The author believes that hundreds of non-hazardous waste could be treated via pyroprocess in cement plants. The Chinese central government started treating municipal garbage and sewage sludge in this way several years ago, and continues to give more support by stimulation policy and regulation in order that cement industry can contribute to the country’s environmental goals. Modifying a cement plant to deal with municipal waste is far cheaper than investing in new incineration plants. However, this new mode of production technology and equipment does increase the stress on the cement plant.
According to the lessons learned from successful projects, the author has found obvious advantages for the cement industry to treat municipal waste as follows:
- Less investment: there is existing preheater and calcinator, rotary kiln, fuel injection and burning system. The plant need only invest in transportation, storage, conveying and lifting system, feeding system, etc, which ranges from about RMB50 million to RMB150 million according to the specifications of the waste.
- The cement incineration system works for a wide range of garbage; the resulting kiln dust can be used as an admixture to benefit the clinker, while the waste can replace some materials and alternative fuels. The cement kiln is quite adaptable to different fuel types. Minor adjustments will not affect the quality of the clinker. The kiln combustion residue of municipal waste, sulfur, chlorine, heavy metals and other harmful substances and so on are trapped in the cement clinker. There are no secondary pollution problems. It will not affect the normal operation of the kiln if it is possible to stabilise the waste coming in.
- With Chinese cement capacity at 65% of total worldwide capacity, there is high potential for development in the field of waste treatment, particularly when cement plants are located close to or are easily accessible to sources of municipal waste.
- High temperatures in the kiln and preheater ensure that municipal waste stays in the high temperature zone (>850°C) for 5 ~ 15 seconds to ensure there are no dioxins.
- The high temperature of the kiln means that the high calorific value of waste materials such as sewage sludge and waste rock can be fully utilised.
Cement grinding process
Commonly, slag and flyash are used as additives in the cement grinding process together with clinker and gypsum. Slag can be ground separately in a vertical mill. The finely ground slag can be mixed directly by percentage to pure Portland cement to produce slag cement or CEM II grade cement. The limitation of these processes is that high levels of fineness cannot be achieved by lower energy consumption. The surface area of cement is less than 4500 cm2/g. In order to maximise quantity output and fineness of the slag, flyash and some others industry wastes coming from high temperature processes, a lot of tests were carried out. When the fineness is high and the surface area is greater than 6000 cm2/g, the chemical activity of the ground powder is increased. It can be mixed by 30 to 50% in the cement. The early strength is lower and the 28-day strength is almost the same with the corresponding cement.
It is time to find an alternative and environment friendly solution to the treatment of municipal waste, which is a fierce problem in China.
- Using raw meal grinding, cement grinding and pyroprocess at the cement plant, it is possible to maximise the application of industry and municipal waste so that the cement plant become a waste treatment centre and the cement industry can achieve its environmental targets.
- Policy and financial support from the Chinese central government for cement industry. According to Environmental Protection Department of China Academy for environmental planning forecast, there is about RMB4 trillion investment planned in the next 5 – 10 years.
- Investment for industry waste and municipal treatment in cement industry will be very low because there is an existing rotary kiln, calcinatory, grinding facilities. The new mill investment for a higher surface area grinding is also very low cost.
- Environmentally friendly solution: there are no dioxins because the burning temperature is more than 850°C and the residence time is more than 2 seconds. AVIC/KHD’s design can ensure residence time of 5 – 8 seconds. There is no pollution, no need for further treatment, and it saves on the use of traditional fuels. The cement industry will save 5 million tpy of standard coal by using 300 million tpy of municipal and industry waste.
This is part three of a three-part article written by Shixiang Chen for World Cement’s October issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the October 2015 issue here.
Read the article online at: https://www.worldcement.com/asia-pacific-rim/30092015/waste-disposal-chinese-cement-industry-3/