What are the indicators to measure coal quality and how to detect these indicators?

First, the characteristics of mineral raw materials

(A) the physical properties of coal

The physical properties of coal are the external manifestations of some chemical compositions and molecular structures of coal. It is determined by original coal-forming materials and their accumulation conditions, transformation process, coalification degree, wind and oxidation degree. Including color, luster, pink, specific gravity and bulk density, hardness, brittleness, fracture and conductivity. Among them, except for specific gravity and conductivity, which need to be measured in the laboratory, everything else can be determined by naked eye observation. The physical properties of coal can be used as the basis for preliminary evaluation of coal quality, and can be used to study the genesis and metamorphic mechanism of coal and solve geological problems such as coal seam correlation.

1. color

Refers to the natural color of fresh coal surface, which is the result of coal absorbing light waves of different wavelengths. It is brownish black and generally deepens with the increase of coalification degree.

glitter

Refers to the reflection ability of the surface of coal under ordinary light. Generally asphalt, glass and diamond luster. The higher the degree of coalification, the stronger the luster; The more mineral content, the deeper the luster; The deeper the wind and oxidation, the darker the luster until it disappears completely.

3. Pink

It refers to the color of grinding coal into powder or the traces left by coal carving on glazed porcelain plates, so it is also called stripe color. Light brown-black. Generally, the higher the degree of coalification, the deeper the pink color.

4. Specific gravity and bulk density

The specific gravity of coal, also known as the density of coal, is the ratio of the weight of a certain volume of coal after removing pores to the weight of water at the same temperature and volume. The bulk density of coal is also called the weight or false specific gravity of coal, which is the ratio of the weight of a certain volume of coal including pores to the weight of water at the same temperature and volume. The bulk density of coal is an important index for calculating coal seam reserves. The bulk density of lignite is generally 1.05 ~ 1.2, that of bituminous coal is 1.2 ~ 1.4, and that of anthracite varies greatly between 1.35 ~ 1.8. The composition of coal and rock, the degree of coalification and the composition and content of minerals in coal are the main factors affecting the specific gravity and bulk density. With the same mineral content, the proportion of coal increases with the deepening of coalification.

5. Difficulties

Refers to the ability of coal to resist external mechanical action. According to the different ways of mechanical external force, the hardness of coal can be further divided into three categories: scoring hardness, indentation hardness and wear resistance hardness. The hardness of coal is related to the degree of coalification, and the hardness of lignite and coking coal is the smallest, about 2 ~ 2.5; The hardness of anthracite is the largest, close to 4.

6.brittleness

Is the extent to which coal is destroyed by external forces. The original coal-forming materials, coal composition and coalification degree all have effects on the brittleness of coal. Among the coals with different metamorphic degrees, the brittleness of long-flame coal and gas coal is smaller, the brittleness of fat coal, coking coal and lean coal is the largest, and the brittleness of anthracite coal is the smallest.

7. rupture

Refers to the cross-sectional shape formed after coal is hit by external force. Common cracks in coal are shell-shaped cracks and serrated cracks. The original material composition and coalification degree of coal are different, and the fracture shape is also different.

8. Electrical conductivity

Refers to the ability of coal to conduct current, usually expressed in resistivity. Lignite has low resistivity. When lignite is transformed into bituminous coal, the resistivity increases sharply. Bituminous coal is a poor conductor, with the increase of coalification degree, the resistivity decreases, and it drops sharply when it comes to anthracite, so it has good conductivity.

(2) Chemical composition of coal

The chemical composition of coal is very complex, but it can be divided into two categories: organic matter and inorganic matter, with organic matter as the main component.

The organic matter in coal is mainly composed of five elements: carbon, hydrogen, oxygen, nitrogen and organic sulfur. Among them, carbon, hydrogen and oxygen account for more than 95% of organic matter. In addition, there are very small amounts of phosphorus and other elements. The elemental composition of organic matter in coal changes regularly with the change of coalification degree. Generally speaking, the deeper the coalification, the higher the carbon content, the lower the hydrogen and oxygen content and the lower the nitrogen content. The content of sulfur is only related to the genetic type of coal. Carbon and hydrogen are important elements that generate heat when coal is burned, and oxygen is a combustion-supporting element, which constitutes the main body of organic matter. When coal is burned, nitrogen does not generate heat, and often precipitates in a free state. However, at high temperature, some nitrogen is converted into ammonia and other nitrogen-containing compounds, which can be recycled to produce ammonium sulfate, urea and nitrogen fertilizer. Sulfur, phosphorus, fluorine, chlorine and arsenic are harmful elements in coal. Sulfur-rich coal combustion produces sulfide gas, which not only corrodes metal equipment and reacts with water in the air to form acid rain, pollutes the environment and harms factory production, but also when coal containing sulfur and phosphorus is used for metallurgical coking, most of the sulfur and phosphorus in the coal are converted into coke during smelting and then into steel, which seriously affects the quality of coke and steel and is not conducive to steel casting and processing. When burning or coking coal containing fluorine and chlorine, all kinds of pipes and furnace walls will be strongly corroded. If arsenic-containing coal is used as fuel in brewing and food industry, high arsenic content will increase the toxicity of products and endanger people's health.

Inorganic substances in coal are mainly water and minerals, and their existence reduces the quality and utilization value of coal, most of which are harmful components in coal.

In addition, there are some rare, dispersed and radioactive elements, such as germanium, gallium, indium, thorium, vanadium, titanium and uranium, which exist in coal in the form of organic or inorganic compounds respectively. Some of these elements are important mineral resources once they reach industrial level or can be comprehensively utilized.

The chemical composition and content of coal can be known through elemental analysis, and the properties of coal can be preliminarily understood through industrial analysis, and the types and uses of coal can be roughly judged. The industrial analysis of coal includes the determination of moisture, ash and volatile matter and the calculation of fixed carbon.

1. moisture

Refers to the water content per unit weight of coal. There are three existing states of water in coal: external water, internal water and crystal water. Generally, the internal moisture of coal is used as an index to evaluate coal quality. The lower the degree of coalification, the greater the internal surface area of coal and the higher the moisture content. Moisture is harmful to the processing and utilization of coal. During coal storage, it can accelerate weathering, cracking and even spontaneous combustion; In the process of transportation, it will increase the volume, waste the capacity and increase the freight; When coking, it consumes heat, reduces the furnace temperature, prolongs the coking time and reduces the production efficiency; When burning, reduce the effective calorific value; In winter in alpine regions, coal will freeze, making loading and unloading difficult. Only when briquettes and briquettes are pressed, a proper amount of water is needed to form them.

Step 2: Grey

Refers to the solid residue left by coal after complete combustion under specific conditions. It comes from the oxidative decomposition of minerals in coal. Ash is extremely unfavorable to the processing and utilization of coal. The higher the ash content, the lower the thermal efficiency; When burning, molten ash will also form slag in the furnace, which will affect the gasification and combustion of coal and make it difficult to discharge slag. When coking, all coke is transferred, which reduces the strength of coke and seriously affects the quality of coke. The composition of coal ash is very complex, and different components directly affect the melting point of ash. Coal with low ash melting point will bring many difficulties to production operation when it is burned and gasified. Therefore, when evaluating the industrial use of coal, it is necessary to analyze the ash composition and determine the melting point of ash.

3. Volatiles

Refers to the combustible gas produced by thermal decomposition of organic matter in coal. It is the main index to classify coal and is used to preliminarily determine the processing and utilization properties of coal. The yield of volatile matter in coal is closely related to the degree of coalification. The lower the coalification degree, the higher the volatile matter, and the lower the volatile matter gradually.

4. Fixed carbon

When determining the volatilization of coal, the remaining nonvolatile matter is called coke residue. Coke residue minus ash is called fixed carbon. It is a non-volatile solid combustible in coal and can be calculated by calculation method. The appearance of coke residue is closely related to the properties of organic matter in coal. Therefore, according to the appearance characteristics of coke residue, the caking property and industrial use of coal can be qualitatively judged.

(3) Technical characteristics of coal

In order to improve the comprehensive utilization value of coal, it is necessary to understand and study the technological properties of coal to meet the requirements of various aspects of coal quality. The technological properties of coal mainly include: cohesiveness and coking, calorific value, chemical reactivity, thermal stability, light transmittance, mechanical strength and washability.

1. Cohesion and coking

Cohesiveness refers to the property that coal particles can be bonded to each other in the process of dry distillation due to the decomposition and melting of organic matter in coal. Coking refers to the ability of coal to form coke during dry distillation. The caking property of coal is a necessary condition for coking. Coal with good caking property must have good caking property, but coal with good caking property may not be able to produce coke with good quality alone. This is why coking needs coal blending. Cohesion is the main index of coal industrial classification, which is generally expressed by the thickness of colloid formed by thermal decomposition and softening of organic matter in coal, and is often called the thickness of colloid layer. The thicker the colloidal layer, the better the cohesiveness. There are many methods to determine adhesion and coking, except for the determination of colloidal layer, Logar index method, Oya expansion test and so on. Cohesion is affected by many factors, such as coalification degree, coal composition, oxidation degree, mineral content and so on. The coals with the highest and lowest coalification degree are generally not caking, and the thickness of colloidal layer is also very small.

2. calorific value

Refers to the heat generated by the complete combustion of coal per unit weight, also known as calorific value, usually expressed as 106J/kg. It is an important index to evaluate coal quality, especially thermal coal. In the international market, thermal coal is priced according to its calorific value. Since1June 1985, the reform in China has followed the pricing method from ash to calorific value for decades. The calorific value is mainly related to the content of combustible elements in coal and the degree of coalification. In order to compare coal consumption, in industrial production, the actual coal consumption is often converted into standard coal with calorific value of 2.930368× 107J/kg for calculation.

3. Chemical reactivity

Also known as activities. Refers to the ability of coal to interact with carbon dioxide, oxygen and water vapor at a certain temperature. It is an important index to evaluate gasified coal and thermal coal. Reactivity directly affects coal consumption and effective components of gas. The activity of coal generally decreases with the deepening of coalification.

4. Heat resistance

Also known as heat resistance. Refers to the ability of coal to maintain its original particle size at high temperature. This is another important index to evaluate gasified coal and thermal coal. Thermal stability directly affects the normal production in the furnace and the gasification and combustion efficiency of coal.

5. Light transmittance

Refers to coal with low coalification degree (lignite, long flame coal, etc.). ). After being treated with a mixed solution of nitric acid and phosphoric acid under specific conditions, the light transmittance of the obtained solution is called light transmittance. With the deepening of coalification, the light transmittance increases gradually. Therefore, it is an important index to distinguish lignite, long flame coal and gas coal.

6. Mechanical strength

Refers to the difficulty of lump coal being broken by external force. When coal with low mechanical strength is put into gasifier, it is easy to break into small pieces and powder, which affects the normal operation of gasifier. Therefore, coal used for gasification must have high mechanical strength.

7. Optional

It refers to the difficulty of removing dirt bands and minerals from coal by washing. See Section 4 for the current coal preparation methods in China.