4 Carbonization Of Coal 332a

Carbonization of coal

carbonization • Coal carbonization is the process for producing metallurgical coke for use in iron-making blast furnaces and other metal smelting processes. • The process of converting coal into coke is called as carbonization of coal. • heat treatment of coal in absence of air so that the coal decomposes thermally .they produced   

Solid fuel-coke Liquid fuel-coal tar Gaseous fuel-coal gas,mixture of CH4,H2,CO,CO2

COKE • Coke are the solid carbonaceous material derived from destructive distillation of lowash, low-sulfur bituminous coal.

• Caking coals Coal that softens and agglomerates on heating and after volatile matter has been driven off at high temperatures; produces a hard gray cellular mass of coke. All caking coals are not good coking coals.

• Coking coals Coal that can be converted into useful coke that must be strong enough to withstand handling.

Depending on the behavior of coal, when heated in the absence of air, the coal is classified into; (i) Non-coking coal which undergoes practically no fusing effect and also called as free burning coal. (ii) Coking coal which gives porous, hard and strong residue after heating in the absence of air. The residue is used for metallurgical purposes and is known as coke. They are quite strong and not crushed under the weight of ore, flux and coal in big furnaces.

Mechanism of carbonization of coal

LTC

HTC

• Carbonization of coal entails heating coal to temperatures as high as 1100◦C in the absence of oxygen in order to distill out tars and light oils . • A gaseous by-product referred to as coke oven gas (COG) along with ammonia, water, and sulfur compounds are also thermally removed from the coal. • The coke that remains after this distillation largely consists of carbon , in various crystallographic forms, but also contains the thermally modified remains of various minerals that were in the original coal. • These mineral remains, commonly referred to as coke ash, do not combust and are left as a residue after the coke is burned

CARBONISATION

LTC

MTC

HTC

(at600˚C) (at800-1000˚C) (at1000-1400˚C)

Low Temp.Carbonisation(LTC) Generally used for production of soft coke or char or semi coke. Normally it is a economical upgradation method of low rank coal like lignite. The major products are semicoke, low temp. liquor,crude low temp.spirit and gas. In low temperature carbonization quantity of gaseous product is less while liquid products are large.

In high temperature carbonization, the yield of gaseous product is more than liquid products with production of tar relatively low

• Low temperature carbonization : It is done at a temperature off about 500-700 C. It produces semi coke due to incomplete carbonization of coal. The coke oven gases produced have a lower calorific value. The tar produced is aliphatic in nature. The coke produced is more reactive, weaker in strength and large in size. High Temperature Carbonization : Done at a temperature range of 900-1100 C. It produces metallurgical coke. The coke gases have a higher calorific value. The tar produced contains more of the aromatics and the coke obtained is denser and less reactive

Difference between LTC &HTC Characteristics

Low Temperature Carbonisation

High Temperature Carbonisation

1. Heating temperature

500-7000C

900-1200°C

2. Yield of coke

75−80%

65−75%

3. Volatile matter content

5−15%

1−3%

4. Mechanical strength

Poor

Good

5. Calorific value

6500−9500 kcal/m3

5400−6000 kcal/m3

6. Quantity of by-product gases

130−150 m3/tone

300−390 m3/tone

7. Coke produced

Soft

Hard

8. Smoke produced

Smokeless

Smoky

9. In gas, percentage of (a) Aromatic hydrocarbons (b) Aliphatic hydrocarbons

Lower Higher

Higher Lower

10. Uses

Domestic

Metallurgy

• Commercially practised in coke oven for coke and in gas retorts

for gas. • Coke oven is of two types; (metallurgical industry)

a. Beehive type (coke completely burnt) b. By-product slot type (coke by product obtained) •

Similarly gas retorts are of three types producing coal gas; (gas industry) a.Horizontal type retort b.Intermittant vertical retort c.Continuous vertical retort

(Beehive Coke Oven)

(Horizontal Gas Retorts)

Coke Oven gas Composition Constituents

Amount(in%)

H2

54

CH4

28

CO

7.4

N2

5.6

CO2

2.0

O2

0.4

CnHm

2.6

Gross calorific value =5020Kcal/Nm3 (dry condition) =4450Kcal/Nm3 (wet condition)

• The commercial coke making process can be broken down into two categories: • By –product coke making • A brief description of each coking process is presented here.The majority of coke produced in the United States comes from wet-charge, by-product coke oven batteries (Figure 1). The entire cokemaking operation is comprised of the following steps: Before carbonization, the selected coals from specific mines are blended, pulverized, and oiled for proper bulk density control. The blended coal is charged into a number of slot type ovens wherein each oven shares a common heating flue with the adjacent oven. Coal is carbonized in a reducing atmosphere and the off-gas is collected and sent to the by-product plant where various by-products are recovered. Hence, this process is called by-product cokemaking. • Non- recovery coke making

Coke oven Batteries •

• •

• • • •

Coke oven plant consists of Coke oven batteries containing number of oven (around 65 ovens in each battery). The coal is charged to the coke oven through charging holes. The coal is then carbonized for 17-18 hours, during which volatile matter of coal distills out as coke oven gas and is sent to the recovery section for recovery of valuable chemicals. The ovens are maintained under positive pressure by maintaining high hydraulic main pressure of 7 mm water column in batteries. The coking is complete when the central temperature in the oven is around 9501000 oC. At this point the oven is isolated from hydraulic mains and after proper venting of residual gases, the doors are opened for coke pushing. At the end of coking period the coke mass has a high volume shrinkage which leads to detachment of mass from the walls ensuring easy pushing. The coke is then quenched and transferred to coke sorting plant. The control of oven pressure is quite important because lower pressure leads to air entry while higher pressure leads to excessive gassing, leakage of doors, stand pipe etc. Proper leveling of coal is important and care is taken so that free board space above (300 mm) is maintained to avoid choking.

BRIQUETTE • A briquette is a block of flammable material used as fuel to start and maintain a fire. • Common types of briquettes are charcoal briquettes and biomass briquettes. Parameter

Value

Briquette density, t/m³

1.0-1.2

Heat content, MJ/Kg

19.3-20.5

Ash content, %

0.5-1.5

Briquetting • Briquetting consists in applying pressure to a mass of particles with or without addition of binders& converting to agglomerate. • Main objective is to convert low grade solid fuel to one of higher quality.

(Generalised Briquetting Process)

Briquetting of Lignites • Briquetting used because of structural quality & as is obtained as fines during mining. • Briquetting requires no binder & comparatively low pressure suffices compaction of particles. • Some important parameters for briquetting; Particle size

8mm(after crushing from 250mm)

Moisture

9-11%(reduced from50-56%)

Pressure

1000kg/m³

Optimum temperature

70˚C

Briquetting of Bituminous Coal • Briquetting of fine grade coals like bituminous takes place with binders like coal,tar,pitch,petroleum bitumen .

( Typical Briquetting Process)