rafaelkandiyoti.presentation

8/10/2019 RafaelKandiyoti.presentation

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* We know a great deal about thermal breakdown in c& we know a lot about reactor design

* But during the pyrolysis of coal (& other solid fuelsour observations are affected by experimental design

* Normal reactor design concepts do not work (directin the case of pyrolytic (i.e. thermal) breakdown.

The problem is

* Thermal decomposition products (of coals) are reacdifficult to keep track of theprimary - secondary - tertiary reaction products

Department of Chemical Engineering * Imperial College London


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* yields & structures of products depend on howproducts are removed (or not) from parent coal partic& from the reaction zone therefore

* different reactor and/or sample configurations,under similar experimental temps & pressures

can give different resultsIn this paper We try to superpose what we know of thermal

breakdown ONTO how reaction products areaffected by experimental design parameters

We aim to explore the value of bench scaleexperiments in investigating fuel behaviour inlarger scale pilot & industrial plant


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we will also explore how

similar concepts are required to observeimportant phenomenasuch as

* the effect of heating rate during pyrolysis* effect of solvent type on coal dissolution* effect of time-at-temperature on thegasification reactivity of charswe will review some of these cases


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Meanwhile(as inANYscientific experiment)

..we require data on the behaviour of solid fuelto beindependent

(or as independent as possible)

from the experimental method (design)


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Let us begin by looking atinitial thermal breakdown in coals

in shortwe need to knowHow experimental design(including reactor design)

can affect product yields and quality


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Coal Elemental C%, dafT1 (

C) T 2 ( C)

Can a 54.2 250 310Burning Star 75.5 220 310Linby 83.0 205 310Point of Ayr 85.4 220 325

Cortonwoodb

87.2 250 340Cynheidre c 95.2 - -

These esr data show the onsetof extensive covalent bond

scission reactions from about310 - 340 Cdepending on coal rank

Fowler, Bartle & Kandiyoti,Carbon27 (1989) 197



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These experiments were carried out between 19

In another set of experiments(Many years later: 2000-2002)

we observed the following,which could be directly linkedto our earlier esr experiments


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Atmospheric pressure wire mesh reactor

Heating ratevariable between

1 C s -1 &10,000 C s -1

Multistage heatingto pre-set temps.

Max. Temp 2,000 C


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0

100

200

300

400

500

600

50 100 150 200Time s)

Tmp

aue(C

Temp-1

Temp-2Temp(Mean)

1 /s

1000 /s

Heating patterns in the wire-mesh reactorFast- hold -Slow hold

Or slow-hold-slow-hold



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20

25

30

3540

45

50

55

60

65

70

0 20 40 60 80 100 120Hold Time s)

E

ra

Yed(ma

,d

1000(400)1(400)

Heat coal particles at 1 C s-1

OR at 1,000 C s-1

to 400 Cthen extract the chars with NMP

Coal A


* NOTE: The internally released extractable mateis stable at 400 C during at least two-minutes


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Thus internally released extractable material

a. may be extracted by a good solvent during liquefacb. or if the temperature is raised without solvent

we get dry pyrolysis

more on pyrolysis a little further on


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Let us see how such information

may help us explain observations fromcoal liquefaction experiments

We have an unusual liquefaction reactordesign:

removes extracts from the reaction zone assoon as they are released from the parentcoal particle


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The flowing-solvent reactor assembly

Xu & Kandiyoti Energy and Fuels 10 (1996) 1115


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Xu & Kandiyoti Energy & Fuels 10 (1996) 1115

Temperature and power control history5 C s

-1

to 450 C with 400s holdingSolvent flow rate : 0.9 ml s-1 at 70 bar



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During liquefaction, we can recover large amounts ofextract above 350 - 375C in a good solvent


But not in

dry pyrolysis

In dry pyrolysis, bond scissionis similar BUT material releasedfrom the solid matrix remainswithin the coal particlesand at 400C the extractables are stable for minutes


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---------------------------------------------------HRate Holding Medium Weight Loss C s-1 time(s) 350 C 450 C---------------------------------------------------

PoA vitrinite:1,000 150 Helium 3.3 20.5 (2)*5 400 Tetralin 28.8*** 77.6 (2)5 400 Q/P** 38.0 73.8 (2)

5 400 Quinoline --- 72.7 (2)5 400 Hexadecane 12.5 27.3 (2)PoA whole coal:5 400 Tetralin 24.6 82.5 (4)5 400 Quinoline 39.5 74.7 (2)5 400 Hexadecane --- 24.0 (1)---------------------------------------------------* Number of repeated runs used for calculating the average value** Q/P: quinoline/phenanthrene (2.5:1 w/w) mixture.*** Holding time: 500 seconds. The weight loss from 100 s experiments under

the same conditions was 29.2 %, within experimental error.All data: (% w/w daf basis)

Conversions in the Flowing Solvent Reactoronversions in the Flowing Solvent Reactorsolvent flow rate of 0.9 ml solvent flow rate of 0.9 ml s-1 at 70 bar(g)t 70 bar(g)


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& what happens when flowing-solventreactor results are compared with extracti

in closed bomb (i.e. batch) reactors ??

so for a good solvent that is not a donor solvent, thconversions are close to those of a donor-solvent

in the flowing solvent reactor


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Gibbins, Kimber, Gaines & Kandiyoti, R., Fuel 70, (1991), 380

a. Effect of solvent type on conversion. Flowing-solvent reactHeating at 5 C s-1; solvent flow rate: 0.9 ml/s at 70 barb. Flowing Solvent & Mini-Bomb reactors.

1-methylnaphthalene solvent; solvent/coal ratio in m-b: 4/1


Flowing solvent reactor Mini-bomb reactor


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The type of solvent thus has a determinanteffect on the course of the liquefaction process

BUT SO DOES THE CHOICE OF REACTOR

Using non-donor (good) solvents in theflowing solvent reactorhas a far less dramatic effect: becauseextract free-radicals dissolve in excess solve& are greatly diluted

This shows us how results can be affected by experimental des


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let us now examine the effect of reactor desigon the results of pyrolysis/gasification experime

but first we need to think about ?-happensduring pyrolysis


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during dry pyrolysis at 400 C, most of the extractables are still

intact within the coal particleWhen the temperature is raised further,- some of the lighter components evaporate- most of the extractables crack, producing lighter t

& light gases-but a significant amount of the extractablesrecombines to form char

That is why we get 50-60 % char during dry pyroly


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We need to keep an eye on two key parameters

1. Effect of heating rate2. Trajectory of the volatiles

D t t f Ch i l E i i * I i l C ll L d


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Effect of heating rate on tar and total volatile yieldLinby coal, atmospheric pressure He, 700C, 30 s hold

Fuel68 (1989) 895

Particle size:106-152 m

Heating rate range:1 C s-1 - 1000 C s-1

We thinkthe effectis due to1. Rapid volatiles ejection2. FR quenching by native

hydrogen

D t t f Ch i l E i i * I i l C ll L d


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Products of pyrolysis are reactive

Tarsmayre-polymerise to char and/orcrack to gas.

Product distributions thus (also) dependon extents of volatiles/solid contact

* The outcome of experiments depend on how volatiles areremoved from the reaction zone* Char gasification reactivity depends on mode of pyrolysifast/slow heating? tars removed/condensed on char?

& what happens when particles are stacked toge


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Resistance heating ofthe high-pressure

reactor tube6 mm id&8 mm id

Fuel 66, (1987), 1413Fuel 77, (1998), 1411

Hot-Rod fixed-bed

reactor

800 C & 100 baror

1,000 C & 40 bar


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Department of Chemical Engineering Imperial College London

and at high pressure?Comparing yields between the

(i) wire mesh reactor &(ii) a fluidized bed reactor

In experiments carried out at 1,000C

& 1 30 bars pressureThe designs of the high-pressure rigs are

somewhat different



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High-Pressure Wire-Mesh Reactor

160 bar at 850Cor

40 bar & 2,000Cat 1 10,000C s-1

steam injectioncapability

5 mg sample

p g g p g

Messenbock, Dugwell & KandiyotiEnergy and Fuels 13 , (1999), 122



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to product recovery

High-PressureFluidized-BedReactor System

1,000 C & 30 bars

Body: Incoloy 800HTsample injected as

a single slugMegaritis, Zhuo,Messenbock, Dugwell, &Kandiyoti,Energy & Fuels 12, (1998), 144

p g g p g



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1

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14 2

4

16

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12

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48 mm o.d. ; 32 mm i.d.504 mm long

Creep Rupture Limit:1000 hr at 1,000 C at 40 bar

Megaritis, Zhuo,Messenbock, Dugwell, &Kandiyoti,Energy & Fuels 12, (1998), 144



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Megaritis, Zhuo, Messenbock, Dugwell, & Kandiyoti, E & F 12, (1998), 14

Pyrolysis of Daw Mill (UK) coal1,000 C between 1 30 barfluidized-bed (FBR) & wire-mesh (WMR) reactors

conversions tar yields



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Megaritis, Zhuo, Messenbock, Dugwell, & Kandiyoti, E & F 12, (1998), 1

CO2-gasification of Daw Mill (UK) coal1,000 C between 1 30 barfluidized-bed (FBR) & wire-mesh (WMR) reactors



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in CO2 gasification

differences show up betweenbetween reactors approximating

* single particle behaviour (+ fast heatin&

* stacked particles ( + slow heating)


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.we have seen that* experimental design also affects thegasification reactivities of chars

* condensed pyrolysis tars in H-R reacan deactivate residual chars

* time of exposure (due to slow heatincan also deactivate chars



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1 10 100 1000 100000.5

1.0

1.5

2.0

2.5

3.0

3.5

C

o m

b u s t

i o n

R e a c t

i v i t y R

m a x

( % / m i n

. , d a

f )

Heating Rate ( oC/s )

0 s 10 s

60 s

Combustion Reactivities; Chars from pyrolysis runDaw Mill(UK) coal: 1000C in atmospheric pressure He

We observe significant loss of reactivityin the first 10 s, even at 1,000C



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At 950 1000 C, chars deactivate by as much as a facto3 within ~ 10 s

Unless particles are consumed quickly, OR the temperatuincreased, char consumption will be much slower after 1

Char deactivation must be quantified and taken intoconsideration in kinetic and reactor modelling

What does this mean for existing kinetic models of coalgasification which contain time-independent reaction rateconstants?



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summing up

* these examples all arise from the reactivity ofproducts formed during thermal breakdown

* test reactor design must therefore attempt totake account of changing sample properties(moving targets); we always need informationon how these properties change

* When attempting to generate data to mimic(larger) pilot or plant scale equipment, we arereally trying to match data from two moving

targets [test reactor and real system]



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Thank you for your attention

rafaelkandiyoti.presentation

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