What Is Coal Combustion?

Coal burning is coal that can be burned, nicknamed coal. Coal is a solid flammable mineral that is gradually formed by ancient plants buried in the ground and undergoing complex biochemical and physicochemical changes.

A kind

Coal is the remains of ancient plants, which were formed after the death of various plants from the lowest level of fungi and algae to higher plants. Due to changes in the earth's crust, these plants were buried underground and gradually evolved into coal due to formation pressure and geothermal effects.

From plant to coal, it has undergone hundreds of millions of years and a series of changes have taken place. Generally speaking, this change can be divided into two phases,

China's coal is classified according to the provisions of GB / T57512009 "China Coal Classification", which is based on the degree of coalification and process performance of coal. According to the degree of coalification of coal, coal is divided into anthracite, bituminous coal and lignite, and the degree of coalification decreases in order.
For bituminous coal, the index characterizing the degree of coalification is expressed by dry ash-free volatile content V _daf ; and the process performance index is mainly based on the adhesion index G _T Accordingly, bituminous coal is divided into the following 12 categories:

As a fuel, coal began as early as 800 years ago. Coal is widely used as a fuel for industrial production, starting from the industrial revolution at the end of the 18th century. With the invention and use of the steam engine, coal has been widely used as fuel for industrial production, bringing unprecedented productivity to the society and promoting the development of the industry. With the development of coal, steel, chemicals, mining, metallurgy And other industries. The heat of coal is high, and the calorific value of standard coal is 7000 kcal / kg. Moreover, coal has abundant reserves on the earth, is widely distributed, and is generally relatively easy to mine, so it is widely used as a fuel in various industrial productions.

In addition to coal as a fuel to obtain heat and kinetic energy, it is more important to produce metallurgical coke and artificial petroleum from it, that is, the liquid product of low-temperature carbonization of coal-

I. Coal desulfurization technology before combustion

It is mainly coal desulfurization, that is, the coal is purified before combustion to remove some sulfur and ash in the raw coal. Divided into physical, chemical and microbiological methods.

1. Physical method: It mainly refers to gravity coal separation, which makes use of the density difference between organic matter and pyrite in coal to separate them. The influencing factors of this method mainly include the crushing size of coal and the state of sulfur. The main methods include jig dressing, heavy media dressing, and wind power dressing.

2. Chemical method: It can be divided into physical chemical method and purification method. Physical and chemical methods are flotation; chemical methods include alkaline desulfurization, gas desulfurization, pyrolysis and hydrodesulfurization, and oxidation desulfurization.

3. Microbial method: a new biological engineering technology applied to the coal industry on the basis of bacterial leaching metals, which can remove organic and inorganic sulfur in coal.

China's current coal washing rate is relatively low, about 20%, compared with 42% in the United States, 94.9% in the United Kingdom, 88.7% in France, and 98.2% in Japan. Increasing the coal washing rate is expected to significantly improve coal-fired sulfur dioxide pollution. However, physical selective washing can only remove 80% of inorganic sulfur in coal, which accounts for 15% to 30% of the total sulfur content in coal. It cannot meet the requirements of sulfur dioxide pollution control for coal combustion, so it can only be used as an auxiliary means for coal sulfur .

Second, coal desulfurization technology

Limestone or dolomite is added as a desulfurizing agent during the combustion of coal. Calcium carbonate and magnesium carbonate are thermally decomposed to form calcium oxide and magnesium oxide, which react with sulfur dioxide in the flue gas to form sulfates, which are discharged with ash. There are two main types of desulfurization technologies used in the combustion process in China: briquette sulfur removal and fluidized bed combustion desulfurization technologies.

1. Briquette sulfur-fixing technology: Screen different raw materials for blending according to a certain proportion, mix with pre-treated binder and sulfur-fixing agent after crushing, and extrude and dry through mechanical equipment to obtain Finished industrial sulfur-fixed briquette with certain strength and shape. Sulfur-fixing agents mainly include limestone, marble, calcium carbide slag, etc., the amount of which is added depends on the sulfur content. Combustion of briquettes can greatly reduce the concentration of sulfur dioxide, carbon monoxide and soot in the flue gas, save coal, and the economic and environmental benefits are considerable. However, in industrial practical applications, it is necessary to solve the fire lag of briquettes, and misfires caused by improper operation problem.

2. Fluidized bed combustion desulfurization technology: Coal and sorbent are added to the bed of the combustion chamber, and the bed is suspended from the bottom of the furnace for fluidized combustion, resulting in turbulent mixing conditions and extended residence time, which improves the Combustion efficiency. The reaction process is the combustion of sulfur in coal to generate sulfur dioxide, and the limestone is calcined and decomposed into porous calcium oxide. The sulfur dioxide reaches the surface of the adsorbent and reacts to achieve the desulfurization effect. The main influencing factors of fluidized bed combustion desulfurization are calcium-sulfur ratio, calcination temperature, particle size pore structure of desulfurizer, and types of desulfurizer. To improve the desulfurization efficiency, the following methods can be used:

(1) Improve the design and operating conditions of the combustion system

(2) Precalcination of desulfurizer

(3) Use additives such as sodium carbonate, potassium carbonate, etc.

(4) Development of new desulfurizer

Third, the flue gas desulfurization technology after combustion

The basic principle of flue gas desulfurization is the acid-base neutralization reaction. Sulfur dioxide in the flue gas is an acidic substance. It reacts with basic substances to form sulfite or sulfate, thereby removing the sulfur dioxide in the flue gas. The most commonly used alkaline substances are limestone, quicklime and slaked lime. Other alkaline substances such as ammonia and seawater can also be used. They are divided into three categories: wet flue gas desulfurization technology, dry flue gas desulfurization technology, and semi-dry flue gas desulfurization technology. They are introduced as follows:

1. Wet flue gas desulfurization technology

Wet flue gas desulfurization technology means that the absorbent is liquid or slurry. Because it is a gas-liquid reaction, the reaction speed is fast, the efficiency is high, and the utilization rate of the desulfurizing agent is high. The main disadvantages of this method are the secondary pollution of desulfurization wastewater; the system is prone to fouling and corrosion; the initial investment cost of the desulfurization equipment is large;

(1) Limestone-gypsum flue gas desulfurization technology

This technology uses limestone slurry as a desulfurizing agent, sprays and scrubs the flue gas in the absorption tower, reacts sulfur dioxide in the flue gas to form calcium sulfite, and simultaneously blasts air into the slurry of the absorption tower to force the conversion of calcium sulfite. It is calcium sulfate, and a by-product of the desulfurizing agent is gypsum. The system includes flue gas heat exchange system, absorption tower desulfurization system, desulfurizer slurry preparation system, gypsum dehydration and wastewater treatment system. Because limestone is cheap and easy to transport and store, it has become the main desulfurizer in wet flue gas desulfurization process. Limestone-gypsum flue gas desulfurization technology has become the preferred wet flue gas desulfurization process. This method has high desulfurization efficiency (greater than 95%) and high working reliability, but it is easy to block and corrode and it is difficult to treat desulfurization wastewater.

(2) Ammonia flue gas desulfurization technology

The principle of this method is to use ammonia as a desulfurization absorbent. The ammonia and the flue gas are contacted and mixed in the absorption tower. The sulfur dioxide in the flue gas reacts with the ammonia to form ammonia sulfite, which is oxidized to produce ammonia sulfate solution. After crystallization, dehydration and drying You can get ammonia sulfate (fertilizer). The reaction speed of this method is much faster than the limestone-gypsum method, and there is no structure and blockage.

In addition, the wet flue gas desulfurization technology includes sodium method, double alkali desulfurization method and seawater flue gas desulfurization method, etc., which should be selected according to the source of the absorbent, local specific conditions and the sales of by-products.

2. Semi-dry flue gas desulfurization technology

The rotary spray drying method is mainly introduced. This method is a joint process developed by the United States and Denmark. Compared with the flue gas desulfurization process, this method has the characteristics of simple equipment, low investment and operating costs, small footprint, etc., and the flue gas desulfurization rate reaches 75% -90%.

This method uses the principle of spray drying to spray the absorbent slurry into the absorption tower. In the absorption tower, while the absorbent chemically reacts with sulfur dioxide in the flue gas, it absorbs the heat in the flue gas to evaporate and dry the moisture in the absorbent, and the waste residue after the desulfurization reaction is completed is discharged in a dry state. The method includes four steps: 1) preparation of the absorbent; 2) atomization of the absorbent slurry; 3) mixing the mist particles with the flue gas to absorb sulfur dioxide and drying; This method generally uses quicklime as an absorbent. The quicklime is matured into slaked lime with good reaction ability. The slaked lime slurry is sprayed into a uniform mist by a high-speed rotating atomizer of up to 15000 20,000r / min. Once the drop comes into contact with the flue gas, a strong heat exchange and chemical reaction occurs, and most of the water is quickly evaporated, resulting in a solid waste with little water content.