China Overview
In China, cement is the most important building material product, with its output value accounting for over 50 percent of the total of all building materials industries. Since 1985, China has been the largest producer and consumer of cement in the world. In 2002, the production capacity of the industry reached 820 million tons, and the real output was 725 million tons.
To meet the demand of economic development while restrained by a limited budget, China built many small cement plants with shaft kiln technologies. The fast economic development since 1990 has dramatically stimulated cement production. Although many modern and large-scale cement plants have been built, outdated shaft kilns still exist due to market demand and imbalanced economic development of different regions. The co-existence of advanced and outdated, large and small sized cement plants presents a distinct contrast in technologies. Take clinker sintering facilities for example, there are advanced kilns, such as new suspension preheater (NSP); and outdated ones, such as wet process kiln, lepol kiln, hollow kiln, and shaft kiln.
The China cement industry is facing an arduous task of structural adjustment. Up to 2002, the production capacity of all NSP production lines only accounted for 17 percent of the total despite an annual increase of about 30 million tons in recent years. Replacing outdated capacity with advanced NSP will be a major task of the industry in the next ten to fifteen years.
In the development process, the industry ability of designing and constructing modern cement plants of different scales has been greatly improved, capable of producing all kinds of advanced electro-mechanical and automatically controlled facilities. The industry manufactures more than 96 percent of the facilities used domestically and can supply cement production line construction at home and abroad. Major technological research forces of the industry include Tianjin Cement Design & Research Institute, Nanjing Cement Design & Research Institute, Chengdu Building Materials Design Institute, China Building Materials Academy, Hefei Cement Research Institute, and machine manufacturing companies in Tangshan and Shenyang .
In consistence with the world trend, China cement industry focuses on energy-saving, consumption reduction, environmental protection, quality and productivity improvement, and clean and intensive production for sustainable development. The main technological developments can be summarized as follows:
1. Types and Quality Standards of Cement
Portland Cements (also known as silicate cements in China) are the major cement products in China , with the output accounting for 97 percent of the total.
China has developed 60 types of special cements with independent intellectual property rights. Most of them are not produced all year round, but rather based on market needs. The special cements only take up three percent of the total output, some of which are more often required, including high strength cement, fast hardening and early strength cement, oil well cement, expansive cement, sulfate resisting cement, low and moderate heat cement, white cement, highway cement, aluminate cement and sul-phoaluminate cement. Among them, sulphoaluminate cement is first developed in China and exported to many countries and areas in the world.
China has 101 cement standards, including 13 compulsory national standards, 27 recommended national standards, 7 compulsory industrial standards, and 54 recommended industrial standards. The standards of Portland cements are compulsory national standards. Regardless of production processes, the cements must meet the quality requirements specified in the standards. The Chinese standards for Portland cements adopt the ISO standards in consistence with international practice.
2. Mining, homogenization of fuels and raw materials, and quality control of raw meal
Due to the huge amount of cement production and consumption, some limestone resources are being exhausted. Rational mining of limestone has been put on the agenda of the industry. The importance has been recognized to employ modern technologies for mining, storage, transportation and pre-homogenization of raw materials. Varieties of pre-homogenizing stacking yards and on-line weighing equipments have been widely adopted in newly built NSP cement plants.
In order to reduce the fluctuation of the composition of finished meal to stabilize clinker sintering, advanced rapid analysis devises (on line and off line), such as fluorescent X-ray photometer, and homogenizing facilities such as automatic raw meal homogenizing silo have been widely adopted in China. On large-scale NSP lines, the acceptance rate of finished meal composition is generally above 95 percent.
3. NSP sintering system
NSP sintering system is the main processing equipment of a NSP cement production line. China development in the NSP technology has been improved since early 1980s. Advanced NSP technologies of 1000-5000 t/d sintering systems have already been developed. The design of 10000t/d NSP sintering system has also been completed and three production lines are under construction. Currently, China has 223 NSP production lines with a daily output of between700 tons to 5000 tons. Among those NSP production lines, there are 19 with a daily output of 4000 tons or more.
§ Main technological progresses achieved in NSP sintering system in China are as follows:
§ Maximum production capacity: 10000t/d.
§ Heat consumption of clinker sintering: 710 X4.1816 kJ/kg.
§ Burner: four-channel burner used at kiln front, three channel burner used in calciner to ensure an efficient burn and good flame formation.
§ Calciner and preheater systems: computer simulated design according to the characteristics of different raw materials and fuels such as anthracite coal.
§ Preheater: five-stage cyclone preheater with a total pressure drop of 4800 + 300Pa.
§ Emission of NOx: as low as 500mg/m3 (with 10% O2) from sintering system.
§ Clinker cooling rate: 74% with grate cooler.
§ Sintering system (5000t/d): electricity consumption lower than 24kW.h/t; heat efficiency 57%; rotary kiln output 5.0t/d.m3 ,annual running rate around 90%.
4. Grinding System
Electricity consumption of cement production largely depends on grinding process in normal circumstances. However, in China the electricity consumption between different plants is not comparable in most cases, and lower power consumption is not necessarily the result of more advanced grinding technology. The combined power consumption of modern cement plants is usually higher than that of shaft kiln plants. As most of the shaft kiln plants do not have their own mines, lack of some processing segments and the massive use of labor instead of machine explain the low consumption of power. The power consumption can be compared among NSP plants, and it reflects the level of the adopted grinding technologies and equipments. In China ,the combined power consumption of NSP plants has decreased from 125kwh/t in the 1980s to 95kwh/t today in some modern plants, mainly due to the adoption of advanced technologies and equipments.
Progresses made in grinding technology are as follows:
(1) Roller mill (vertical mill) is widely used to grind raw meal, integrating intermediate crushing, grinding, drying, and separating processes, which simplifies the production process, increases grinding efficiency and saves power. China can make various vertical mill equipments for 1000-5000t/d production lines. Under comparable conditions, the power consumption of the raw meal grinding system is below 18kwh/t; the power consumption of the mill is less than 8kwh/t; and the life span of both roller covering and back linings are above 8000 hours.
(2) Equipment making and operation technology of roller mills have been well developed and widely used when building new plants and updating existing plants as a means to save power and increase output especially in cement grinding process. Currently, China cement industry uses semi-finish or combined grinding system, which is composed of roller mill, ball crusher and separator in different combinations. The output of cement is significantly increased and power consumption decreased compared with when only ball crusher (open circuit or close circuit) is used. Take the 160t/h grinding system for example; power consumption of 31kwh/t can be achieved for a specific surface area of 3400cm2/g, saving power by 15%~30%.
(3) Use of vertical mill and ball mill in combination in cement grinding systems is adopted in newly built production lines, saving power by 10%~15%.
(4) High efficiency separator is widely adopted in ball mill systems and the output can be increased by 20%~30%.
(5) Division surface used for sieving is installed in ball mill to increase grinding efficiency and to save power.
5. Equipment Manufacturing
Since the 1980s, China has purchased production equipment considered most advanced at the time as well as key equipments manufacturing technologies and licenses. NSP plants of Jidong, Ningguo, Zhujiang, etc. purchased the whole set of production equipment from overseas.
Meanwhile, China has been carrying on technology innovation based on imported technologies. Currently, China is able to manufacture complete sets of facilities for 1000-5000t/d NSP plants, and the manufacturing technologies for plants with a daily output of 10000t/d or more are under development. In 2001 and 2002, 90 NSP production lines were built with most facilities made domestically. As a result, the investment needed for a NSP production line has been reduced by over 50 percent compared with adopting imported equipment.
Some China research institutes have been researching the thermodynamics of NSP systems, combustion dynamics of fuel, heat exchange mechanism, energy-saving grinding theory, environmental protection enhancement mechanism, etc. to develop their own technologies in equipment manufacturing.
6. Wastes Utilization, Environmental Protection and Sustainable Development
The China cement industry utilizes industrial wastes, mainly slag and flyash, of more than 120 million tons every year. Most waste is used as blended materials or additives of cement while a small amount is used in batching raw meal. How to use combustible waste as alternative fuel and urban garbage as alternative raw material is still in the stage of research and development by two cement plants in Beijing and Shanghai respectively.
While the environmental protection in shaft kiln and old rotary kiln plants is not satisfactory, NSP plants generally perform well in this respect. The NSP plants in China are able to meet the dust emission standard of 50 mg/m3. Beijing Cement Plant is one of the examples that meet the international advanced standard of environmental protection and it has won the honor of garden plant? The reduction of NOx emission and the pollution caused by the use of gypsum and fluorite mineralization agent in shaft kiln plant have also been brought to attention in the industry. In recent years, China industry has made the following progress for environmental protection and sustainable development:
§ Use of low-grade limestone
§ Use of sandstone and shale to replace clay materials
§ Use of industrial waste, flyash and calcium carbide sludge for raw material
§ Use of sewer waste as raw materials
§ Use of anthracite low-smoke coal in NSP kilns
§ Burning of combustible, poisonous and hazardous waste in rotary kilns
§ Promotion of bulk cement, which has been increased to 21.7%
§ Promotion of nylon-braided bag.
Summary
China is a developing country with a large population and few resources per capita. Though the cement industry has developed rapidly since the reform and opening up in 1980s and the output has satisfied the construction demand, serious problems exist with regard to industry structure, including shaft kiln production, small production scale, outdated equipment, environmental pollution and exhausted resources. These problems present not only serious challenges but also tremendous opportunities for the China cement industry.