Fuel Gasification

16 A Kinetic Study of the Reaction of

Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on November 19, 2016 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0069.ch016

Coal Char with Hydrogen-Steam Mixtures C. Y. WEN and O. C. ABRAHAM West Virginia University, Morgantown, W. Va. 26506 A. T. TALWALKAR Institute of Gas Technology, 3424 South State St., IIT Center, Chicago, Ill. 60616

A design procedure is developed to estimate the product gas composition at different levels of carbon conversion for a continuous one-stage coal hydrogasifier. The coal char reactions with steam and hydrogen are treated as two simultaneous reactions taking place without interfering with each other. The initial (first phase) rapid reaction owing to hydrogen is found to be proportional to its effective partial pressure and to the reactive carbon left in the char. The first-phase steam-char reaction is independent of the steam partial pressure and is proportional to the amount of residual reactive carbon. The second-phase reactions are considered to take place at solid-gas interface and are strongly affected by gas velocity at elevated temperatures in the hydrogasifier.

'Tp he gasification of coal with hydrogen-steam mixtures to produce a gaseous product of high heating value in a practical continuous reactor is of paramount importance to the gas industry. Several research programs are now under way to develop an efficient and economically feasible process for producing high heating-value gas from coal. One of these, a study at the Institute of Gas Technology, has led to the work reported here. The reaction of coal with a steam-hydrogen mixture represents a system of heterogeneous reactions in which solid and gas phases are present. The study and interpretation of two such phase reactions are inherently more complex than those of homogeneous reactions in which only a single phase is present. For a reaction to proceed between a 253

Schora; Fuel Gasification Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

254

FUEL GASIFICATION

fluid and a solid, a combination of diffusional and kinetic processes with adsorption and surface-reaction steps is considered to occur.


The literature contains a number of papers (2, 4, 13, 14, 16, 20, 22, 24, 25, 26, 28, 31, 32, 34, 35, 36, 37) concerned with the kinetics of these reactions. The carbon studied in these papers ranges from coal to graphite. Because it requires many possible variables, such as temperature, pressure, the nature of chemical reaction, and the character of the solid surface, and because it incorporates many constants which require experimental evaluation, the general mathematical model to estimate the product gas distribution for different levels of carbon conversion can become exceedingly complicated. Practical application of this model is particularly difficult when a choice has to be made between reaction mechanisms, each of which can generate complex functions with a sufficient number of arbitrary constants to fit any given experimental curve. The purpose of the work discussed in this paper was to study the influence of temperature and the partial pressure of hydrogen and steam on the rate of steam-hydrogen and coal char reactions based on the previous pilot plant data obtained at IGT (10, II) and to develop a correlation to estimate the performance of a hydrogasification reactor in terms of its product gas distribution for different levels of carbon conversion. General Considerations Most coals are made up of a number of macérais. Carbons derived from different macérais differ in reactivity. As gasification proceeds, a decline in rate is expected since carbon of progressively lower reactivity remains. Differences in the chemical reactivity of macérais have been reviewed by Brown (3) and Channabasappa and Linden (5), who found, in increasing order for hydrogénation, that fusain, durain, and vitrain have different reactivities. In their study of coal pyrolysis, Chermin and van Krevelen (6) showed that upon heating, coal first becomes metaplastic and then gives off volatile matter, leaving a rather stable coke. Thus, coal char may be considered to consist of two portions which differ greatly in reactivity. The highly reactive portion is related to the amount of volatile matter, characterized by the aliphatic hydrocarbon side chain, and to the oxygenated functional groups present. The low reactivity portion is the residual, carbonaceous, stable coke. Thus, the gasification of coal by simultaneous reactions with steam and hydrogen at elevated temperatures is divided into first- and second-phase reactions, each reaction representing one of the two distinctly different reactivities of carbon present in coal.


16.

255

Reaction of Coal Char

WEN E T AL.

First-Phase Reaction C* + 2 H - > C H 2

C* + H 0 - > C O + H 2


(1)

4

(2)

2

where C * is the reactive carbon present in the first phase. Of course, the actual reactions are much more complex than indi cated by the above simple schemes. It has been demonstrated (10) that the gaseous products from the first-phase reactions are predominantly methane, carbon monoxide, and hydrogen. The experimental data of the same investigations show that the hydrogen and steam reactions in the first phase occur independently of each other and probably undergo pyrolysis followed by the vapor-phase reaction of hydrogen and steam. The rates of the two first-phase reactions, therefore, are assumed to be additive. Let X and X o be the conversions of carbon from hydrogen and steam, respectively; then the over-all conversion of carbon is simply: H 2

H 2

X =

X H

2

+

(3)

XH O 2

Hydrogen-Char Reaction. The hydrogen-char reaction in the first phase may be represented by: "dXu ~\ dS J i ~~ 2

or

[dXnJ

=

(4) -

h(f

X)

,

wherefciis the rate constant obtained in (atm.-hr. )'\ and / is the fraction of carbon that reacts according to the first-phase reaction. P * is the partial pressure of hydrogen in equilibrium with reacting char and meth ane. This mechanism states that the first-phase reaction occurs in such a way that the rate of reaction at any time is proportional to the mass (or volume) of the unreacted portion of the volatile carbon still present in the particle and to the effective partial pressure of hydrogen. E Q U I L I B R I U M . The coal char-hydrogen reaction has been shown to exceed the carbon-hydrogen-methane equilibrium at low conversion and to reach the carbon-hydrogen equilibrium at nearly complete conversion (36,37). From the equilibrium composition of the hydrogen-char system, a pseudo-equilibrium constant, K , is defined as: H 2

p

Ρ

*

which has been calculated as a function of the carbon conversion level. Figure 1 indicates the general trend of the K values at 1300°F. with respect to the fraction of carbon converted. The empirical relationship p


256

FUEL

§ -

0.8

oc Β ω -J

σ ο.

0.6

OlS

0.4

S§

0 2

Lu ^


GASIFICATION

α. ο

ο• 0

• 0.1

• 0.2

• • • 0.3 0.4 0.5 0.6 0.7 0.8 Χ, CONVERSION, FRACTION 1

1

1

1

1

0.9

1.0

Figure 1. Approximate trend of the equilibrium constant as a function of conversion for the hydrogen-char reaction at 1300°F. and 2000 p.s.i.g. total pressure developed to convert the equilibrium constant from one temperature to another is: Ί8,400\ (Kp) 1300 F. (7) 34,713 exp , Τ ) where Τ is in degrees Rankine. Figures 2, 3, and 4 compare the proposed rate equation with the experimental results obtained from a semiflow system (10) at three temperature levels. Although the first- and second-phase reactions must be considered to occur simultaneously throughout the gasification, a sharp delineation exists between the two phases. It has been observed that the rate falls by a factor of 10 or more in a short period of gasification and thereafter remains relatively constant. Therefore, the experimental measurements of the rate in semiflow tests (10) during the short initial period may be considered to correspond roughly to that of the first-phase reaction pro posed. Thus, the data points at the conversion level above 0.25 shown in Figure 3 represent the portion of the second-phase reaction period. E F F E C T O F H Y D R O G E N P A R T I A L P R E S S U R E . Apart from the equilibrium hindrance, the gasification rate is affected by the partial pressure of hydro gen. The rates of reaction of coal with hydrogen in the first phase are shown by Figures 2,3, and 4 to be roughly proportional to the partial pres sure of hydrogen. A similar type of reaction mechanism was proposed for hydrogen-char reactions (36, 37). This has been shown to agree with the majority of previous investigators, who found that the reaction is approxi mately first-order with hydrogen. Goring et al. (16) showed that the methane-formation rates with pure hydrogen and hydrogen-steam mix tures at the same partial pressure as hydrogen are a strong function of the (Κρ)

Γ

=



16.

257


W E N E T AL.

partial pressure of hydrogen. Wen and Huebler (36, 37) demonstrated that the rate of methane formation is roughly proportional to the différence between the partial pressure of hydrogen and the hydrogen partial pressure in equilibrium with the char. Von Fredersdorff (33) observed that an increase in hydrogen partial pressure would increase the rate of hydrogenolysis. This relationship is further confirmed by Moseley and Paterson (29) who reported that the rate of hydrogénation of coal to methane is directly proportional to the hydrogen partial pressure, even in the early stages of the reaction. 1.0 0.8

pi VJ

0.6

NOMINAL TEMP, I600°F Run Steam, Hydrogen,

Symbol N0.ffc2.atm % Ο

1

69 0 k, (f-X)

0.4

l-X

S n

-1

k^Otatm-roV f

0.2

%

100

8 0 2 4

CM

0?

0.1 0.08

0

0.06 0.04

0 Ο

tf-r

0.02 0

w

πI

0.01 0.008 0.006 0.004

• • 1 0

0.1

0.2

0.3

X, OVERALL CONVERSION (CONVERSION OF CARBON FED)

Figure 2. Rate of hydrogen-char reaction during the first-phase reaction at 1600°F. E F F E C T O F T E M P E R A T U R E . Figures 2, 3, and 4 show that the fraction of the carbon that reacts according to the first phase, /, increases with temperature. Moseley and Paterson (29) pointed out that the fraction of carbon that reacts according to the first phase is a function of both tem perature and pressure. Since the total pressures used in the present investigation were nearly constant at approximately 70 atm., total pres sure had no significant effect on the value of /. Previous investigators


258

FUEL GASIFICATION


(11) observed that an increase in pressure broadens the range of the initial high rate period. However, for a small range of pressures, the fraction of carbon that goes into the first-phase reaction can be assumed to be a function of only the temperature. The values of / for various temperatures are shown as follows: Temperature, °F. 1300 1500 1700

/ 0.22 0.23 0.25

Above 1800°F., the rate of the hydrogen reaction is not substantially affected by the temperature, as shown in Figure 4. This is probably because at these high temperatures (1800°-2050°F.), the chemical reaction is so rapid that the rate becomes controlled by diffusion. In

ο ο φ w Ο .ο M

0.01

I

0

I 0.1

I 0.2

I 0.3

I 1 0.4

.

0.5

1

0.6

Χ, OVERALL CONVERSION (CONVERSION OF CARBON FED)

Figure 3.

Rate of hydrogen-char reaction dunng the firstphase reaction at 1700°F.

addition, at such high temperature, the reaction rate for the remainder of the relatively stable carbon is no longer small enough to be neglected. Thus differentiation of the first-phase reaction from the second-phase reaction becomes so difficult that an accurate value of / cannot be obtained.


16.

259


W E N E T AL.

Steam-Char Reaction. The rate of the steam-char reaction in the first phase is represented by:

[^],- 'm

x )

8

and [dX o] j

q

2

ι - x

=

K

)

where k is the rate constant in hr." , and / is the fraction of carbon that will react according to the first-phase reaction. As in the hydrogen-char 1

2

Nominal

Run

Steam, Hydrogen,

TempfF Symbol No. P 2 > Π 14,19 69 1800 # 18 34.5 Δ 8 69 1900 A 20 34.5 • 7,30 69 • 26 51.75 2000 0 19 34.5 a t m

H

χ ο CO

Ο *D Φ Ο

σ

Φ

w

CM

(hr)


(l-x)de

/χ

k (f — X)

H2

0.8 0.6 0.4

*σ Φ

χ> 0.2

Ο

-Ο

•

I T •

k

100 50 100 50 100 75 50

Λ

LA

Δ •

\ 0

1

A

CM d? 0.1 Χ Χ CD 0.08 Χ ι

β / β

Δ

A

ΙΛ

V•

° 0 50 0 50 0 25 50

β /

ο

Δ

Δ

•

0.06

\

0.04

•

0.02

• 0.01

>

0.2

0.4

0.6

0.8

1.0

X, OVERALL CONVERSION (CONVERSION OF CARBON FED)

Figure 4.

Rate of hydrogen-char reaction during the firstphase reaction above 1800° F.



260

FUEL

GASIFICATION

reaction, the first-phase reaction of steam-char is a volume reaction, whose rate is proportional to the amount of the unreacted portion of the volatile carbon still present in the particle. E F F E C T O F P A R T I A L PRESSURE O F S T E A M . Contradictory conclusions have been reported in the literature regarding the effect of the partial pressure of steam. The reaction has been variously reported to be zero order (31), fractional order, first order (12, 15, 17,18, 21), and between first and second order (21,31) with respect to steam. As shown in Figures 5, 6, 7, and 8, the rate of the steam-char reaction is not affected by the partial pressure of steam, so far as detectable by the experiment. This agrees with the previous investigators (13, 26, 27, 30). 10.0 Ν : * 8.0

\

6.0 θ

NOMINAL TEMP, Ι 7 0 0 Τ Run Steam, Hydrogen, Symbol No. H o,atm % % Ο 42 69 100 0

m

° ο Υ

D r

2

Δ

•

25

•

k 2

75

f

* 40 hr' «0.25

1

i

•Ξ 1.0 \ ê0.8

ΧΧ χ

25

2

2.0

ο

17.25 k (f-X) i-x

1

0.6

π Ο

I

0.4

0.2

•

ιI

•

0.1 0.08 0.2

0.4

0.6

0.8

1.0

X (OVERALL CONVERSION)

Figure 5. Rate of steam-char reaction during the first-phase reaction at 1700°F. EFFECT OF TEMPERATURE. The fraction of carbon that will react according to the first-phase mechanism for steam also increases with increasing temperature. As shown in Figures 5, 6, 7, and 8, the tempera ture dependence of / and h> can be evaluated by the proposed mechanism. However, these correlations must be treated with caution since the second-phase reaction may become so rapid at temperatures above 1900°F. that the isolation of the first-phase reaction from the secondphase reaction would become difficult; thus, the evaluation of / might no longer be accurate.


16.

W E N E T AL.

261



Second-Phase Reaction The second-phase reaction is heterogeneous and occurs at the surface of the particle. The reaction causes the reacting surface to shrink and to leave an ash layer as the particle moves through the reactor. Unlike the first-phase reaction, which is only slightly affected by temperature, the second-phase reaction is quite sensitive to variations in temperature for tests conducted in a semiflow system (10). Since a high gas flow rate was maintained in semiflow tests, gas diffusion probably does not affect the rate. At temperatures below 1700°F., the first-phase reaction rate is an order or two larger than the second-phase reaction rate, but as the temperature approaches 2000°F., the two rates become comparable. This is, of course, true only when the reaction is controlled by the chemical step.

0.2

0.4 0.6 X(0VERALL CONVERSION)

0.8

Figure 6. Rate of steam-char reaction during the first-phase reaction at 1900°F. Hydrogen-Char Reaction. As discussed previously (36, 37), the reaction under experimental conditions in the pilot-plant, continuous flow reactor may be affected by gas diffusion since the gas flow rates employed were extremely low and there was evidence that considerable gas channeling, predominantly along the vessel wall, occurred in this type of reactor.


262

FUEL GASIFICATION

Under such circumstances, the reaction rate of the second-phase reaction: C + 2H ^CH 2

(10)

4

may be characterized by: (ID

where ( K g ) , the effective mass transfer factor for hydrogen, is defined as (23): H2

(Kg)n =

3fcg

2

(12)

Pc

and can be obtained from the moving-bed data by the relation: (Kg)

H 2

=

(13) avg

401

•

•

•

NOMINAL TEMP, 2 0 0 0 T Run ρ Steam, Hydrogen, Symbol No. ^o, atm % %

20

Ο

43 24 26

Δ

Ο ο JD

1

6

%

4

JO

ο

JQ

ο

69 100 34.5 50 17.25 25 k (f-X)

0 50 75

2

ΔΥ Δ

•

c

— - "S XP


= ^ g ï ! d " /)

Fuel Gasification

Recommend Documents