The vertical shaft kiln, or vertical kiln, is a type of calcination equipment used in lime or cement production. It can complete the comprehensive mechanized operation of feeding, calcining, crushing, and discharging.
The vertical shaft kiln is the earliest invented cement kiln and was widely used in cement plants around the world before the rotary kiln appears. In the 20th century, the vertical kiln in most developed countries is gradually replaced by a type of more advanced and environment-friendly cement kiln, the rotary kiln. However, in countries like India, vertical kilns are still operating in many cement plants until now.
Vertical Kiln Construction
- High efficiency
- Low investment
- Wide waste utilization scope
- Intelligent automatic control
- Small coverage area
- Low energy consumption
By introducing the working principle and production mode of shaft kilns, we will answer why they are abandoned by the industry.
Vertical Shaft Kiln Structures & Working Principles
In cement plants using vertical kilns, the grinded raw materials are first made into wet pellets by adding 12-14% water to the pelletizing pan of the disc pelletizer. Then the wet pellets are fed from the kiln top and discharged from the bottom after calcination.
According to the temperature inside during operation, the space inside the vertical kiln is divided into three zones from top to bottom: the preheating zone, the calcining zone, and the cooling zone.
1. The Preheating Zone
The wet pellets are heated by the hot air that flows from bottom to top when they enter the kiln, so that the water inside the pellets evaporates, turn the wet pellets into dry pellets.
At the same time, due to the rising temperature, the volatile matter in the fuel is constantly escaping. However, because the hot air flow lacks oxygen in itself, the volatile matter cannot be burnt and is directly discharged into the atmosphere with the exhaust gas.
Under the action of gravity, the material ball continues to sink in the kiln, and its temperature continues to rise. When the material temperature reaches 500-600℃, the kaolin is dehydrated; when the temperature continues to rise, the carbonate begins to decompose and part of the solid-state reaction occurs.
The material height in the preheating zone accounts for 5-10% of the total height of the kiln body, and the temperature range of the material is 20-1000℃.
2. The Calcining Zone
When the raw material sinks into the calcining zone, its temperature continues to rise above 1000 ℃.
At this time, most of coal in the raw material starts burning, which makes the material temperature rise rapidly, and leads to a large number of solid-state reactions; the exothermic solid-state reaction makes the temperature of the material further rise to beyond 1300 ℃, and the material appears liquid phase and enters the sintering stage.
The height and position of the calcining zone in the vertical kiln are affected by the coal in the raw material, the quality of the coal, the pellets size, the air flow condition, the pre homogenization condition of the raw material, the discharging speed, and the operation level of workers. Generally, the calcining zone locates below the kiln expansion port, its height accounts for 10-15% of the kiln body.
3. The Cooling Zone
In the cooling zone, the calcined clinker exchanges heat with the cold air blown from the bottom. The clinker is cooled, and the air is heated and then rises into the calcining zone to serve as combustion-supporting air.
Why Are Vertical Shaft Kilns Diminishing
The reason why shaft kiln is adopted by less and less cement plants is that the modern dry process rotary kiln cement production technology is far superior to the traditional shaft kiln cement production technology in terms of production capacity, product quality, energy efficiency, and environmental protection.
Compared with rotary kilns, vertical kilns require lower initial investment, but lag behind in all other aspects. Their disadvantages include:
- The product cement is low-grade quality, unsuitable for large structures or infrastructure such as bridges, airports, etc.
- The production process is complex, requires more personnel, the production cost is also high.
- High environmental pollution. The use of fluorite and other mineralizers during the production process causes high fluorine emission, which has a great impact on crops and plants.
- Higher heat consumption (up to 105% more per ton of clinker) and therefore, higher fuel costs.
- Higher percentage of free lime (1.5% to 5%).
- Unable to use alternative fuels such as municipal waste.
- It is harder to obtain production license in many countries due to its heavy environmental pollution.
As a professional cement plant constructor, AGICO has always been endeavoring to offer our customers with the most advanced and cost effective cement production solutions. The dry process cement plant using a rotary kiln is our current most recommended cement plant solution. But we still have the technology and equipment to design and build a vertical kiln system cement plant, and can offer this product If customers insist on choosing vertical kilns for their cement production.