COAL PREPARATION, CARBONIZATION, LIQUEFACTION & GASIFICATION

PREPARED BY: SANJAY SHARMA (16-MRE-001) AKHIL SHARMA (16-MRE-003) SHAIVYA MANHAS (16- MRE-005) AMAN SHARMA (16-MRE-007) OSHIN CHATTA (16-MRE-009) VASUNDHRA GUPTA (16-MRE-011) AFSHAN RASHID (16-MRE-013)

COAL PREPARATION

What Is Coal Preparation?

• Coal preparation is the removal of undesirable material (ash, Sulphur, moisture)from the Run-of-Mine (ROM) coal by employing separation processes which are able to differentiate between the physical and surface properties of the coal and the impurities.

• Coal preparation is also called as Washing, Cleaning, Processing of Coal.

• Coal processing technologies play an important role in the electrical power supply chain by providing high-quality fuel for coal-fired utilities and industrial boilers.

cont…

• The upgrading and cleaning of the coal is achieved by using following separation process :

i) Low cost separation. ii) Solid-solid separation. iii) Solid-liquid separation

• The separation process removes waste rocks and water from the mined coal.

Why is Coal Preparation Needed?

• Coal preparation is required because freshly mined coals contains: i) Heterogeneous mixture of organic (carbonaceous) matter. ii) Inorganic (mineral) matter.

• These impurities reduce coal heating value, leave behind an undesirable ash residue.

• Increase the cost of transporting of coal to market.

• Reduces heating value and can lead to handling and freezing issues for consumers.

Coal Separation Mechanism

• The coal separation mechanism includes:

SCREENING. CLASSIFICATION. DENSE MEDIUM SEPRATION. GRAVITY SEPRATION.. CENTRIFUGATION. FILTERATION. THICKENNG.

Coal Processing Operations

Particle Sizing: • Run-of-mine coal produced by mechanized mining

operations can contain particles as small as fine powder and as large as several hundred millimeters.

• There are different type of sizing equipments used to produce desired size of coal which are as:

i)Vibrating screens ii)Cyclones

Vibrating Screens• Screening is used to separate different sizes

of crushed coal. In this process coarse and fine coal is separated so to accommodate for specific markets and industrial usage.

• Screens are mechanical sizing devices that use a mesh to sort particles into fine (particles that pass through the screen openings) and coarse (particles that are retained on the screen surface).

• Vibrating screens, use a shaking rotating mechanism to segregate particles and to move material along the screen surface.

• High-frequency screens vibrate very rapidly to enhance the passage of fine particles and are normally used for dewatering fine coal or rock.

Cyclones• A cyclone is a conical vessel in which coal along with finely

ground magnetite is pumped tangentially to a tapered inlet and short cylindrical section at a predetermined flow rate and pressure followed by a conical section where the separation takes place.

• The higher specific gravity fractions being subject to greater centrifugal forces pull away from the central core and descend downwards towards the apex along the wall of cyclone body and pass out as rejects.

•  The lighter particles are caught in an upward stream and pass out as clean coal through the cyclone overflow outlet.

Cont…

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• Cyclones are used where conventional screening or sieving becomes impractical.

• Classifying cyclones are commonly applied to size (cut) at 0.10 to 0.15 mm.

• And represent the only practical option for sizing ultrafine particles (at a cut of 0.045 mm).

• This sizing device exploits differences in the settling rates of particles of different size (i.e., smaller particles settle slower than larger particles).

Solid - Solid Separation The separation of valuable carbonaceous material from waste rock is typically accomplished using low-cost processes that exploit differences in physical properties that vary with mineral content.

The common properties that are used to separate coal and rock are•Size•Density•Wet ability

What is Dense Medium Separation?

• Dense medium separation or heavy medium separation is used in coal preparation to produce a commercially graded end product ,clean coal being separated from heavier shale or high ash coal.

• A popular process for cleaning coarse coal which is having a size greater than 12.5 mm.

PRINCIPLE

It is the simplest of all gravity processes and has long been a standard laboratory method for separating minerals of different specific gravity.Heavy coal particles of suitable density are used ,so that those minerals lighter than the heavy particles will float, while those denser will sink to bottom .

Water-Based Density Separators

• Water-Based Density Separators is a variety of density-based separators available for separating coal and rock in the particle size range between 0.2 and 1.0 mm.

• The most common methods include water-only cyclones and spirals.

• A water-only cyclone (WOC) is similar to a classifying cyclone, but typically has a broad wide angled conical bottom .

• Separation of coal and rock occurs because of the formation of dense suspension created by the natural fines already in the feed slurry.

• A spiral consists of a corkscrew-shaped device that sorts coal from rock by selective segregation that occurs as particles move in the flowing film along the helical trough.

• Because of the low unit capacity (two to four tons per hour), spirals are

usually arranged in groups that are fed by an overhead distributor. WOCs and spirals are often employed in two stages or in combination with other water-based separators to improve performance.

Cont…

Banks of spirals used to separate coal from rocks

Solid-Liquid Separation

• Solid-liquid separators are used downstream of coal cleaning processes to remove unwanted surface moisture that lowers heating value, leads to handling/freezing problems, and increases transportation costs.

• Several different types of mechanical dewatering methods are required, depending on the size of particles to be treated.

• The removal of water from the surfaces of coarser (> 5 mm) coal is predominantly carried out using simple screens.

• Fine particles, which have a higher surface area and tend to have correspondingly higher moisture content, are typically dewatered using centrifugal methods or filtration system.

Centrifugal Dewatering• Centrifugal dewatering systems, which use centrifugal force to pull water

away from the surfaces of coal particles, operate in much the same fashion as the spin cycle in a home washing machine.

• For coarse particles, centrifugal dryers that use either a rotating scroll or

vibratory action to transport solids are commonly used.

• For fine particles (< 1 mm), another popular design, known as a screen-bowl centrifuge, may be used.

• These units are capable of providing low moisture products, although some ultrafine solids can be lost as waste effluent with the bulk of the water.

Filtration Dewatering• Filtration processes may be used to dewater fine coal in cases where high coal

recovery is desirable.

• Filtration involves the entrapment of fine solids as a cake against a porous filtering medium.

• Traditionally, flotation concentrates have been dewatered using some form of vacuum filtration.

• These units are capable of maintaining high coal recoveries (greater than 97 percent) while generating product moisture contents of approximately 20 to 30 percent.

• The most popular type of vacuum filter used in the United States is the disc filter.

CARBONISATION

Carbonization

• The process of converting coal into coke is called as carbonization of coal.

• When a coking coal is heated in the absence of air, the porous , hard and strong residue left is called coke.

• Coke making process is multistep complex process and variety of solid liquids and gaseous products are produced which contain many valuable products.

Contd…

• Coke is white , lustrous, dense porous mass. Depending on the behavior of coal , when heated in the absence of air, the coal is classified into:

1. Non-coking coal which undergoes practically no fusing effect and also called as free burning coal.

2. 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.

Types of coal• Depending upon the extent of carbonization , coal can

be classified into four types as follows:

Classification of coal• Peat 11%• Lignite 38% (soft coal/ brown coal)• Bituminous 65% (household coal)• Anthracite 90% (hard coal)

Carbonization at various temperature

• Low temperature carbonization is used to produce liquid fuels while high temperature carbonization is used to produce gaseous products.

• Low temperature carbonization (450-7500 C): In low temperature carbonization quantity of gaseous product is less while liquid products are large.

• High temperature carbonization (above 9000 C): In high temperature carbonization, the yield of gaseous product is more than liquid products with production of tar relatively low.

LIQUEFaCTION

Conversion of Coal to Liquid(CTL) Fuels

• Several very old processes… since WWII

• Same general theme: increase H/C ratio

• Can make a wide variety of hydrocarbon products (e.g. synthetic crude or synfuel)

• Two basic methods: direct and indirect

Advantages• CTL Improves national and economic security

• Lessens dependence on foreign oil

• Uses domestic resources and produces more jobs for Americans

• Provides environmental benefits- Cleaner fuels that reduce NOx and particulate emissions- Enables use of higher efficiency engines

• Is capable of capturing CO2 emissions

• Provides geographic diversity as energy source

Coal Liquefaction• Very expensive

• Liquefaction attractive for transportation fuel

• Indirect liquefaction commercially proven (>50 years)

• Acid gas removal by amines (CO2, H2S)

• S removal by Claus ProcessH2S + O2 = H2O + SO2

H2S + SO2 = S + H2O

Direct Liquefaction Features

• Low transportation cost

• Less chemical transformation required

• Higher efficiency than high Btu gas production

• Easy to store

• Less water required for manufactureHowever, processing slurries at high temperature and pressure presents difficulties with equipment life and solid/liquid separation (still not commercial)

SASOL in South Africa

• South Africa’s SASOL Co. developed a commercial coal liquids industry (fuel plus chemicals)• The plant produces about 150,000 barrels daily at its second plant

Indirect Coal liquefaction is proven technology

Conversion Approaches

1. Direct Liquefaction:• Dissolves coal in a solvent at elevated temperature and pressure• Combined with hydrogen gas and a catalyst

2. Indirect Liquefaction:• Involves first gasifying coal, followed by reacting carbon monoxide and hydrogen together

nCO + (2n+1)H2 = CnH2n+2 + nH2O

Adds hydrogen to break down the coal

Dissolves in a solvent followed by hydrocracking

Operates at 450 C and 170 bars Light products are distilled Medium and heavy distillates

obtained from vacuum distillation Liquid yields of 70% of the dry

weight of coal feed Further upgrade is needed for use

as transportation fuels

Complete breakdown of coal with steam and oxygen

Sulfur is removed from the syngas Syngas reacted over catalyst at

300 C and 20 bars Produces a lighter suite of

products; high quality gasoline and petrochemicals

Oxygenated chemicals

Comparison of ProcessesDirect

LiquefactionIndirect

Liquefaction

Indirect Liquefaction

• Fischer-Tropsch Indirect Liquefaction Process- Yields high quality transportation fuels plus other

products

• Coal-based liquid fuel becomes viable when the per-barrel price of oil is expected to exceed the $70-100 range for 20+ years.

• CTL has high front-end capital cost- A 50,000 barrel-a-day plant would cost over $3 billion to construct.

• The product refinement process is three to four times more expensive than refining an equivalent amount of oil.

• The cost of sequestering the captured CO2 would increase the price of the end product by $10-20 a barrel.

• The imposition of a carbon cap and trade policy would also raise the cost of fuel produced with CTL technology

CTL Costs

Liquids Fuels Summary….

• Proven technologies

• All processes require adding hydrogen

• All processes remove sulfur and ash

• Product include liquid, gas, and combustible solid(char)

• Several long-standing commercial processes (e.g., S. Africa - SASOL)

• So far no large-scale use without government support

Coal to Liquids Summary….

• Good promise because of very large coal deposits in the world

• Numerous processes that can generate a wide variety of products

• Environmental issues include some hazardous wastes and byproducts

• Does not yet effectively compete with natural gas or oil production … requires some form of subsidy

COAL GASIFICATIO

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Introduction :-- Gasification – conversion of carbonaceous fuel to gaseous product.

Dominant chemical process – partial oxidation.

Coal gasification – process for producing syngas.

1PRE – TREATMENT OF

COAL 2GASIFICATION

GAS CLEANING3

3 steps of coal gasification process :--

SELECTION OF SUITBLE COAL FOR GASIFICATION

Lignite, bituminous, sub-bituminous, anthracite – different applications

Factors Recommended specificationCoal rank Sub-bituminous, bituminous

Ash content <(8-12)% & max. 25%Volatile content No limitCoal reactivity Moderate - highAsh viscosity < 250 poise at operating temp.

4 ranks of coal – based on the carbon content

Gasification process

1PRE – TREATMENT OF

COAL

Gasification - coal - properly preheated

High moisture coal - drying required

Caking coals - partial oxidation required

Fluid or entrained bed gasifier - crushing & pulverizing required

Fixed bed gasifier - briqueting of fines coal particle required

Gasification process

2GASIFICATION

Pretreated coal – charged in gasification reactor Temp. :- 800-1900 °C & Pressure :- 10 MPa

Combustion reaction :- C + 0.5O2 CO C + O2 CO2

Gasification reaction :- C + CO2 2CO C + H2O CO + H2

Water – gas shift reaction :- CO + H2O CO2 + H2

Methanol production :- CO2 + 3H2 CH3OH + H2O

Gasification process

Low heat content product gas – coal burning - < stoichiometric quantity of air with or without steam

Medium heat content product gas – oxygen replaced by air

Water-gas shift conversion – additional hydrogen

Ash – removed in dry condition

Main product – synthesis gas – 1:2 = CO : H2

Gasification process

GAS CLEANING 3

Gas products – CO, CO2, H2, CH4, H2S, other organic vapour, etc. - Impurities – particulate matters - Gas product purification – prior to combustion –removal of impurities

Tar & oil removal – quenching method

Ammonia removal – scrubbing process - either aqueous or organic liquid

Acid gases removal – solvent treatment technology

Water-gas shift conversion process – CO2 removal - passing through absorber – carbon capture technology

Different gasification technique

Updraft moving bed gasifier – coal from the top & oxidants from the bottom

Downdraft moving bed gasifier – coal from the top & air from the side

Entrained flow gasifier – reaction between fined coal particles & oxygen & steam

Fluidized bed gasifier - for low rank coals like lignite

Underground coal gasification

Process – under ground – within the coal seam - 2 vertical walls drilled to the coal seam – horizontally linked – combustion process & syngas formation – same as above ground gasification

Economic benefits - no need to mine the coal - no need for coal handling - no need to prepare the coal to be fed into a reactor

Environmental benefits - minimum land use - coal seams below the fresh drinking water supplies - no landfill disposal required for ash or slag

Applications

Synthesis gas production – electricity generation, transportation fuels (e.g. gasoline & diesel)

Methanol production

Hydrogen formation – ammonia, powering hydrogen economy, upgrading fossil fuels

Ammonia - fertilizer

Hydrocarbon – methane, natural gas from coal gasification – liquefied natural gas – can replaced gasoline & diesel

Current Scenario

INDIA OTHER

Rising population – increasing energy demand – CO2 emitted from gasification – released in atmosphere – green house effect - research on carbon capture technique – capture & compression – transportation – storage

China - Recent development in converting coal to clean fuels – advance domestic gasification technology – energy efficient – Fischer – tropsch synthesis combined – moving bed gasification – high efficiency

USA - Transport Integrated

Gasification (TRIG) technology – low rank coals can also used – power generation