Desulfurization gypsum burning process and burning equipment

                                Desulfurization gypsum burning process and burning equipment

1 desulfurization gypsum calcination process
1.1 low temperature slow calcination
Low temperature slow calcination means that the materials stay in the calcination equipment for a long time, and the feeding and discharging time is at least dozens of minutes. The surface temperature of gypsum particles is in a low state, and the internal and external temperatures of particles are relatively close. For example, frying pan, indirect rotary kiln and indirect fluidized bed furnace are low-temperature slow calcination equipment. These calcination methods (equipment) indirectly heat the desulfurization gypsum through the built-in heating pipe, and the heat source is generally steam, heat transfer oil or hot flue gas. Because it is indirectly heated, the heat transfer speed between the material and the heat source is slow. After the gypsum is slowly heated and heated, it is slowly dehydrated to form hemihydrate gypsum. The most outstanding advantage of low-temperature slow calcination is that the product quality is uniform and stable. Due to low-temperature calcination, most of the calcined products are hemihydrate gypsum HH (about 88%), a very small amount of anhydrite a Ⅲ (about 2%) and dihydrate gypsum DH (less than 1%), and the content of crystal water is 5.5% ～ 6.0%. Therefore, the quality of building gypsum powder generated by low-temperature slow calcination is stable, the phase composition is stable, the setting speed is slow, and the water demand for standard consistency is small.
1.2 high temperature rapid calcination
During high-temperature rapid calcination, the heat source temperature is usually greater than 600 ℃, and the materials stay in the calciner for only a few seconds. The surface temperature of gypsum particles is high, and the internal temperature of particles varies according to the size of agglomerated particles, such as air flow calcination and direct rotary kiln calcination. The high-temperature rapid calcination method (equipment) is that the desulfurized gypsum directly contacts with the high-temperature flue gas for heat exchange and rapid dehydration and calcination. Due to the rapid temperature rise of fine gypsum powder, anhydrous gypsum a Ⅲ and anhydrous gypsum a Ⅱ are easily formed; Gypsum powder with medium fineness mainly produces hemihydrate gypsum HH; However, the agglomerated coarse gypsum powder is calcined on the surface and remains in aqueous phase before reaching the calcination temperature inside. Its phase components are generally hemihydrate gypsum HH of about 70%, anhydrite a Ⅲ of about 10%, dihydrate gypsum of about 3%, showing that anhydrite a Ⅲ accounts for a large proportion, and even a certain amount of anhydrite a Ⅱ will be produced, reducing the active components of gypsum. Therefore, it is difficult to control the rapid calcination process, the quality of building gypsum fluctuates greatly, the product phase composition is unstable, the setting speed is too fast, and the water required for standard consistency is high, but the production efficiency is high. Anhydrous gypsum a Ⅲ is an unstable phase, which can easily absorb moisture in the air to form a semi-aqueous phase. Therefore, aging cooling device shall be set in the system. The most outstanding advantages of high-temperature rapid calcination are low equipment investment and high production efficiency
1.3 composite calcination
Compound calcination is a new process developed in recent years, especially in the gypsum board industry. Composite calcination mainly simulates slow calcination, lengthens the process and time of gypsum calcination, avoids centralized heating, and changes the "one-step calcination" method to "two-step calcination" method. Desulfurized gypsum is heated successively in two different calcining equipment. First, all free water and some crystalline water are removed, and then the remaining crystalline water is removed to generate qualified building gypsum. This can reduce the material temperature and realize the cascade utilization of energy, so as to achieve the same effect as low-temperature calcination. At present, the common practice is to combine air flow rapid drying and calcination with indirect heating fluidized bed furnace or air flow rapid drying and frying pan calcination.
2 desulfurization gypsum calcination equipment
2.1 direct heating equipment
2.1.1 air flow calcination equipment
The air flow calcination equipment is used to directly contact the hot gas flow with gypsum powder in the air duct, and the dihydrate gypsum is rapidly dehydrated to produce hemihydrate gypsum. The hot gas flow can be the hot flue gas generated after the combustion of coal or natural gas. This equipment is generally equipped with a dispersing device at the feed end of the desulfurization gypsum. On the one hand, the high-speed rotating hammer is used to break up and throw up the materials to achieve particle dispersion. On the other hand, the narrow and long gap between the rotors is used to evenly distribute the hot air flow, so as to ensure that the gypsum particles are in uniform contact with the hot air flow to complete the heat exchange. During the gas flow calcination process, gypsum particles are heated in a suspended state. The gas-solid two-phase heat exchange area is large. The material moves in the same direction with the hot flue gas. The initial temperature difference is large, and the heat exchange efficiency is high. At the same time, it can also be changed from a straight pipe gas flow to a pulse gas flow or a cyclone can be arranged to disturb the gas-solid two-phase flow, so as to generate a relative velocity difference between the particles and the gas flow, and enhance the heat transfer efficiency. In the swirl section or diffusion section, the air velocity decreases, which is equivalent to increasing the heat exchange drying time between large particle gypsum and hot flue gas, which is conducive to the calcination process. The main features of air flow calcination equipment are compact and simple equipment, low equipment cost and high heat exchange efficiency.
2.1.2 direct heating rotary kiln
The direct heating rotary kiln is that the hot gas flow and raw gypsum powder are in direct contact in the rotary kiln barrel, and the dihydrate gypsum is dehydrated into hemihydrate gypsum. The gypsum powder rotates slowly in the inclined rotary kiln barrel, and the thermal medium generally moves in the same direction with the gypsum powder. Because the gypsum powder is partially in the accumulation state in the rotary kiln, the heat exchange efficiency is relatively low, and the hot gas flow can be the hot smoke generated after the combustion of coal, natural gas and heavy oil. The direct heating rotary kiln is generally equipped with a lifting plate or a baffle plate to ensure that the gypsum is evenly dispersed in the cylinder section and contacts with the hot air flow, reduce the generation of wind tunnel and improve the heat exchange efficiency. At the same time, due to the high evaporation intensity and high humidity of the materials at the inlet, in order to reduce the adhesion of the wet materials to the cylinder wall, the temperature control of the inlet hot air flow is high. The whole kiln equipment is long and a rotating body, and the system moisture removal and dust collection are the control difficulties. The direct heating rotary kiln has the advantages of simple equipment structure and price advantage.
Air flow calcination equipment and direct heating rotary kiln are used as direct heat exchange methods. The calcination process mainly involves convective heat exchange between materials and hot gas flow, so that the free water and some crystalline water in wet desulfurized gypsum vaporize and absorb heat. The temperature of the air flow drops, releases heat and takes away water vapor. The mass transfer and heat transfer processes are completed in the same air flow. Therefore, the air flow must have a certain air volume, and the temperature of the tail gas must be higher than the calcination temperature, This leads to the increase of energy consumption of the whole system.
2.2 indirect heating equipment
2.2.1 continuous frying pan
Continuous frying pan is a circular pan with a cross-over internal heat exchange tube. The drum, bottom and heat exchange tube indirectly transfer heat energy to dihydrate gypsum in the pan. The water vapor disturbance generated by mechanical mixing and gypsum dehydration makes gypsum particles in a fluidized state. Gypsum particles are evenly contacted with the heat exchange tube for heat exchange, heated and dehydrated to form hemihydrate gypsum, which is discharged from the pan through the overflow port. The heat energy can be provided by the hot flue gas generated after the combustion of coal, natural gas and heavy oil. The material temperature in the boiler is low, and the material calcination time is mostly about 1H, belonging to the low-temperature slow calcination mode. The continuous frying pan generally needs to be equipped with air drying equipment to be applied to the calcination of desulfurization gypsum, and the oversize of frying pan equipment should be avoided. The influence of the arrangement of heat exchange pipe and mixing device should be considered when selecting the type. The overall cost is high, but the calcination quality is stable.
2.2.2 inner heating tubular steam rotary kiln
The inner heating tube steam rotary kiln is suitable for places with steam. Compared with the direct heating rotary kiln, there are many heat exchange tubes inside. The tubes are filled with steam. The gypsum is indirectly heated in contact with the heat exchange tubes in the cylinder. The steam condenses into water after heat release in the tubes. The internal heating tubular steam rotary kiln adopts paddle conveyor for feeding, and moves forward with the slow rotation of the rotary kiln (1~5r/min). The gypsum is distributed in the lower section of the cylinder, in a stacked state, and cannot be in contact with all heat exchange tubes at the same time. The heat exchange efficiency is low. The design of lifting plate or retaining plate can ensure the uniform dispersion of gypsum in the cylinder section and improve the heat exchange efficiency with the heat exchange tubes. The whole kiln has long equipment, complex steam system and high cost.
2.2.3 internal heating tube type (heat transfer oil, steam, flue gas) fluidized calciner
The inner heating tube type (heat transfer oil, steam and flue gas) boiling calciner is a vertical straight cylindrical container, which adopts the principle of fluidized drying. Its main structural features are as follows: first, the bottom of the container is equipped with a porous gas distribution plate or a hood, which is designed to support solid materials. At the same time, the Roots fan generates blast air flow from the distribution plate into the bed evenly, and the air distribution plate can be single or multi block unit combination; The second is that the vessel is equipped with a clapboard, so that the materials can be heated in the internal long stroke; Thirdly, a large number of heating tubes are installed in the vessel, and the thermal medium in the tubes is saturated steam, heat transfer oil or hot flue gas; Fourth, the upper interface of the vessel bed is provided with a feed port and an overflow port. Gypsum enters from the feed port and enters another area of the diaphragm after dehydration to discharge at the overflow port. The fluidization of gypsum in the whole fluidized bed calciner is mainly realized by the steam generated by gypsum dehydration. This equipment can adopt low-temperature or high-temperature heat source. The balance and dispersion of gypsum feed, the design of heat exchange tube, blast preheating and air flow distribution, the position and height of internal partition, the horizontal or vertical flow direction arrangement of thermal medium, etc. are the key points to be considered in the design and application of internal heating tubular fluidized bed calciner. The fluidized bed furnace with flue gas as the thermal medium is equipped with an air flow drying device, which has the advantages of low equipment cost, stable calcination quality and wide application.
2.2.4 main characteristics of indirect heating equipment
The heat of the indirect heating equipment is directly transmitted through the material and the heating pipe. The scouring of the material on the heating pipe eliminates most of the boundary layer on the surface of the heating pipe, reduces the heat transfer resistance and enhances the heat exchange efficiency.
The calcination process does not require the contact heat exchange between the atmospheric flow and the materials. The quantity of the discharged humid and hot air is small, and the temperature of the discharged humid and hot air is low. Therefore, the thermal energy of the whole system can be used step by step, greatly reducing the comprehensive energy consumption of production
3 principles for selection of desulfurization gypsum calcination process and equipment
(1) The appropriate calcination process and equipment shall be determined according to the type of building gypsum terminal supplies. If it is used to produce gypsum building materials such as gypsum board and gypsum block, the rapid or composite calcination equipment can be selected. The building gypsum produced by the rapid method has fast setting and hardening, which can improve the production efficiency and is suitable for assembly line production; If it is used to produce gypsum cementitious materials such as plastering gypsum and bonding gypsum, slow or composite calcining equipment should be selected. The building gypsum with slow method has slow setting and hardening and stable performance.
(2) The appropriate calcination process and equipment shall be determined according to the heat source. Different equipment has different requirements for heat source. Due to the rapid calcination at high temperature, anhydrous gypsum a Ⅲ and a Ⅱ will inevitably be produced in the construction gypsum products, resulting in the instability of the performance of the construction gypsum products. Therefore, when there are low-temperature heat sources such as steam or heat transfer oil, priority can be given.