efficiency, large rotating power, and large shape, which makes it difficult to significantly reduce heat consumption, cost, and other indicators
To realize low-carbon cement production, it is necessary to change the current cement clinker production process. If anything is most likely to replace the rotary kiln in the future, it is natural to first promote the calcination of fluidized cement clinker. Under the condition of fluidized calcination, the efficiency of heat and mass transfer is high, and the material can be rapidly heated to about 1400 ℃ in a few seconds. It is an ideal cement clinker production equipment. Compared with rotary kiln, calcining cement clinker under fluidization conditions can reduce coal consumption by more than 20%, reduce carbon dioxide by more than 25%, and reduce nitrogen oxide emissions by more than 40%. It is the next generation of new low-carbon cement kiln technology.
The cement kiln R & D team of the Institute of Engineering Thermophysics, Chinese Academy of Sciences, based on the principle of fluidized burning, combined with the laboratory’s research foundation in circulating fluidized bed for more than 40 years, innovatively proposed the pre calcination calcining process of low-carbon cement. This process can realize the decomposition of raw meal, calcination of dicalcium silicate, and calcination of tricalcium silicate in separate areas, and has many advantages in energy saving and carbon dioxide emission reduction. At present, the mechanism and small-scale test research have been completed, and the 10t/d pilot test technology development is being carried out.