When it comes to coal use in cement industry, it’s predominantly employed as kiln fuel in coal fired kiln and ball mill. Among these, coal fired kiln play a pivotal role in cement and various industrial productions, where the quality of coal fuel directly impacts kiln efficiency and product quality. Understanding the fundamental requirements of coal fuel for rotary kilns is paramount for prospective buyers of coal fired kilns. This article, from the perspective of the cement industry, delves into the key quality prerequisites of coal fuel for coal fired kilns.
1. Calorific Value: Ensuring Efficient Combustion
Calorific value is a fundamental property of coal fuel and is crucial for combustion efficiency. A higher calorific value is desirable as it enhances heat generation capacity and calcination temperature. Coal with higher calorific value generates more heat energy, leading to higher calcination temperatures, thereby promoting the calcination reaction in coal fired kilns. On the contrary, lower calorific value coal leads to increased unit heat consumption, impacting the kiln’s thermal efficiency and output. Generally, the lower heating value of coal should exceed 21,772 kJ/kg (equivalent to 5,200 kcal/kg).
2. Volatile Matter: Impact on Combustion Stability
Volatile matter constitutes the combustible component of coal fuel, significantly influencing combustion stability and flame control. Insufficient volatile matter content could hinder coal ignition and result in issues like unstable combustion zones. To ensure stable flames and uniform calcination, the volatile matter content is typically maintained between 22% to 32%. Depending on circumstances, achieving optimal combustion may involve blending coal fuels with varying volatile matter content.
3. Ash Content: Affecting Reaction and Thermal Efficiency
Ash content, a non-combustible component of coal, directly affects chemical reactions and thermal efficiency within coal fired kilns. High ash content coal decreases the kiln’s heat-generating capacity, impacting calcination reaction temperature and efficiency. Additionally, ash content influences the quality and composition of post-calcination products, subsequently affecting the final cement quality.
Due to increased coal ash, the rise in liquid phase during firing leads to increased viscosity and clinker rings formation, impacting kiln ventilation and raising power consumption of exhaust fans. This increase in coal ash corresponds to a rise in coal powder usage, altering material balance in the facility. This can affect coal mill output, sometimes necessitating relaxed coal powder fineness standards, resulting in incomplete combustion and a negative cycle.
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In conclusion, the impact of coal ash on cement production is multifaceted. Considering cement industry equipment and control conditions, it’s generally required that coal powder ash content be less than 27% for ex-kiln calciners.
4. Moisture: Balancing Combustion Rate
The moisture content in coal directly influences combustion rate and flame temperature. High moisture content slows down combustion rate and reduces flame temperature, thereby affecting calcination efficiency. However, a small amount of moisture promotes the oxidation of coal and enhances flame radiative capacity. Typically, coal fuel moisture content should be controlled below 2.0%, preferably between 0.5% to 1.0%.
5. Coal Powder Fineness: Optimizing Combustion
When using coal as kiln fuel, grinding chunky coal into suitable coal powder is essential for efficient combustion. Coal powder fineness impacts combustion integrity and uniformity. Excessively coarse coal powder results in incomplete combustion, raising fuel consumption. Conversely, overly fine coal powder can generate ash deposits on clinker surfaces, leading to uneven composition and reduced product quality. Incomplete combustion of coal powder generates a reducing atmosphere, affecting product quality. Therefore, coal powder fineness is best maintained around 88μm, with residual content below 15%. If bituminous coal volatile matter is ≤15%, coal powder fineness should be controlled below 6.0%.
In conclusion, understanding the quality requirements of coal for coal fired kilns is crucial for the cement industry. High calorific value, appropriate volatile matter, ash content, moisture, and coal powder fineness are key factors ensuring the efficient and stable operation of coal fired kilns. As prospective buyers of coal fired kilns, it’s imperative to consider not only the technical specifications of the product but also the quality requirements of coal fuel, ensuring kiln performance and desired output.
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