The burning out rate of waste in the calciner will affect its final effect on reducing coal consumption. What factors will affect the burn out rate of waste in the calciner?
The burnout rate of waste in the calciner can be simplified as the reaction degree of a solid substance in a certain volume container
Burnout rate = chemical reaction rate × reaction time
Therefore, the factors affecting the chemical reaction rate and reaction time will affect the burnout rate of waste in the calciner.
1. Factors affecting the rate of chemical reaction
To identify the factors that affect the rate of chemical reaction, the type of chemical reaction must be identified first. Compared with the combustion of pulverized coal, the waste has higher moisture content, larger particle size and more types, and its combustion process is more complex.
The combustion process of different kinds of waste is summarized in the table below. It can be seen from the table below that due to the difference of components, the combustion process of wastes is quite different. The specific performance is that the moisture drying of wastes containing fixed carbon starts from the inside of particles after inert heating, then decomposes at high temperature when the shape of particles remains unchanged, and finally the combustion of fixed carbon; the waste without fixed carbon starts moisture drying from the surface of particles after inert heating, and then It’s a melting and decomposition process.
Generally speaking, when the shape of waste particles remains unchanged (mass combustion), the reactant O2 needs to enter the particle for reaction, and the product CO2 needs to diffuse out of the particle, so the particle size (or size) and porosity (internal channel) of the waste are very important; if it is said that the shape of waste particles will change with the combustion process (surface combustion), such as splitting, splitting, etc The diffusion rate of O2 to the surface of waste, the specific surface area of particles and the rate of chemical reaction are very important. In addition, inert heating is required before combustion, that is, the temperature of the waste is heated to the ignition point. At this time, the moisture content of the waste will be greatly affected, because the moisture content of the waste must be heated and evaporated first. Therefore, the following three points are important factors affecting the combustion rate of waste
(1) Waste disposal capacity
With the increase of waste disposal capacity, more waste will be surrounded around a waste. Unlike pulverized coal, these wastes can not be well dispersed, so it will affect the diffusion of O2 to the surface of waste and the diffusion of evaporated water and combustion products. At the same time, more waste means that under the premise of the same flue gas enthalpy, the waste will burn The initial temperature increases slowly before sintering, which will affect the overall reaction rate when the reaction time is fixed. In addition, with the increase of waste disposal capacity, when the waste can not replace the pulverized coal well, the amount of oxygen needed for combustion and the amount of flue gas generated will inevitably increase, which requires the increase of high temperature fan, and the residence time of waste in the calciner will also be shortened.
(2) Moisture content of waste
It is unnecessary to say much about the effect of moisture content. Other reactions, such as volatilization analysis and fixed carbon combustion reaction, can continue only after the water is evaporated or almost evaporated. Therefore, the higher the moisture content, the longer the time required for water evaporation. The evaporation rate of water is affected by particle temperature, moisture content in flue gas and specific surface area of particles.
(3) Size of waste
It can be said that the size of waste is the key of the key! Different from pulverized coal, the size of waste is usually mm or even cm. For mass combustion, the increase of waste size will seriously affect the diffusion of O2 to the interior of waste and the diffusion of product CO2; for surface combustion, the increase of waste size will reduce the specific surface area of particle reaction, thus reducing the impact on its combustion rate. The results show that the main factors affecting the burning out process of polypropylene (PP) are the diameter of the material, the burning rate of the particle and the burning out time of the tire.
2. Factors affecting reaction time
For the waste fed into the calciner, if the small-sized waste is carried away with the flue gas, the residence time is basically equivalent to that of the flue gas, usually within 5S; if the large-scale waste falls into the cone of the calciner and circulates in the calciner, the reaction time is affected by the wind speed of the cone and the structure of the calciner, and the reaction time is usually within 30s; If the large-scale waste directly falls into the smoke chamber, the reaction time will be affected by the rotary kiln speed and other factors (this scenario will be discussed separately later, which will cause greater harm). For the first two scenarios, the residence time of 5-7s or 30s is obviously not enough for the waste with high moisture content and large size.
3. Summary
(1) The burn out rate of waste in calciner is affected by chemical reaction rate and reaction time
(2) In terms of chemical reaction rate, the reaction path of different wastes is quite different, but for the disposal of waste in calciner, the amount of waste disposal, the moisture content of waste and the size of waste are the main factors affecting the reaction rate
(3) For the reaction time, if the waste is carried away with the flue gas, the residence time is within 5-7s; if the recycle occurs in the calciner, it is usually within 30s, which is far from enough for the waste with high moisture content, large size and low specific surface area
(4) As far as the actual situation is concerned, we hope to increase the amount of waste disposal. If other conditions remain unchanged, it will be at the cost of sacrificing the burn out rate of waste. It is very difficult to further reduce the moisture and size of waste (higher economic cost is required). Therefore, how to extend the reaction time of waste is the key to improve the burn out rate of waste!
