lignite

Mineral coal with the lowest degree of coalification

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Lignite (coal); brown coal ；wood coal）， It is the mineral coal with the lowest degree of coalification. A type between peat Brown black, dull low-grade coal between asphalt coal and asphalt coal. Strong chemical reactivity, easy weathering in the air, difficult storage and transportation, and serious air pollution during combustion. However, as high-quality coal is almost mined out, lignite has become the main use in China coal However, due to the low degree of coalification of lignite, a large amount of black ash will float in the air when burning. If the coal is not washed and refined, the use of a large number of inferior lignite will lead to increasingly serious haze problems in China.

Chinese name: lignite
Foreign name: Lignite (coal)； brown coal ；wood coal
Alias: Diesel coal

Appearance: Brown black, matt
Calorific value: 23.0-27.2 Megajoule /Kg
Density: 1.1-1.2

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physical property

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Because it is rich in volatile matter, it is easy to burn and smoke. The traces of the original wood can be clearly seen on the section (from Gymnosperms Forming). It contains humic acid soluble in alkali liquor. Carbon content: 60%~77%, density: about 1.1-1.2g/cm three The volatile component is more than 40%. nothing Thickness of colloid layer 。 Constant humidity ash free basis gross calorific value It is about 23.0-27.2 MJ/kg (5500-6500 kcal/kg). It is mostly brown or brown black, relative density 1.2～1.45g/cm three 。

Lignite has high moisture content (15-60%), high volatile composition (>40%) and free humic acid. It is easy to be weathered and broken in the air, with a low ignition point (about 270 °). It is easy to ignite and self ignite after storage for more than two months, and the stacking height should not exceed two meters, which determines that lignite is not suitable for transportation. Therefore, the market of lignite is also limited.

lignite

Drying method

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Before and after lignite drying

Air drying medium Lignite dryer Under the action of induced draft fan, it forms convection heating mode with heat source, so that lignite and high humidity negative pressure enter the dryer for mass transfer and heat transfer exchange. The wet lignite moves forward slowly by stirring, dispersing and crushing the lignite, and is discharged from the drying drum to complete the process of drying the lignite. The dried lignite is mainly used as fuel for power plants, and can also be used as chemical raw materials Catalyst carrier Adsorbent, purified sewage, metal recovery, etc.

Lignite dehydration process can generally be divided into evaporation drying and non evaporation dehydration. Lignite drying technology can be divided into the following six categories:

(1) Rotary drum drying technology - the core of rotary drum drying is a slightly inclined cylinder that can rotate. The inclination of the cylinder can be adjusted, generally ranging from 2o to 10o; According to the contact mode of wet material and heat carrier, the rotary drum drying device for lignite developed and used in industry mainly has direct heating Rotary dryer Rotary tube dryer and steam tube indirect heating rotary drum dryer.

(2) Belt drying technology - lignite is evenly distributed to the conveyor belt from the feeding end through the feeding device. The conveyor belt is usually made of perforated stainless steel sheet, driven by the motor through the gearbox, which can adjust the speed. The most commonly used drying medium is hot air or hot flue gas.

(3) Air drying technology - air drying is also called "instant drying", which is the application of fluidized dilute phase transportation in drying; According to the difference of drying medium and operating temperature, airflow drying can be divided into straight tube type and bed mixed type.

(4) Fluidized bed drying technology -- lignite developed and applied in industry Fluidized bed drying equipment Superheated steam or air is mainly used as the fluidizing medium (or drying medium), and the fluidized bed is equipped with a heat exchanger inside; Lignite fluidized bed drying technology mainly includes superheated steam fluidized bed drying technology and steam air combined drying technology.

(5) Vibration drying technology - Vibration drying is the use of mechanical vibration to realize the flow of solid particles in the dryer, and at the same time, the drying process is realized under the action of drying media (such as hot flue gas). The vibrating drying equipment for lignite mainly includes vibrating mixed flow dryer and vibrating fluidized bed dryer. Rotary tube drying technology in rotary drum drying technology is the most mature lignite dehydration and drying technology in industry.

(6) K-fuel technology is the most non evaporative drying technology. This technology uses the direct contact between raw coal and steam in the upgrading device to remove the moisture in raw coal in the form of liquid water by adjusting the three factors of time, temperature and pressure. While "squeezing" the moisture in coal, it changes the pore structure and hydrophilic performance of coal, and improves the quality of low-grade coal. stateside Wyoming Build a quality improvement plant with an annual processing capacity of 750000 tons.^[1]

Pyrolysis of lignite

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Coal pyrolysis refers to a complex process of a series of physical changes and chemical reactions that occur when coal is continuously heated to a higher temperature in an inert atmosphere, also known as thermal decomposition or dry distillation. Coal pyrolysis is an important intermediate process in coal processing and conversion, such as combustion, gasification, liquefaction and other processes. Compared with other coal conversion methods, coal pyrolysis is only a thermal process, which can realize partial gasification and liquefaction of coal under normal pressure without hydrogenation and oxygen, and produce gas and coke. Compared with gasification or liquefaction process, coal pyrolysis process is simple, with mild processing conditions, less investment and low production cost.

Coal macromolecules are rich in carbon and lack of hydrogen. During pyrolysis, hydrogen in coal is enriched into coke gas and gas, and carbon rich semi coke is obtained at the same time.

Macro changes

When the coal material is heated under the condition of air isolation, with the gradual increase of heating temperature, the water first separates from the coal; When the temperature reaches 350~400 ℃, the originally dispersed coal particles (coking coal) begin to form colloid, the number gradually increases, and then gradually solidify into semi coke with the increase of temperature; In the semi coke stage, there is still a large amount of gas separated out, and then the semi coke gradually shrinks, cracks and fragments appear, until it becomes coke at about 1000 ℃. In the process of high temperature pyrolysis, coal changes from dispersed coal particles to massive coke, and its internal structure has undergone great changes, that is, coal pyrolysis at high temperature mainly goes through two processes; One is the bonding process between 300~500 ℃; The second is the coking process of shrinkage, crack and fracture of semi coke after 500 ℃.

Product and use

After lignite pyrolysis, the obtained products are semi coke, coal tar and gas. Its purpose is as follows:

Semifocal

Semi coke is the main product of lignite pyrolysis upgrading, and its calorific value is higher than that of the original lignite (generally 50% - 80% higher). It is mainly used for:

a. Preparation of activated carbon from semi coke

Pyrolysis semi coke has low volatile matter, few heteroatoms and compact microstructure, which can be used as raw material for producing low ash and high strength activated carbon.

Activated carbon is made from semi coke produced by multi-stage rotary furnace with lignite as raw material. It shows that lignite semi coke can be activated to produce properties

Better activated carbon.

b. Semi coke as reducing agent for ferroalloy

The smelting of ferrosilicon, ferromanganese, ferrochrome and other alloys requires reducing coke. Taking smelting ferrosilicon as an example, 0.8~1t reduction coke is required for each ton of ferrosilicon. High quality reduction coke requires less harmful impurities, high content of fixed carbon, high reactivity and high specific resistance. SiO in ash composition two And Fe two O three It is a useful ingredient for smelting, Al two O three And P (phosphorus) are harmful impurities, Al two O three Affect ferroalloy quality and increase power consumption. Ferroalloy production is a high energy consumption industry. Due to the high specific resistance and good reactivity of lignite semi coke, it can reduce power consumption and improve product quality in ferroalloy production. The actual production data shows that using semi coke as reducing agent to produce 1t ferrosilicon can reduce electricity by 500kW · h, and ferrosilicon output is increased by 1.5%. The pyrolysis semi coke of multi-stage rotary furnace has low ash, low aluminum and high reactivity, The characteristic of high specific resistance is high quality ferrosilicon coke.

c. Semi Jiaozuo Blast furnace fuel injection

Pulverized coal injection into blast furnace can replace part of metallurgical coke, and is beneficial to ironmaking operation control and production efficiency. Anthracite is the main injection material used. Some people have studied the semi coke as injection material and pointed out that the semi coke obtained from pyrolysis of low metamorphic coal is superior to anthracite in chemical reaction, mechanical crushing, combustion performance, etc. The volatile matter of anthracite is mostly 5%~10%, which is similar to that of semi coke (pyrolysis at 750 ℃), while the ignition point of semi coke (300~400 ℃) is generally lower than that of anthracite (470~560 ℃), and the combustion performance is good. Previous studies have shown that the grindability coefficient of semi coke is more than 60, which is easier to grind than the general anthracite (30-50). Semi coke has little volatile matter precipitation at low temperature and is non explosive. It is suitable for pulverizing 200 meshes, accounting for 30%~40%), storage and transportation.

d. Semi coke for calcium carbide

Calcium carbide production is also High energy consuming industry , which requires Coke reactivity Good, high specific resistance. Production practice shows that semi coke is more suitable for calcium carbide than metallurgical coke fragments, which can not only improve the quality of calcium carbide products, but also reduce power consumption and electrode paste consumption.

e. The semi coke used as gasification raw material has large reactivity, good thermal stability when it is in block shape, and good grindability when pulverizing. It can be used as fixed bed, fluidized bed, and entrained flow bed gasification raw materials to obtain the syngas required by users.

f. As oil absorbent

Semi coke has fast oil absorption speed and large oil absorption capacity, which can be used to deal with marine oil pollution

g. As carrier of denitration agent

The semi coke has developed pores, and when used as the carrier of denitration agent, it has a large reaction specific surface area, good denitration effect, and is easy to

Combustion treatment is carried out at.

h. As smokeless fuel

The coke has low volatile matter and good combustion performance. It can be directly used for iron ore powder sintering, boilers, cement kilns, ceramic kilns, etc., and can also be used for civil and molding fuel production.

i. As coking thinner

Semi coke has high carbon content, high microstructure strength, and is inert during coking. As a thinner, it can expand coking coal resources, reduce coke ash content, and improve coke lump size and strength.

Coal tar and gas

As an important chemical recovery product in coking process, coal tar is a complex mixture of tens of thousands of components, from which about 500 single compounds have been separated and identified, accounting for 55% of the total amount of coal tar, including 174 neutral components such as benzene, xylene, naphthalene, etc; Phenol, cresol and other 63 acidic components and 113 basic components. Many compounds in coal tar are plastics, synthetic fibers synthetic rubber , pesticide, medicine High temperature resistant material And valuable raw materials for national defense industry Polycyclic aromatic compounds It cannot be produced and replaced by petrochemical industry. Coal tar is mainly used to process and produce light oil, phenol oil, naphthalene oil and modified asphalt, and then to produce benzene, phenol, naphthalene, anthracene and other chemical raw materials after deep processing. The products are numerous and widely used. The fast pyrolysis gas by solid heat carrier method is medium calorific value gas, which can be used as city gas, industrial fuel, and chemical raw materials.^[2]

National production

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Output of each country (unit: million tons)^[3]

country	one thousand nine hundred and seventy	one thousand nine hundred and eighty	one thousand nine hundred and ninety	two thousand	two thousand and one
Germany	three hundred and sixty-nine point three zero zero	three hundred and eighty-eight	three hundred and fifty-six point five zero zero	one hundred and sixty-seven point seven zero zero	one hundred and seventy-five point four zero zero
Russia	one hundred and twenty-seven	one hundred and forty-one	one hundred and thirty-seven point three zero zero	eighty-six point four zero zero	eighty-three point two zero zero
U.S.A	five point four zero zero	forty-two point three zero zero	eighty-two point six zero zero	eighty-three point five zero zero	eighty point five zero zero
Australia	twenty-four point two zero zero	thirty-two point nine zero zero	forty-six	sixty-five	sixty-seven point eight zero zero
Greece	eight point one zero zero	twenty-three point two zero zero	fifty-one point seven zero zero	sixty-three point three zero zero	sixty-seven
India	twenty-four	twenty-four	twenty-four	twenty-four	twenty-four
poland	thirty-two point eight zero zero	thirty-six point nine zero zero	sixty-seven point six zero zero	sixty-one point three zero zero	fifty-nine point five zero zero
turkey	four point four zero zero	fifteen	forty-three point eight zero zero	sixty-three	fifty-seven point two zero zero
Czech Republic	sixty-seven	eighty-seven	seventy-one	fifty point one zero zero	fifty point seven zero zero
The People's Republic of China	thirteen	twenty-two	thirty-eight	forty	forty-seven
Yugoslavia	twenty-six	forty-three	sixty	-	-
Serbia	-	-	-	thirty-five point five zero zero	thirty-five point five zero zero
Romania	fourteen point one zero zero	twenty-seven point one zero zero	thirty-three point five zero zero	seventeen point nine zero zero	twenty-nine point eight zero zero
Democratic People's Republic of Korea	five point seven zero zero	ten	ten	twenty-six	twenty-six point five zero zero
Total	eight hundred and four	1,028.000	1,214.000	eight hundred and seventy-seven point four zero zero	eight hundred and ninety-four point eight zero zero

Opencast lignite mine near Grevenbroich, Germany

It is mainly used as fuel for power plants, as well as chemical raw materials, catalyst carrier Adsorbent , purifying sewage and recovering metal, etc.

There are usually two types:

(1) Earthy brown coal, loose and soft;

(2) Dark brown coal (lignite) is dense and hard. It can be directly used as domestic fuel, industrial heat source fuel and power generation fuel, as well as gasification and low-temperature Dry distillation And other raw materials.

In some parts of the world, lignite is used as heat source fuel and Chemicals And raw materials for gaseous and liquid fuels. In the United States, first in the northern Great Plains（ North Dakota , Eastern Montana Wyoming Northeast and South Dakota And the Gulf Coast (Alabama Arkansas , Kansas Louisiana Oklahoma and Texas) and the Pacific states（ California 、 Oregon and Washington State ）Lignite was discovered.

The amount of lignite in China is only 211.8 billion tons, which is the same as that in the world Resources (2622.9 billion tons) About the world coal resource 24.4% of the total (10753.9 billion tons), accounting for about 13% of the total national coal resources, mainly distributed in eastern Inner Mongolia and eastern Yunnan, northeast and south China There are also a few.

The combustion value of lignite is low, 3000 kcal or less. The latest development technology at home and abroad is lignite drying, which can increase its combustion value to 4800 or even 6000 kcal.

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