process design for coal liquefaction

8/11/2019 Process Design for Coal Liquefaction

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I S H A R U S T A G I ( 2 0 0 9 C H 7 0 7 2 1 )

M A T T H E W G E O R G E ( 2 0 0 8 C H 7 0 1 1 7 )

N I K H I L G O Y A L ( 2 0 0 9 C H 7 0 1 4 7 )

Process Design for Coal

Liquefaction1


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Different Routes for Coal to Liquid2


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World Oil and Coal Reserves3


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Coal Liquefaction

Portion of Organic Coal Substance converted toliquid products

Key Objectives: Producing liquids by chemically altering the coal structure

Increasing the content of hydrogen relative to carbon

Simultaneous Removal of nitrogen, sulfur, oxygen, and

mineral matter

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Liquefaction Methods

Pyrolysis: Coal conversion at a T> 400 deg. C in a non-oxidizing atmosphere to

gases, liquids, and char.

Indirect: Coal reaction with O2 & H2O high T to produce a mixture of CO &

H2 (synthesis gas), which can be catalytically converted to liquidproducts.

Direct: Coal reacts at high P & T with gaseous H2 and a solvent (Hydrogen-

donor) followed by conversion to liquids using or without addedcatalysts.

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Indirect Liquefaction

Step-1: Coal Gasification

Includes the endothermic reactions of carbon with steam andcarbon dioxide and the exothermic reaction of carbon with

oxygen.

Additional reactions include coal de-volatilization, the watergas shift reaction and the methanation reaction

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Coal Gasification

Important Considerations:

H2/CO ratio in the exit gas- an important parameter in FTsynthesis (step-2)

WGS Catalysts:

High T (440-700oC): Cr/Fe oxide or Zn/Cr oxide

Low T (230-350oC): Cu/Cr Oxide

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Importance of Catalysts in Coal Liquefaction

Stabilization of Free-Radicals formed during thermaldecomposition takes place by hydrogen abstraction fromeither hydrogen donor substances or molecular hydrogenactivated by a catalyst

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Catalyst Functions

Activate molecular hydrogen and transfer it to coalfragment radicals

Increase hydrogen transfer to the coal by hydrogenatingsolvent species to produce H-donor compounds

Increase coal conversion and yield of oil fractions

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Step 2: Syn-Gas Catalytic Conversion

Fischer-Tropsch Synthesis Wide range of products hydrocarbons, alcohols, aldehydes,

ketones, and acids

Common catalytic route: High Temperature Fischer Tropsch (HTFT): 330-350 0C

Fused Iron catalysts used*

Product fraction gasoline and light olefins

Low Temperature Fischer Tropsch (LTFT): 220-250 0C Precipitated alkali promoted Iron catalyst or supported Cobalt catalyst

used*

Product fraction waxes, diesel fuel

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Reaction Mechanism11

H2 adsorption:

Dissociative in nature with heats of adsorption in the orderCo> Ni > Fe

Alkali metals and oxides used as promoters affect hydrogen-

metal bond strength

CO adsorption:

Both dissociative (on Cr, Mn, Fe) and molecular (Co, Ni, Ru) On Ni, CO easily dissociates, forming methane while on Co and

Fe, strong metal-carbon bonds lead to C2+ compounds


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Carbide Mechanism for Fischer Tropsch SynthesisRef: A. T. Bell, Catal. Rev.Sci. Eng., 23, 203 (1981)

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Reaction Mechanism


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Advantages of Coal based liquid fuels14

Energy independence: Consumption = 14 MB-liquid fuels/day

Import = 55% Oil

Recoverable Coal Reserves = 270 billion tons

Potential Oil Production = 2 MB/day

Use in gasoline engines

ICTL process plants can easily be converted tohydrogen fuel cell production plants, once fuel celltechnology becomes more viable.


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Process Flow Sheet15


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Exergy Analysis

For the quantification of the environmentalperformance and comprehensive energy efficiencyof the system

Based on the second law analysis of the system

Maximum efficiency is attained for a process inwhich exergy is conserved

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Exergy Analysis

The process was divided into 5 CVs including:

ASU

Gasifier

Gas Cooling and scrubbing section

WGS and gas leaning

Fuel synthesis, and steam cycle.

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Calculation Steps20

The sum of the work in and out of a control volume is calculated.

The heat that crosses a control volume and the averagetemperature related to the heat streams are calculated.

Mass Conservation is verified to confirm that the system isoperating at steady state.

The flow rates that cross a control volume are identified andcalculated.

The specific exergy of each flow crossing a control volume iscalculated.


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Definitions21


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Results

Flow Streams/ ParametersSingle-Stage 2-Stage

Consumables

Coal Fed (ton/day) 11,400 10,800

O2 Fed (ton/day) 8800 7560

Water Consumption (ton/day) 1890 2230

Power

Steam Turbine (MW) 210 266

Parasitic Load -201 -236

ASU -130 -124

Acid Removal -8 -8

Others -63 -104

Net Power Output 0 30

Products

FT-Oil (m3/hr) 146 146

Energy Efficiency (LHV) 43 47

Exergy Efficiency 41 44

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F-T Unit

Consists of: F-T Reactor

Water Separation Unit

Tail Gas Recycle Loop

F-T YieldF-T Selectivity

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F-T Reactor Mass Balance24


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According to the Mass Balance, CO conversion = 50%

H2 conversion = 60 %

Also, a large amount of water is produced.

A steam system is installed.

F-T Reactor Mass Balance25


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Exergy Analysis

Exergetic efficiency for the reference case (taking theyield to be 90%) comes out to be 62%

Output energy is contained in the hydrocarbonproduct (95%) as well as the tail gases that are usedto derive power through the steam system (5%)

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Lost Work

Gasifier FT-Unit Power PlantASU

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Process Simulation using ASPEN Plus28

Gasification Process:

Chemical reactions in coal gasification (adapted from Perry et al., 2008)


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Gasification29

Reaction Sequence

Reaction sequence for the gasification of coal (adapted from Higman et al., 2003)


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ASPEN PFD31

OXYGEN

GASIFIER SYNGAS

MIXER

COAL

WATER

SLURRY

B1

SYNAPP

F


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Yield as a function of P & T32

Yield: No. of k-moles of Syn-gas per kg of MAF-Coal

0.0000

0.0100

0.0200

0.0300

0.0400

0.0500

0.0600

0.0700

800.00 1000.00 1200.00 1400.00 1600.00

Syn-Gasyield

Temperature (K)

P=30

P=10

P=50

P=70


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Assumptions: Equilibrium Operation

of the reactor undergiven T/P conditions

H2/CO ratio = 2 atT= 670 K (400 oC)

WGS Reactor: H2/CO ratio as a function of T

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0

0.5

1

1.5

2

2.5

3

3.5

400 600 800 1000 1200


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WGS Reactor Model35

Mass Balance: Equilibrium conversion (Xe) in terms of equilibrium constant (Keq)A: H2O, B: CO


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WGS Reactor: Non-Isothermal Adiabatic Model

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Assumptions: Specific heat (cP) of species and reaction heat (HRX) is

independent of pressure at which reaction takes place.

cP of species remain constant at the average of the reactor feed

temperature and calculated equilibrium (or reactor outlet)temperature.

Adiabatic reactor: Q = 0.

No work in reactor: W = 0.

All reactants enter the reactor at the same temperature,therefore Ti0 = T0

No phase changes occurs in reactor


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Non-Isothermal Adiabatic Model (contd..)37

Energy Balance: Relationship between temperatureand equilibrium conversion (XEB)


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Maximum Conversion38

Intersection of the Mass & Heat Balance curves


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WGS reactor model results39

Comparison of model results for different H2O/COfeed ratios


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WGS Reactor Model Results40

Comparison of model results for different feedtemperatures


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LTFT vs. HTFT41

LTFT Flow-sheet

HTFT Flow-sheet


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LTFT vs. HTFT43

FT step is marginally more expensive for the HTFTtechnology and nearly double the cost for the LTFTroute when compared to natural gas conversion

Comparison between the HTFT and LTFT options, itis clear that the LTFT route is more efficient

The main reason for the lower efficiency of the HTFT

option is that the acid gas removal step becomesmore utility intensive.


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Poly-generation System in FT process44


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MEB, Electricity produced and Utilities balance

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Items Units FT-100 FT-70 FT-30 FT-00Feedstocks

Coal t/h 1180 1180 1180 1180

Limestone t/h 130 130 130 130

Oxygen for the gasification t/h 984 986 984 984

Steam for the gasification t/h 142 142 142 142

CO2 for coal feeding in the gasification t/h 185 185 185 185

Scrubbing water in the gasification t/h 325 325 325 325

Steam into the WGS reactor t/h 139 432 821 1114

Air used in the Claus process t/h 6 6 6 6

Air used in the gas turbine t/h 1130 6495 13557 18878

Main products

Diesel t/h 242 170 73 0

Naphtha t/h 86 61 26 0

LPG t/h 47 33 14 0

Sulfur recovered t/h 3 3 3 3

Utilities

Making-up water t/h 4474 5066 6005 6710Electricity consumption MW 601 595 586 579

Electricity

Generated by the gas turbine MW 136 830 1749 2435

Generated by the steam turbine MW 458 833 1209 1494

Sub-total of electricity MW -8 1068 2374 3350

EFF % 48.3 44.9 39 34.6


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References

F. Fischer and H. Tropsch, Brennst. Chem., 4, 276 (2003).

J. M. Fox III, Catal. Rev.Sci. Eng., 35, 169 (1998).

J. H. Gregor, Catal. Lett., 7, 317 (1990). O.P.R. van Vliet et al. FischerTropsch diesel production in a well-to-wheel

perspective: A carbon, energy flow and cost analysis. Energy Conversion andManagement 50 (2009) 855876.

M. E. Dry and J. C. Hoogendoorn, Catal. Rev.-Sci. Eng., 23, 265 (1981).

B. Jager and R. Espinoza, Catal. Today, 23, 17 (1995).

Andre P. Steynberg, Herman G. Nel. Clean coal conversion options using FischerTropsch technology. Fuel 83 (2004) 765770

G.W. Yu et al. Process analysis for polygeneration of FischerTropsch liquids andpower with CO2 capture based on coal gasification. Fuel 89 (2010) 1070-1076.

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process design for coal liquefaction

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