Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

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Chemical Engineering : Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

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Gasification of Coal, Petrocoke and Biomass 

Gasification of coal, petrocoke, biomass and other carbonaceous materials to produce synthesis gas has gained significant economic and environmental importance in the recent years. Although the technology of gasification or partial oxidation of heavy feedstock is quite established for over half century, however the technology of gasification has undergone continuous developments over years to tackle higher ends of refinery bottoms, petrocoke, coal and biomass

 Gasification of coal to produce coal gas goes to the end of 18th century when coal gas was used for heating and lighting. History of gasification is given in Table M-II 3.1.However with the availability of natural gas and petroleum products at cheaper rate the interest in coal gasification dwindled. However due to increasing cost of oil and gas and the availability of petrocoke and look for alternative feed stock for gasoline and petrochemical, interest was renewed in coal gasification. Coal gasification is now used by surface and underground gasification. Gasification of biomass is also getting interest during recent years because of availability of biomass in abundance

 India has estimated coal reserves of 253 billion tons and lignite around 28 billion tons and lignite around 28 billion tons. Indian coal is mainly non-coking anthracite and bituminous in nature with ash content of over 35percent. Gasification of coal offers as promising source of chemicals as well liquid fuel. The gasification of coal involves the conversion of coal to high content gas (CO and H2)

Gasification technology offers the cleanest and most efficient way to convert low and/or negative value carbon based feed stock to syngas. Indian context of gasification is given in Table M-II 3.2. Synthetic gas can ultimately replace natural gas for Industrial uses, electrical power generation and basic raw material to produce chemical and fuel oil. Commercial Success of Gasification Technology will depend on the advancement of technologies such as:

  • Low cost oxygen production
  • Syngas cleanup
  • Cost effective separation of Hydrogen from C2

Gasification is the process of converting organic part of solid fuel to combustible gases of high heat value by interaction with steam and oxygen. Gasification converts the low value fuel to high heat value gas. Typical gasification flow diagram is shown in Figure M-II 3.1.

The feed for Gasification can be

  • Gas   (e.g., Natural gas)
  • Liquid  (e.g., Light or Heavy oils)
  • Solid   (e.g., Petroleum Coke, Coal, Lignite or Biomass).  

Table M-II 3.1: History of Gasification   

Period

Technology

Before 1700

Major fuels were Wood and Charcoal

1700-1750

Industrial revolutions - Coal as fuel

1800-1900

Coal Pyrolysis - Town gas supply, Water gas, Producer Gas

1920

Cryogenic air separation - Oxygen replaces air

1926

Winkler Fluidized Bed Gasifier

1931

Lurgi Moving Bed Gasifier

1940

Koppers-Totzek Entrained Flow Gasifier

1950s

Texaco and Shell develop Oil Gasification

1970s

Oil crisis

1973

Texaco develops Slurry Process for Coal Gasification

1974

Shell and Koppers-Totzek Pressure Gasification JV

1981

High Temperature Winkler Gasification

1984

Lurgi Slagging Gasifier (together with British Gas)

1999

Shell/Krupp-Uhde develops Pressurised Entrained; Flow (PRENFLO) Gasifier

Beyond 2000

Shell Gasification, GE Quench/PHR/FHR, Siemens, Chinese, GPE, Plasma, Headwaters

                     

 Table M-II 3.2: Gasification – Indian Context  

1940s

Wood Gasification

Wood

FACT - Cochin

1945-1950

Lurgi Fixed Bed

Coal

Sindri

1960s

Winkler Fluidized Bed

Lignite

Neyveli

1960s

Texaco

Naphtha

FACT - Cochin

1970s

Krupp-Koppers Entrained Bed Atm.

Coal

RamagundamTalcher

1970s

Shell

Fuel oil

Sindri

1980s

Shell

Fuel oil

NFL - Bhatinda,Panipat, Nangal

1980s

Texaco

Fuel oil

GNFC - Bharuch

 

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev  

Figure M-II 3.1: Typical Flow Diagram of Gasification process and its Products. 

Under Ground Coal Gasification (UCG) 

Under the process of UCG, gasification of coal happens in situ by controlled burning. About 350m3 of gas can be produced per tone of coal. Bye products of significant importance are hydrocarbon, phenols, anhydrous ammonia, and clean water. In UCG overcomes hazard of underground and open cast mining and ash/slag removal is not required as they remain in the cavities. Some of the advantages of UCG are tapping of energy from vast coal reserves which are not economically viable to mine, increased worker safety, low environmental impact, increased worker safety

Surface Gasification

Surface gasification involves reaction of coal with hot steam and controlled amount of air or oxygen under high temperature and pressure. Produces synthetic gas which can be used be the substitute of Natural gas, for power generation using low Btu gas as fuel, manufacture of chemicals and fertilizer and conversions to liquid fuel by GTL liquid.

 Process Steps in Gasification
Various process steps involved in gasification are :

Feed Preparation: Wet feed system or dry feed system may be used in gasification. Wet feed system employ grinding and slurring with water and pumping to gasifier while dry system consist of grinding of the feed and employ lock hopers to pressurize the feed

Gasification: Gasification is carried out at temperature between 900-1100oC. carbon react with oxygen and steam to raw synthesis gas( CO and H2) and some minor by products which is removed to produce clean synthesis gas which can be used as fuel , for generation of steam and electricity or may be used as chemical feed stock. 

Syn gas cooling: High temperature syn gas produced from gasification is cooled Air Separation Unit: The process produces oxygen for gasification. The process is based on cryogenic separation of air. The process involves liquefaction of air and fractionation to get oxygen, nitrogen and other gases.  
 
Acid Gas Removal: This involves removal of impurities like H2S, COS, NH3 and HCN by absorption using various commercial solvent like mono-ethanol amine (MEA), methyl diethanol amine (MDEA), methanol rectisolvent and Selexol . The solvent is regenerated by stripping the acid gases and recycle.

Slag Handling System: The gasifier may be either slagging or non slagging.

Gasification in Petroleum refinery  With increasing use of heavier crude and processing of heavy residue has resulted in increased production of tar, petroccoke and asphalt by various residues upgrading technologies processes like visbreaking, coking and deasphalting. These residues can be gasified for production of hydrogen, syn. gas, electricity ammonia and chemicals.. During recent years petrocoke gasification is getting interest by petroleum refiners for production of syn. gas which can be further process to separate hydrogen or can be used for production of useful chemicals.

Gasifier Configurations  

Various types of gasifier used are mention in Table M-II 3.3 and shown in Figure M-II 3.2 below: 

Table M-II 3.3: Various Type of Gasifier  

Fixed bed

Fixed bed involves an upward flow of reaction gas through a

relatively stationary bed of hot coal. The gas velocity is low.

Moving bed

gasifier

Moving bed gasifier operates counter currently where the coal

inter the gasifier at the top and moves downward and slowly

heated with the product gas. Gasification takes place in the gasification zone.

Fluidised bed gasifier e.g., Winkler/KBR/U-

GAS

Fluidised bed operates at higher gas velocities than fixed bed

and uses smaller particle. The gasifier operates at atmospheric

pressure and moderate and uniform temperature.

Entrained flow

gasifier

Entrained bed operates with parallel flows of reaction gas and

pulverized coal which minimize the reaction time and maximize throughput of product.. The residence time is few seconds. Ash

is removed as molten slag

Gasification Reactions:

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Where for gas as pure methane m=4 and n=1 hence m/n=4

For oil m/n≈2 hence m=2, n=1

For coal m/n≈1 hence m=1,n=1 

It is exothermic and produces a gas containing mainly CO and H2. The raw synthesis gas contains small quantities of CO2, H2O and H2S and impurities, such as CH4, NH3, COS, HCN, N2, Argon and ash, the quantities being determined by the composition of the feedstock, the oxidant and actual gasification temperature (1300-1400oC). A small amount of unconverted carbon is also present and ranges from 0.5 to 1.0 by percent wt in liquid feedstock or 50-200 ppm wt in gaseous feedstock. The various reactions can be summarized as follows

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Figure M-II 3.2: Various Type of Gasifier Commonly Used 

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Figure M-II 3.3: Fluidised bed gasifier  \

NEW GASIFIER DESIGNS 

GE Global/ Unmixed Fuel Processor (UOP):

  • Elimination of Air Separation Unit (ASU)
  • High Temperature Syngas Clean up
  • Higher efficiency
  • Lower cost

KBR Transport Gasifier (TRIGTM

  • Low rank, high-ash, high-moisture coal compatible
  • For power generation, air can be employed as the oxidant
  • Lower cost predicted
  • Higher availability predicted
  • Non-slagging, and refractory issues should therefore be minimal
  • Higher predicted efficiency
  • Lower emissions (due to higher efficiency)
  • Large scale up of the technology still required, by a factor of ~30
  • By 2010, this technology will be operating at the scale of E-Gas which has >20 years   experience already in 2007.
  • Lower temperatures and short gas residence time may lead to some methane formation, which is detrimental in chemical applications.
  • Ash disposal problem if carbon conversion predictions are not met in commercial apparatus.

Catalytic Coal Gasification - BluegasTM

  • Elimination of oxygen plant
  • For SNG objective, little or no catalytic methanation required
  • High thermodynamic efficiency potential
  • Catalyst cost and recoverability
  • Carbon conversion and methane production yields in the gasifier
  • Cost of applying the catalyst effectively to the coal
  • Inherently must be done in a fluidized bed which have not been scaled up to larger capacities of entrained gasifier (yet)
  • Interactions of catalyst with coal ash
  • Separation costs of syngas and methane - cryogenic process
  • Excess steam requirements
  • Unsuitable for chemical synthesis processes due to CHreforming requirement.  

 Coal Gasification in fertilizer Industry: Coal gasification fro production of synthesis gas and ammonia has being used many part of the world where large quantity of coal is present. However, higher energy consumption in coal based ammonia plant has been major constrain.  Relative energy consumption per tonne of ammonia is given below.

Gas   Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

There has been extensive research for improvement of coal gasification technology to improve the efficiency and reduce the cost.

 Rapid Thermal Processing (RTP TM) Process for Conversion of Biomass to Liquid Fuels 

RTP TM is fully heat integrated technology that yields over 70percent liquid products from typical biomass feed stocks. It is a fast thermal process where biomass is rapidly of heated in the absence of oxygen. The biomass is vaporized and the vapour cooled to generate high yields of pyrolysis oil. Typical yield of various biomass is given in Table M-II 3.4. Chemical and biochemical conversion of biomass to biofuel is shown in Figure M-II 3.4 and Figure M-II 3.5 respectively.

Table M-II 3.4: Typical Yield of Various Biomass 

Biomass Feed stock

Typical Pyrolysis oil yield ,wt% of dry feed stock70-75

Hardwood

70-80

Softwood

70-80

Hardwood bark

60-65

Soft wood bark

55-65

Corn fiber

65-75

Bagasse

70-75

Waste Paper

60-80

                          

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Figure M-II 3.4: Chemical Conversion Route of Biomass to Biofuels 

Gasification of Coal, Petrocoke And Biomass Chemical Engineering Notes | EduRev

Figure M-II 3.5: Biochemical Conversion Route of Biomass to Biofuels  

 Co-gasification of Coal and biomass for methanol synthesis: Cogasification of coal and biomass combined with power and methanol production can be of considered as a potential fuel-base for gasification and further production of synthesis gas and methanol. Some of the advantage of integrated biomass-coal cogasification system is possible continuous operation of the coal gasification reactor, lower costs of electric-energy production

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