Low-carbon transition technology path: fuel substitution

The development process of fuel substitution

Fuel substitution technology is a mature carbon reduction technology in cement industry. From a worldwide point of view, Europe is in the leading position, with an alternative fuel replacement rate of about 39%, followed by Brazil and North America, about 5%, while China's cement industry fuel replacement rate is less than 2%, which is at the bottom.

At present, the development of fuel substitution technology in China's cement industry is relatively backward, and it is still in the stage of using cement kiln collaborative disposal technology. Due to the low quality of waste pretreatment and disposal, the calorific value of the "alternative fuel" produced by the collaborative disposal line of cement kilns constructed by most cement enterprises fluctuates greatly, which has a great impact on the operation of the cement clinker production line, and fails to meet the internationally recognized quality standard of alternative fuel.

1. Biomass fuel

Biomass is a general term for all plants, microorganisms, some animals and their waste. Biomass fuel refers to solid, liquid or gaseous fuels extracted from biomass. The biomass substitute fuels commonly used in cement industry are mainly: straw, rice husk, waste wood, agricultural and forestry waste, etc. Unlike traditional fuels such as oil and coal, this new biofuel is a renewable fuel and is less likely to cause environmental pollution than other fuels.

Biomass fuel has the characteristics of carbon neutral, renewable and environmentally friendly (low carbon content, no sulfur, heavy metals and other elements), and has become one of the important directions for the development of cement companies in the future.

China is rich in biomass resources, which can produce about 829 million tons of straw every year, of which about 694 million tons can be collected; 9.604 million tonnes of the forestry residues produced could be used for energy in the future; In the future, when combined with CCUS, it can become a carbon-negative technology. In short, China's biomass fuel carbon potential is huge, but due to the biomass fuel has high calorific value volatility, difficult recovery, poor combustion stability and other characteristics, biomass alternative fuel in China's cement industry has not been large-scale application.

Foreign biomass fuel replacement process started earlier, Holcim, Cemex, Heildelberg biomass fuel (animal bone meal, crops, wood chips and other crops) in the alternative fuel accounted for more than 15%. Heidelberg Cement alternative fuels will account for 26.4% of the fuel mix in 2021. Of these, waste-based biomass used accounts for about 42% of the alternative fuel mix. (Holcim and Lafarge were not merged until 2017.)

2. RDF, a refuse derived fuel

RDF stands for Refuse Derived Fuel, that is, refuse derived fuel, which is a harmless solid fuel made by sorting waste plastics, waste paper, waste wood, textiles, etc. from domestic garbage and then breaking, drying and forming them. The calorific value of RDF is 14595 ~ 20016 kJ/kg, which is much higher than that of coal (about 10870kJ/kg), while the average calorific value of China's general municipal solid waste is about 4000kJ/kg. [5] Mr. Gao Min, Technology Center of Chengdu Building Materials Industrial Design and Research Institute Co., LTD., wrote in the article "Technology for Collaborative Disposal of Urban and rural domestic waste in Cement kilns" : "(1) Urban and rural domestic waste is divided into three parts after pretreatment: primary RDF, detrite residue, inorganic heavy matter... ; (2) The organic combustibles obtained by air separation are further broken and processed in multiple stages to make the organic combustibles into primary RDF, without adding any additives (lime powder, pulverized coal, etc.) in the preparation process, and without traditional necessary processes such as drying and molding, simplifying the disposal method."

The American Society for Testing Materials (ASTM) classifies RDF into seven categories based on the degree and nature of waste processing. [6] RDF started early in foreign countries and is now relatively mature. More than 30 RDF power stations have been built in the United States, and many companies in Japan have productized RDF. In China, it is still in the initial stage, and there are few RDF landing projects. In recent years, Chengdu Building Materials Institute and Huaxin Cement have designed and put into operation a number of domestic waste preparation RDF and cement kiln collaborative disposal projects.

At present, the alternative fuel in China's cement industry is in a low stage of development, and the concepts of "RDF derived from garbage" and "combustible waste" are confused. Some people mistakenly refer to the technical model of using cement kilns to cooperatively dispose of raw ecological urban domestic waste in Liyang, Huasinhao Ruiwu, Jinyu Handan, Finland, and so on, as "RDF". (The collaborative disposal technology of cement kiln is to formulate targeted disposal plans according to the physical form, packaging method and chemical characteristics of the hazardous waste to be disposed of, respectively from the raw mill, decomposition furnace and kiln door cover, etc., to realize the harmless disposal process of hazardous waste while producing clinker or cement.) In the fuel substitution actions of 8 cement head companies at home and abroad, there are actions to use cement kiln collaborative disposal technology for fuel substitution and solid waste treatment. Jiang Xuchang (2021) collected and sorted out the technical models of ten different scale cement kilns for collaborative disposal of urban domestic waste developed by 12 existing units in China, and showed that so far, none of them has applied "RDF" that meets the European standard in a true sense.