Tunnel Kiln Structure and Energy Consumption

一、Kiln insulation & Energy consumption
Fuel, electricity and labor are three primary costs in the sintered product production process. However, fuel waste occurs easily due to improper construction and mishandling. Therefore, reducing energy consumption becomes a long-term goal for the sintered brick production line.

The insulation performance of kiln is crucial for reducing energy consumption. When the kiln operates at high temperature all the time, about 30% - 40% of the heat is absorbed and dissipated by kiln body. As fuel price continues to rise, improving kiln insulation is increasingly important.

The kiln body consists of two parts: wall and roof. External wall, which is exposed to the atmosphere, usually requires to have the insulation cotton with thickness of 6 - 10cm in order to minimize heat loss. Most heat loss is from kiln roof heat dissipation, thus the roof insulation is more important. In addition to using insulation cotton in refractory arch brick, materials like perlite and others should also be added to further enhance insulation function and reduce heat loss.

Common materials such as aluminosilicate fiber, rock wool, perlite and lightweight insulation brick, are all characterized by good insulation performance. Kilns with insulation material can reduce energy consumption by over 50 kcal per kilogram of product compared to those without insulation.
The national standard stipulates that the temperature rise of external wall should not exceed 15℃, and kiln roof should not exceed 25℃. When a kiln meets this standard, its energy consumption could be greatly reduced. Obviously, material with good insulation performance is a must for such good performance. For example, a 4.6m tunnel kiln requires an additional investment of over 100,000 yuan.

二、Kiln cart insulation & Energy consumptio

Kiln cart heat dissipation is also another way of heat loss. Many kiln carts bottom temperature reaches 300℃, which not only causes serious heat loss, but also easily damage the bearing. The main factors affecting kiln cart heat dissipation are the insulation performance of cart construction material and the sealing performance at the contact surface between adjacent cart. A well-designed kiln cart should has insulation wool, perlite and lightweight insulation brick laid on its bottom,  followed by a layer of refractory brick. Additionally, two-stage sealing with insulation wool should be applied at the joint to reduce heat transfer effectively.

三、Kiln cart sand seal & Energy consumptio

The poor performance of sand seal not only leads to heat loss but also causes airflow disturbance inside the kiln, which are major causes of unfired brick. The cold air entering through the sand seal directly affects the green brick on both sides of the kiln cart. However, the temperature on both sides is already lower than those in the center due to heat absorption by kiln wall, and with the added influx of cold air, the temperature drops even further, leading to the unfired brick cases.

四、Tunnel kiln ventilation & Energy consumptio

Fuel combustion requires sufficient oxygen, which is combustion agent for material burning. It requires approximately 30m³ - 40m³ of air for the combustion of one kilogram of pure carbon. Although the air entering the kiln is formed by smoke exhaust fan, the size of cross-sectional area of ventilation duct is the key to ensure sufficient airflow. Without adequate airflow, the fuel can not be fully combusted. Under optimal oxygen condition, one kilogram of pure carbon can produce 8,500kcal of heat and release carbon dioxide. However, under oxygen shortage condition, the fuel can only produce 1,700kcal of heat, with incompletely Oxidized Carbon converting to carbon monoxide (coal gas) and being exhausted from the kiln.

Based on the information provided above, 10,000 bricks require approximately 1.1 tons of pure carbon. A tunnel kiln that produces 200,000 standard bricks per day has an hourly output of about 8,000 bricks, which requires approximately 0.88 tons (880kg) of pure carbon. The ventilation duct needs to supply 35,200m³ of air per hour, assuming an air velocity of 8m/s, the duct area should be approximately 1.22m². However, the cross-sectional area of the ventilation duct should be 1.5 times larger than the calculated area because the internal and external coal used for brick firing is composed of pure carbon, which has a high ash content and low calorific value. Therefore, the required oxygen is significantly greater than the amount consumed during the combustion of pure carbon.
五、Kiln insulation & Green Brick dryin

The heat for drying brick comes from smoke and waste heat of kiln. The waste heat is generated during the cooling process of the green brick. A well-insulated kiln not only minimizes heat loss during the firing process but also extracts heat from cooling section and feeds it into drying chamber, ensuring that the chamber has sufficient heat for drying brick.
六、Kiln length & Heat utilization rat

Kiln length increase not only boosts output and product quality, but also improves heat utilization rate. By increasing the length, the firing time gets prolonged, realizing a low temperature long firing. Extending insulation time of brick at relatively lower temperature helps to balance kiln temperature, enhancing product strength and reducing unfired brick, and allows for a increase in loading speed, thus boosting the output. More importantly, by extending the kiln length, it is possible to fully extract the waste heat from the cooling zone of the brick and send it to the drying kiln for brick drying. If the kiln is too short, the temperature of the brick that comes out of kiln remains high, leading to significant heat waste. Therefore, it is essential to increase the firing length appropriately.


七、Output & Energy consumptio

The heat absorbed is determined by time rather than output. Throughout the year, the kiln consumes a fixed amount of heat each day, thus, so increasing daily output is the most effective strategy to reduce energy consumption. Enhancing ventilation volume to facilitate fuel combustion is a essential condition for higher output, which is also a good way to reduce energy consumption per brick.