The view of sludge carbonization was first spread out many years ago. It was intuitively judged that this kind of high-temperature disposal technology must first exceed the threshold of drying, and its disposal energy consumption may be very high. Since the beginning of this year, the term sludge carbonization has become more and more popular, and the author has nothing to do to clarify and explore it.
1. Law
Actually, carbonization is a type of process for thermal differentiation and disposal of organic waste. According to process procedures and product characteristics, such processes may have different names, such as carbonization, carbonization, pyrolysis, cracking, dry distillation, coking, gasification, thermal cracking, thermal cracking, high-temperature cracking, etc. No matter how different these names are, its fundamental rule is to heat and differentiate organic matter under controllable conditions. The same characteristics of the process can be summarized in three sentences:
1. High temperature: Under the function of high temperature, some organic matter depolymerizes and turns into combustible gas;
2. Hypoxia: In the process of high temperature treatment, limited incineration is achieved by controlling the amount of oxygen supplied;
3. Low moisture: the waste material (such as sludge) should first reduce the moisture (pre-drying) before starting the thermal decomposition treatment;
This thermal decomposition process of organic matter is extremely similar in principle to industrial coking. The coking reaction is divided into the following stages:
(1) Drying and preheating (20-200 degrees): precipitation of moisture;
(2) Thermal decomposition begins (200-350 degrees): It produces water, carbon dioxide, carbon monoxide, methane and other gases and a small amount of tar vapor and liquid;
(3) Colloid production and solidification (350-500 degrees): Liquids such as tar and asphalt are produced, which are in a colloidal state; with polycondensation and synthesis reactions, volatiles are precipitated to form solid substances and semi-coke;
(4) Semi-coke shrinkage and coke formation (500-950 degrees): a large amount of volatiles are produced, mainly hydrogen and methane, and the hydrogen gas continues to precipitate and the coke becomes harder;
The goal of coking to obtain coke (solid part) and coal chemical products (from the further purification of gas) is very different from the treatment of waste. The pyrolysis of sludge also produces two parts: solid and gas. When it is emphasized that solid is a product with calorific value, it may be called "carbonization" or "carbonization". When emphasizing the gas from the pyrolysis of sludge as a product (used in a gas turbine), it may be called "gasification". As a waste treatment process, the gas produced by pyrolysis is burned as part of the energy source of the entire pyrolysis system to reduce the need for external supplementary energy.
Second, the main process flow and its analysis
The carbonization furnace is relatively complicated, with 4 thermal devices (preheating furnace, reburning furnace, backup furnace, deodorizing furnace), a thermal device using sludge as fuel (carbonization furnace), and a drying device (dryer) , 2 indirect gas-gas heat exchangers (air preheater and odor preheater), 1 condenser.
The key to solving the problem in this process lies in the amount of flue gas produced by combustion and its moisture content. The amount of flue gas and the moisture content of flue gas are related to the value of fuel and excess air coefficient. By calculating the organic matter and excess air coefficient of heavy oil and sludge, the final calculated dry flue gas volume and moisture value should be related to the flue gas volume and moisture at the inlet and outlet of the dryer. That is, since the flue gas generated by the preheating furnace, carbonization furnace, reburning furnace and standby furnace all enter the dryer, the high-temperature gas at the inlet and outlet of the dryer should be the same as the dry flue gas produced by the four thermal devices and the amount of moisture equal.
Three, exhibition review
As mentioned above, the thermal differentiation of sludge is a skill. There are different titles, but the general rules are similar. They all need to be dried first, and then pyrolysis. The energy of pyrolysis is used. The lack of energy needs to be made up. In the essence of energy balance, it is actually the same as drying + burning.
From the perspective of the use of organic matter, the final gasification gas is still burned after pyrolysis, so there is a question about the disposal of incineration flue gas. Of course, this application system is different from the typical oxygen-enriched incineration.
From the perspective of how balanced the system is, the sludge carbonization process is relatively appropriate. After all, the dry air consumption of the dryer's volatility is only 4.3kg/kg.H2O. Naturally, a sufficient high temperature is required, otherwise the drying is It can't be achieved.