Coal Conversion Processes: Gasification and Liquefaction

4

Coal Conversion Processes: Gasification and Liquefaction 1.

Introduction

62

2.

Gasification 2.1 Basic Principles 2.2 Commercial Processes 2.3 Development Trends 2.3.1 Improved processes 2.3.2 New processes 2.4 Emission Sources of Gasification Plants

65 65 72 76 76 76 79

3.

Liquefaction 3.1 Direct Liquefaction 3.1.1 Basic principles for and development of direct liquefaction processes 3.1.2 Developments in the Federal Republic of Germany 3.1.3 Developments in Great Britain 3.1.4 Developments in Poland 3.1.5 Developments in the U.S.A. 3.1.6 Developments in the U.S.S.R. 3.1.7 Future development 3.2 Indirect Liquefaction 3.3 Emission Sources of Liquefaction Plants

80 80 81 82 82 83 83 84 85 85 86

Control Aspects 4.1 Gaseous Emissions 4.1.1 In-process sulphur removal and control 4.1.2 Flue gas control 4.2 Liquid Emissions 4.2.1 Phase separation 4.2.2 Neutralization 4.2.3 Removal of dissolved inorganics

89 89 89 91 91 91 92 92

4.

61

62

Environmental

Implications

of Expanded

Coal

4.2.4 Removal of dissolved organics by extraction or adsorption 4.2.5 Removal of organics by biological oxidation

Utilization 92 93

4.3 Solid Emissions

93

5.

References

94

1

Introduction

There are several factors which make the potential for coal processing plants to discharge hazardous materials different from other fossil or nuclear processing facilities. One of these is the nature of the raw material. Coal has a complex heterocyclic structure which must be broken down to produce desirable products. Cleavage of these cyclic linkages, and the chemical form of the resultant compounds, are sometimes difficult to analyse and predict. In addition, raw coal has less homogeneity than gas or oil in terms of physical structure and the distribution of trace elements. Its structure also presents materialshandling problems, with attendant environmental discharges, not associated with the movements of fluids. For example, coal feeds usually require crushing and grinding into small particles which are then often incorporated into oil or water slurries in order to be uniformly introduced into the liquefaction or gasification process. Another factor of significant concern is the size of projected coal processing plants. Commercially feasible facilities typically will reduce in excess of 20 000 tonnes of coal per day to products and wastes (NIOSH, 1978b; Battelle, 1973; Ghassemi, Crawford and Quinlivan, 1979). Because of the large amounts of material being processed, the potential exists for environmental discharge of considerable quantities of hazardous materials. These include hydrogen cyanide, phenols, cresols, carbonyl and hydrogen sulphides, ammonia, mercaptans, thiocyanides, aniline, arsenic, trace metals, and various polycyclic hydrocarbons (Schmidt, Sharkey and Friedel, 1974; Moeller, Roberts and Britz, 1974; Cooke and Graham, 1965; Forney et al, 1974; Sexton, 1960a, 1960b; Ketcham and Norton, 1960). In this regard the environmental implications of in situ conversion, where even the quantities of coal converted are not well defined, are also of concern. If coal is converted to a secondary energy (gasoline), or feedstocks for chemicals, several process routes are possible, and it seems to be an important aim for future developments to find out the best route with a minimum of pollution. This can be exemplified by looking at the use of coal as

Coal Conversion

Processes:

Gasification

63

and Liquefaction

T A B L E 4.1 Energy use for heat: percentage efficiency specific emissions for secondary energy options from

values coal

and

T y p e of s e c o n d a r y e n e r g y C o a l gas Conventional a electricity SNG T o w n gas

C o a l oil

100

100

100

100

Transport and preparation

98

98

98

98

P r i m a r y e n e r g y at point of conversion

95

95

95

95

Secondary energy

36

57-69

48-63

48-57

Energy at c o n s u m e r

33

55-67

46-61

47-56

Useful e n e r g y as heat

33

47-57

39-52

38-45

Primary energy

Emission kg G c a l useful e n e r g y Dust sox NOx CO, a

-1

0 0 0.5-0.8 0 0.04 0.04 0.76 3.8-5.2 Depend s on combustion cond itions 960 760 890 1220

S N G = Substitute natural gas.

heat supply for space heating and process heat, in the domestic or industrial sectors. For providing heat from coal there exist the routes via electricity production, via gasification and via hydrogénation, and of burning the coal gas or the coal oil at the producers' end. To compare these different routes, both efficiency and specific emissions related to the production of useful heat energy have to be determined for the individual routes. Table 4.1 sets out the efficiency values and specific emissions for various applications of hard coal for producing heat. The characteristics of each coal conversion process must be considered in any attempt at hazard evaluation, as so many different processes have been attempted or projected (see reviews by Parker and Dykstra, 1978; Ghassemi, Crawford and Quinlivan, 1978; NIOSH, 1978a). Each needs to be analysed specifically for the types of wastes produced and how these are distributed to water, air and soil. However, some general observations can be made about the nature of the conversion process. Most processes require energy addition at elevated temperatures and pressures in order to break

64

Environmental 40 Γ

3 0

Implications

of Expanded

Coal

· methane

Utilization methanol*

) ethane

• ethanol

U I • gasoline

ο

20

ο L U

"E ο <

«Γ| ! j e t fuel l ll * I diesel fuel II

"Ivs/oocM

ι ο IV

1-0

s^bjbjt urn inous 3

/

lignite

anthracite

0

0-2

04

0-6

Atomic ratio FIGURE 4.1 finished

0-8

10

θ/θ

The atomic compositions of several products involved in coal conversion

raw materials processes

and

down the coal. Figure 4.1 indicates the atomic compositions of various raw materials and finished products. The necessary energy is ordinarily derived from oxidising a portion of the incoming coal. Hydrogen is supplied either through direct or indirect utilization of the hydrogen atoms in water, or through use of a hydrogen donor solvent. Figure 4.2 shows a generalized flow sheet combining a number of processing schemes. The intent of the flow sheet is to emphasize the elements common to most thermochemical conversion processes.

Coal Conversion

Processes:

Gasification

and Liquefaction

65

Each particular process is considerably more complex and requires careful analysis of its products and wastes. In general, reaction conditions for gasification are more severe than those for liquefaction, although the unit operations are similar. The gasification processes will be treated in more detail in section 2 and the liquefaction processes in section 3. At present, only gasification processes have been utilized on a large-scale and are operating commercially. 2

Gasification

Gasification is a well-known technique of coal conversion. The first commercial application of modern gasification techniques was in 1926 with the commissioning of a plant using the Winkler process at Leuna. Since the 1930s coal gasification has been applied world-wide for the production of ammonia and hydrocarbons and also for the generation of medium-Btu gas. During the 1960s and 1970s intensive research work has been devoted to the further development of gasification techniques. In the first place this means improving conventional methods with a view to greater economy. To cope with the eventuality of a shortage of natural gas, there are plans to convert coal directly into substitute natural gas (SNG) to be fed into existing natural gas pipeline networks. An area which appears highly promising, but as yet is in its infancy, is the application of catalysts in coal gasification. Another area of development is the design and operation of combined processes of coal gasification, with subsequent combustion of the coal gas under pressure, for the environmentally acceptable generation of electricity with high thermal efficiencies. Lastly, efforts are being made to reduce the quantities of coal needed for conversion into gas by using process heat from high-temperature nuclear reactors. (A great number of survey articles have been published; von Fredersdorff, Elliott and Lowry, 1963; Jüntgen et al, 1980; von Gratkowski, 1958; Falbe, 1977; Schmidt, 1966; Dolch, 1936; Schilling, Bonn and Krauss, 1979; VGB Konferenz, 1979; Elgin, 1976; Robson, 1977; Jüntgen, 1977, 1979b; van Heek, 1978; Peters and Schilling, 1977; Schulze, 1974, 1978). 2.1

Basic

Principles

Coal gasification means the complete conversion of coal into gas using heterogeneous gas solid reactions. The main process is the reaction of the carbon of coal with steam, at pressures below 100 bar

66

air,H20

(

- low BTU gas, 100-250BTu/scf

gascleaning

ash

tar^ H2S

Winkler, WellnnanGalusha )

Og.HgO

methanation

i gasification 1-35 atm

MVG 250-550 BTu/scf gas cleaning & treatment (tars)

ash

H20

I

*

F i scherTropsch synthesis

H2S

(Winkler. HTW. Lurgi, KopperTotzek, ShellKoppers, TexaC O)

methanol synthesis

r ~—recycle gas — j

Ã

catalytical gasification 35 atm

gas cleaning & treatment

ash ^

H2S _

(Exxon catalytical gasification) (a)

F I G U R E 4.2 conversion

Generalized

processes

flow

(Considine,

diagram 1977;

Rndle and

combining

Hoffman,

Vvas.

1975)

a number

1978;

Wadden,

of

coal 1976;

^

mcthonol

Environmental Implications of Expanded Coal Utilization

1 gasification 1 atm

— SNG, fuel oil (Coalcon)

separation — H2S, NH3, ash

pyrolysis 1 atm

char

H2

hydrogιnation 150 atm

extraction 10 atm

char

H2

hydrocracking 200 atm

a- svncrude syncruae

)

(

c

q

e

— H2S

». fuel oil1 e °

. __ (NCB processes)

coal H2

catalytical hydrogιnation 100-300atm

svncrude fuel oil

m

separation

^ash H^S ' r

I H2

solid fuel

non-catalytical hydrogιnation 70 atm

separation

I H2

º

(Η-coal, Synthoil, Saarbergwerke, VEBA-Oel)

f u le

m

non-catalytical hydrogιnation 50atm

solvent hydrogιnation 100 atm

ash, H2 m m H2S.NH3 rec ycle solvent——— J

ol i

—ash, H2S, char

(SRCI, SRC I )

fuel oil (EDS)

d

Coal Conversion Processes: Gasification and Liquefaction

flash hydropyrolysis 20-170atm

(b)

continued

67

F I G U R .E 4.2

68

Environmental

Implications

of Expanded

Coal

Utilization

and temperatures above 750°C, to form a "synthesis gas" containing mainly CO and H 2 with smaller amounts of C 0 2 and C H 4 depending on process conditions. To perform this strongly endothermal reaction in technically feasible systems the introduction of process heat is necessary, which is possible by transfer of heat from outside (allothermal processes) or by burning part of the coal inside the reactor using oxygen (autothermic processes). This latter process is the basis for the production of hydrogen, ammonia, methanol, hydrocarbons using the Fischer-Tropsch synthesis and methane using the so-called catalytic methanation reaction. Another method for converting coal into a gas is the direct exothermal conversion of coal with hydrogen into methane at a similar range of pressure and temperatures as mentioned above. This reaction can also be combined with the steam carbon reaction and in this way leads to a direct conversion of coal into SNG. Coal contains not only carbon but also volatile matter, which is released during the heating up of coal in the gasification reactor by the so-called pyrolysis reactions. The interaction of pyrolysis and gasification of coal is mainly dependent on grain size and rate of heating of the feed coal. At low rates of heating the pyrolysis products are released first and the remaining char reacts with the gasification agent later on at high temperatures, yielding gaseous products from the carbon. In this case, if a countercurrent of gas and solids in the gasification reactor is induced, the product gas contains pyrolysis products such as tar, phenols and higher hydrocarbons. At high rates of heating, the pyrolysis reaction is shifted to higher temperatures and in this way both - pyrolysis products at the outer grain shell and carbon - react with the gasifying agent to form the gasification products H 2 , CO, C 0 2 , C H 4 and less or no pyrolysis products are found in the product gas. The nature of the reaction products of coal gasification is primarily determined by the chemical equilibria which are established, especially at high temperatures, and sufficient residence times of coal and products in the gasification reactor. Thus a high pressure and a low temperature increase the methane yield and decrease the hydrogen and the carbon monoxide yield. Much information exists on this topic (Hedden, 1962; Hedden and Mienkina, 1965; Peters, 1976; van Heek, Jüntgen and Peters, 1973; Feistel, van Heek and Jüntgen, 1978; Yoon, Wei and Denn, 1978; Jüntgen and van Heek, 1979a; Jüntgen, 1979a).

Coal Conversion

Processes:

T A B L E 4.2

Typical

Gasification commercial

69

and Liquefaction

gasification

processes

O p e r a t i n g principle

T y p i c a l commercial process

M o v i n g bed

F l u i d i z e d bed

Entrained bed

L u r g i pressure gasification

Winkler process

Koppers-Totzek process

up to 35 bar Dry pyrolysis 400°C Τ Gasification 800-1000°C Τ Combustion 1000°C

Ρ = 1 bar

Ρ = 1 bar

Τ = 1000°C

Τ -

Coal type

Lignite Medium-caking bituminous

Lignite Low-caking b i t u m i n o u s coal

Lignite Bituminous

Particle size of coal

Relatively large; o p t i m u m size range 6-40 mm

Large to m e d i u m ; Fine crushing o p t i m u m size required; size range 80-90% 0-8 m m <0.1 m m

Ash removal

Dry

Dry

Reaction conditions

Ρ Τ

1800°C

Slag

Steam consumption Oxygen

consumption

Gas composition CH4 CO Tars, phenols Gas production per gasifier

Application

3

1

46 000 N m h " raw gas 3 _1 30 000 N m h purified gas (Sasol II) T o w n gas production

Synthesis gas for F - T s y n thesis + C H 3 O H synthesis

17 000 N m 20 000 N m

3

3

Synthesis gas for chemical industry

h~ h

-

20 000 N m (2 burners) 50 000 N m (4 burners)

3

3

Synthesis gas for N H 3 production

fT

fT

1

1

70

Dry ash

Lurgi commercia

:/ installations and

using the West German Quinlivan, 1979)

Lurgi

technology1

(Ghassemi,

Crazν ford

Gasifier ID ( m )

Capacity (Nm3 d a y -1 x 1 0 6)

N o . of gasificrs

Location

Year

T y p e of coal

1

Bohlen, Central G e r m a n y

1940

Lignite

2.6

0.25

5

2

Bohlen, Central G e r m a n y

1943

Lignite

2.6

0.28

5

3

Most, C . S . S . R .

1944

Lignite

2.6

0.21

3

4

Z a l u z i - M o s t , C . S . S . R .,

1949

Lignite

2.6

0.25

3

5

Sasolburg, South Africa

1954

S u b - b i t u m i n o u s w i t h 3 0 % ash and m o r e

3.7

4.20

9

Dorsten, West Germany

1955

Caking sub-bituminous with high chlorine content

2.7

1.54

6

0.62

6

Plant N o .

6 7

Morwell, Australia

1955

Lignite

2.7

8

Daud Khel, Pakistan

1957

H i g h volatile coal w i t h high s u l p h u r content

2.7

0.14

2

9

Sasolburg, South Africa

1958

S u b - b i t u m i n o u s w i t h 3 0 % ash and m o r e

3.7

0.53

1

Environmental Implications of Expanded Coal Utilization

T A B L E 4.3

1960

Westfield, Great Britain

11

Jealgora, India

12

Westfield, Great Britain

1962

Coleshill, Great Britain

14 15

13

W e a k l y caking subbituminous

2.7

0.78

3

not available

0.03

1

W e a k l y caking subbituminous

2.7

0.25

1

1963

Caking sub-bituminous with high chlorine content

2.7

1.30

5

N a j u , Korea

1966

Sub-bituminous with 3 0 %

3.7

2.10

3

Sasolburg, South Africa

1966

S u b - b i t u m i n o u s w i t h 3 0 % ash and m o r e

3.7

2.10

3

3.4

1400 x 1 0 6 Btu h r _ 1

5

5.32

3

Different grades

16

Luenen, F.R.G.

1970

Sub-bituminous

17

Sasolburg, South Africa

1973

S u b - b i t u m i n o u s w i t h 3 0 % ash and m o r e

3.7

Sasolburg, South Africa

1978

S u b - b i t u m i n o u s w i t h 3 0 % ash and m o r e

4.0

18 a

42

36

A parallel d e v e l o p m e n t of the L u r g i process has been carried out b y the G e r m a n D e m o c r a t i c R e p u b l i c since the Second W o r l d W a r and several c o m m e r c i a l plants using the East G e r m a n t e c h n o l o g y c u r r e n t l y exist in Eastern E u r o p e a n c o u n t r i e s . E x a m p l e s i n c l u d e the G a s k o m b i n a n t S c h w a r z e P u m p e , East G e r m a n y w i t h 24 gasifiers (3.6 m I D )

Coal Conversion Processes: Gasification and Liquefaction

10

71

72 2.2

Environmental Commercial

Implications

of Expanded

Coal

Utilization

Processes

Table 4.2 outlines the gasification processes most commonly used and Tables 4.3 and 4.4 list some of the full-scale gasification plants that have been built since 1940. The moving-bed type reactor is used in the Lurgi pressure gasification process - the only process which has so far been carried out using high pressure - the fluidized-bed principle is used in the Winkler process and the entrained-bed type reactor in the Koppers-Totzek process. These processes are classified according to the conditions of the reaction, which leads to very different temperatures in the gasifiers. As a result the throughput rate per unit volume of the reactor increases with temperature but does not allow any assessment of the processes. In the moving bed, changes in temperature and pressure have a considerable influence on the development of the reaction. In the Winkler gasifier the influence of pressure and temperature is less marked. In the Koppers-Totzek gasifier, increases in temperature have hardly any influence. The Lurgi pressure gasification method has proved exceptionally versatile in its applications to date. In particular, large quantities of synthesis gas are produced world-wide for Fischer-Tropsch synthesis. A contract has also been concluded for the production of synthesis gas for methanol. There are also plants producing gas of town-gas quality. The Winkler method is generally used for producing synthesis gas in the chemical industries, and is operated in the CMEA countries predominantly on lignite. The Koppers-Totzek process is used all over the world with widely differing types of coal wherever gas has to be produced for the synthesis of ammonia. The modern plants are geared to an ammonia production capacity of 1000 - 1 tonnes d a y . A number of articles have been published on special gasification processes and are itemized appropriately here (Moving bed: Hiller, 1975; Hoogendorn, 1976; Röbke, 1978; Flmdized bed: Flesch and Veiling, 1962; Davy Powergas, 1973; Banichek, 1973; Franke, 1978; Anwer and Bögner, 1976; Entrained bed: Göke, 1979; Linke and Vogt, 1979; Völkel et ai, 1979; Franzen, 1977; Staege, 1976; Seipenbusch and Ruprecht, 1978; Kraaijveld, 1978; Rossbach et al., 1978; Use of nuclear reactor: Arndt et al, 1979; Jüntgen and van Heek, 1977, 1979b).

Gasification

plants using the Koppers -Totzek

process (Gh,assemi, Crawford

and Q uinlivan,

1978)

N u m b e r of gasifier units

Capacity (CO + H2 in 24 hr)

U s e of synthesis gas

Year of order

Fuel

C h a r b o n n a g e s de France, Paris, Mazingarbe Works (P.d.C.) France

C o a l dust, coke oven gas, tail gas

1

75 0 0 0 150 000 N m 3 2 790 0 0 0 5 580 000 S C F

Methanol and ammonia synthesis

1949

T y p p i O y , O u l u , Finland

C o a l dust, oil, peat

3

140 000 N m 3 5 210 000 S C F

Ammonia synthesis

1950

N i h o n Suiso Kaisha, L t d , Tokyo, Japan

C o a l dust

3

210 000 N m 3 7 820 000 S C F

Ammonia synthesis

1954

Empresa N a c i o n a l " C a l v o S o t e l o " de Combustible Liquidos y Lubricantes, S.A., M a d r i d , nitrogen w o r k s in Puentes de Garcia R o d r i q u e z , C o r u n n a , Spain

L i g n i t e dust

3

242 000 N m 3

Ammonia synthesis

1954

T y p p i O y , O u l u , Finland

C o a l dust, oil, peat

2

140 000 N m 3 5 210 000 S C F

Ammonia synthesis

1955

S.A. U n i o n C h i m i q u e Belge, Brussels, Zandvoorde W o r k s , B e l g i u m

Bunker-C-oil plant convertible: for coal dust gasification

2

176 000 N m 3 6 550 000 S C F

Ammonia synthesis

1955

A m o n i a c o Portugues S . A . R . L . , Lisbon, Estarreja Plant, Portugal

H e a v y gasoline, plant extendable to lig nite and anthracite dust

2

169 000 N m 3 6 300 000 S C F

Ammonia synthesis

1956

The G o v e r n m e n t of Greece; The M i n i s t r y of C o o r d i n a t i o n , A t h e n s ; nitrogenous fertilizer plant, Ptolemais, Greece

Lignite dust, bunker-C-oil

4

629 000 N m 3 23 450 000 S C F

Ammonia synthesis

1959

73

Location

Coal Conversion Processes: Gasification and Liquefaction

E1ECU - D

T A B L E 4.4

74

Continued N u m b e r of gasifier units

Capacity ( C O 4- FL in 24 hr)

U s e of synthesis gas

Year of order

Location

Fuel

Empresa N a c i o n a l " C a l v o S o t e l o " de Combustibles Liquidos y Lubricantes, S . A . , M a d r i d , nitrogen w o r k s in Puentes de Garcia R o d r i q u e z , C o r u n n a , Spain

Lignite dust or naphtha

1

175 000 N m 3 6 500 000 S C F

Ammonia synthesis

1961

The General O r g a n i z a t i o n for Executing the Five Year Industrial Plan, C a i r o ; nitrogen w o r k s of Sociιtι el N a s r d'Engrais et Chimiques, Attaka, Suez, U . A . R .

Refinery off-gas, L P G , and light naphtha

3

778 000 N m 3 28 950 000 S C F

Ammonia synthesis

1963

C h e m i c a l Fertilizer C o m p a n y Ltd, T h a i l a n d , synthetic fertilizer plant at M a o M o h , L a m p a n g , T h a i l a n d

Lignite dust

1

217 000 N m 3 8 070 000 S C F

Ammonia synthesis

A z o t Sanayii T . A . S . , A n k a r a , Kutahya Works, Turkey

Lignite dust

4

775 000 N m 3 28 850 000 S C F

Ammonia synthesis

Chemieanlagen Export-Import G m b H , Berlin fόr VEB G e r m a n i a , C h e m i e a n l a g e n and A p p a r a t e b a u , K a r l - M a r x - S t a d t , V E B Zietz W o r k s

V a c u u m residue a n d / o r fuel oil

2

360 000 N m 3 13 400 000 S C F

R a w gas to 1966 p r o d u c e h y d r o g e nß for h y d r o g e n generation

Kobe Steel Ltd, Kobe, J a p a n , for Industrial Development C o r p . , Zambia, at Kafue near L u s a k a , Zambia, Africa

C o a l dust

1

214 320 N m 3 7 980 000 S C F

Ammonia synthesis

1966

1967

Environmental Implications of Expanded Coal Utilization

T A B L E 4.4

165 000 N m 3 6 150 000 S C F

Ammonia synthesis

1969

C o a l dust

4 (1 of them as s t a n d b y )

2 000 000 N m 3 74 450 000 S C F

Ammonia synthesis

1969

The Fertilizer C o r p o r a t i o n of India L t d , N e w Delhi, T a l c h e r Plant, India

C o a l dust

4

2 000 000 N m 3 74 450 000 S C F

Ammonia synthesis

1970

N i t r o g e n o u s Fertilizers I n d u s t r y S . A . , A t h e n s , n i t r o g e n o u s fertilizer plant, Ptolemais, Greece

Lignite dust

1

242 000 N m 3 9 009 000 S C F

Ammonia synthesis

1970

The Fertilizer C o r p o r a t i o n of India L t d . , N e w Delhi, Korba Plant, India

C o a l dust

4 (1 of them as s t a n d b y )

2 000 000 N m 3 74 450 000 S C F

Ammonia synthesis

1972

A E & C I Ltd, J o h a n n e s b u r g , Modderfontein Plant, South Africa

C o a l dust

6

2 150 000 N m 3 80 025 000 S C F

Ammonia synthesis

1972

Indeco C h e m i c a l s L t d , L u s a k a , Kafue W o r k s , Zambia

C o a l dust

1

220 800 N m 3 8 220 000 S C F

Ammonia synthesis

1974

Indeco C h e m i c a l s L t d , L u s a k a , Kafue W o r k s , Zambia

C o a l dust

2

441 660 N m 3 16 440 000 S C F

Ammonia synthesis

1975

L i g n i t e dust

The Fertilizer C o r p o r a t i o n of India Ltd, N e w Delhi, R a m a g u n d a m Plant, India

(1 of them as s t a n d b y )

S C F = Standard cubic feet N m 3 = N o r m a l cubic metre (0°C anLd 1 a t m )

Coal Conversion Processes: Gasification and Liquefaction

1

N i t r o g e n o u s Fertilizers I n d u s t r y S . A . , A t h e n s , n i t r o g e n o u s fertilizer plant, P t o l e m a i s , Greece

75

76 2.3

Environmental Development

Implications

of Expanded

Coal

Utilization

Trends

2.3.1

Improved processes

Although the gasification processes described are being applied successfully world-wide for the industrial production of gas from coal, they still need to be improved to make them more economic. There is also a need to extend the scope of fluidized-bed technology to include different coal types. Individual research projects in this field are in progress, notably in the U.S.A., the U.K., and the F.R.G., these are listed in Table 4.5. They are categorized according to the different measures introduced to improve techniques. 2.3.2

New processes

Use of

catalysts

If it proved possible to generalize the use of catalysts, considerable advantages could be expected for the implementation of the gasification processes. For example, increasing the rate of reaction would lead to a bigger unit throughput per reactor and the lowering of the gasification temperature would result in energy savings, fewer materials problems and higher methane concentrations in the product gas. Moreover, the heat from the hydrogasification could be used directly for endothermic steam gasification, obviating the need to use oxygen for heat generation in autothermic gasification processes. At present, however, there are still problems with their application on a commercial scale since this requires the use of large quantities of catalyst, and this means extra cost. Also, most catalysts cause appreciable corrosion and are extremely difficult to recover from the coal ash. Use of nuclear

reactor

In autothermic coal gasification the necessary reaction heat has to be generated by a partial combustion of coal with oxygen, which limits the maximum gasification efficiency to between 60 and 70%. A reduction in the specific coal consumption can only be achieved by providing the necessary process heat from other energy sources. One possible source is the high-temperature nuclear reactor with a helium outlet temperature of 950°C. This technique offers advantages over

T A B L E 4.5

development

Increased gasification temperature

projects

catagorize'd by the different

Increased pressure

Increased pressure and temperature

measures

to improve

techniques

Increased pressure to 100 bar. C h a n g e in procedure

(t =

tonnes)

Gasification system 2-stage

Anticipated advantages

Increased t h r o u g h p u t and increased steam decomposition

N o need for compression before gas preparation

Increased throughput

Increased C H 4 content. Improvement in gas preparation

U s e of c a k i n g coals

Examples of process development

Lurgi "slagging gasifier"

ShellKoppers SaarbergOtto

HT-Winkler

Lurgi pressure gasification

Synthane Bigas

F l u i d i z e d bed

G r a v i t a t i n g bed

Combination of fluidized and e n t r a i n e d bed

Texaco Techniques applied

Status

Gravitating bed

Pulverized coal b u r n e r s

L i q u i d slag removal

Burners with a coal suspension feed

Operational (bituminous coal)

Commissioned 6 t hr"1 Underconstr. 11 t h r - 1

Commissioned 1 thr"1 ( D r y lignite)

Under construction 7 t h r -1

Operational 3.1 t h r - 1 Operational 10 t h r " 1 (bituminous coal)

77

Commissioned 6 t hr"1 (bituminous coal)

Separate gas removal

Coal Conversion Processes: Gasification and Liquefaction

Measures

Industrial

78

Environmental

Implications

of Expanded

Coal

Utilization

STEAM

AIR S T E A M

F I G U R E 4.3

Simplified

flow diagram of combined electricity generation

gasification

and

autothermic coal gasification processes, including the generation of more gas per unit of coal, reduction in size of the gas preparation plants, together with greater economy of the process when more expensive European coals are used. Use of combined

cycles

Since tighter environmental legislation makes the conventional coal-based processes of electricity generation more expensive, investigations have been made to establish whether a non-polluting electricity generation process can be developed based on a combination of gasification and combustion of the coal gas. The following advantages were identified: the use of an additional gas turbine means that this process is likely to be more efficient than conventional methods of electricity generation; the dust and sulphur are removed from a relatively smaller quantity of gas before combustion, in contrast to the conventional processes where flue gas desulphurization takes place downstream of combustion; there is potential for using proven methods of H 2 S and dust removal in cases where the process gas is cooled before combustion; the process is generally less polluting in terms of dust, sulphur and NO* emissions than conventional processes of electricity generation.

Coal Conversion

Processes:

Gasification

and Liquefaction

79

A flow diagram of the type of pressure gasification process used by Steag is shown in Figure 4.3. 2.4

Emission Sources of Gasification

Plants

Dust emissions may be generated during stocking, handling, crushing, grinding and screening of the raw coal, and handling or stocking the ash produced in the plants. These problems are well-defined in all coal conversion plants and not specific for gasification plants. There are accepted measures for keeping pollution below the statutory limits. Dust-generating equipment has to be housed in dust-tight casings with suction facilities for dust-laden air. This dust-laden air is then treated in special separation units comprising cyclones, bag-type filters, electrostatic precipitators or wet dust separation systems. Also prevention measures against emission risks during stocking are possible, such as spraying with water or chemicals, partial or total tree planting or the provision of shelter. Differences in the pollution control of different coal gasification processes can arise from the different grain size of coal needed in the gasifier. Both secondary energy conversion and heat production emit dust, S 0 2 , N O x and C Ô 2 . As far as S 0 2 is concerned, emissions are more serious in coal-based electricity generating, despite stack gas desulphurization, than is the case with the use of either coal oil or coal gas for heating purposes. Coal gas meets particularly well the demands of pollution control in regard to sulphur emissions, requiring that the sulphur (as converted into H 2 S ) has to be removed virtually completely. This is standard practice. N O x emissions cannot be fully assessed yet as they are governed more by the firing conditions than by the type of fuel. When conventional energy sources are used to produce heat, C 0 2 is inevitably emitted and the C 0 2 emissions are inversely proportional to the cumulative efficiency pattern. Emissions are thus at their lowest with gasification, but reach their peak when coal is used to generate electricity. Other sources of emission in gasification plants are off-gas streams of different stages which depend on the special gasification process and on the design of special gas purification systems used. They are exemplified in Chapter 8 by discussion of a Lurgi plant. Finally, the properties of the residual coal ash depend on the gas atmosphere and on the maximum temperature in the gasifier. Therefore, questions of the distribution of trace elements and

80

Environmental

Implications

of Expanded

Coal

Utilization

disposal of gasifier ash can only be discussed considering both the raw coal used and the temperature-time history of the solids moving through the gaseous atmosphere of the gasifier. In this context it should be stressed that only part of the trace elements in coal ash is environmentally relevant: that part which is soluble in water under particular environmental conditions. On the other hand, it must be expected that due to the reducing atmosphere in the gasifier, a part of the trace element content is converted into volatile products such as hydrides, carbonyls or sulphides and in this way is entrained in the raw gas. Therefore, there may be enrichment of the dust removed from the raw gas downstream of the gasifier, and in the case of a low-temperature gasification process this will occur with the tar. If the dust removal is by wet scrubbing, trace elements may also be found in the waste water. A great number of articles have been published on the subject of pollution control related to gasification (Magee, Jahnig and Shaw, 1974; Shaw and Magee, 1974; Jahnig, 1975a, 1975b; Hall, 1974; Jüntgen and Klein, 1976; Beckner, 1975; Massey and Dunlop, 1976; Becker, 1979; Rolke, 1979).

3

Liquefaction

There are three major considerations that have made coal liquefaction an attractive proposition: (1) the energy-consuming pattern in many countries is optimized for oil as an energy raw material; the most appropriate way therefore to introduce coal is to convert it to an oil; this implies adding hydrogen to increase the H : C ratio and removing oxygen (Figure 4.1). (2) sulphur and nitrogen in coal yield noxious emissions when coal is burnt; liquefaction by hydrogénation removes sulphur and also much of the nitrogen. (3) liquefaction of coal makes it easy to separate inorganic, often sulphur-rich, fraction (e.g. by filtration). 3.1

Direct

Liquefaction

Solid coal can be converted directly to a liquid by thermo-chemical treatment, at high temperature and high pressure. This route is called direct liquefaction in contrast to indirect liquefaction involving coal gasification to synthesis gas and subsequent synthesis to liquids.

Coal Conversion 3.1.1

Processes:

Gasification

and Liquefaction

81

Basic principles for and development of direct liquefaction processes

Chemistry

Coal is made up of a broad spectrum of molecules with a high molecular weight, these are called clusters and are partially crosslinked. The heteroatoms O, S and Ν are often present in heterocyclic structures. Schematically, the mechanism for liquefaction consists of the following steps: (1) pyrolysis at about 450°C to break the C - C and the C-O bonds between clusters, resulting in formation of gases, liquids and free radical intermediates; (2) free radical recombination to form coke, or if hydrogen is present at a sufficiently high concentration, radical transformation to compounds rich in hydrogen; (3) reaction of the hydrogen to form H 2 S , N H 3 and H 2 0 , which explains the reduction in heteroatom concentration in the product. Both the pyrolytic cracking and the hydrogénation reactions can be catalysed. Sn, Fe, Mo, Co, Zn and Ni compounds have been used as catalysts. The hydrogen consumption is in the range 2 - 1 1 % of the coal weight. The more hydrogen that is added, the lighter is the product achieved. Technology

From a technical point of view, liquefaction generally involves the following procedures: hydrogen production, coal hydrogénation and product refining. The key step is the hydrogénation and the many processes proposed for direct liquefaction differ in the way this is carried out. In the direct liquefaction processes, hydrogénation is always carried out with the coal in a slurry, acting as a heat carrier and solvent for hydrogen and the subsequent products. The choice of a proper solvent for the slurry preparation is crucial to the process. The process usually operates at a high pressure (100-700 atm) when molecular hydrogen is used for the hydrogénation. A more moderate pressure (about 10 atm) is sufficient when a donor solvent is used instead. A typical residence time for the coal in the chemical reactor is 1-2 h. The use of catalysts in the hydrogénation step allows a lower pressure and shorter residence times. El ECU - D*

82

Environmental

Implications

of Expanded

Coal

Utilization

The hydrogen production is an important step from a financial point of view. It accounts for as much as 30% of the net investment in, for example, an Η-coal plant. Unreacted carbon and product gases from the hydrogénation step are used as hydrogen sources. The char is gasified (e.g. in a Texaco generator) and subsequent gas treatment (shift - C 0 2 removal) results in hydrogen production. Product refining always starts with the separation of ash and unreacted coal. Several techniques are used for this: filtration; centrifugation (hydrocycloning); precipitation with an appropriate solvent; distillation. The raw liquid product oil is often further hydrotreated to yield a lighter product suitable for transportation purposes. Recent developments of the direct liquefaction techniques in different countries are presented briefly in sections 3.1.2-3.1.6, giving examples of different projects. 3.1.2

Developments in the Federal Republic of G e r m a n y

Saarbergwerke

AG

process

This is a modified Bergius process. An iron catalyst is used and a recirculated, refined liquid product is used as solvent. The operating pressure is <300 atm and the hydrogen consumption 5-7%. A 6 t -1 6 1 day plant was started at Voelklingen in 1980. A 2 X 10 t y r " plant is planned for start-up in 1987 (Peters, 1979; Strobel, Romey and Koelling, 1979; Würfel and Jorzyk, 1979). VEEA-Oel

AGIRuhrkohle

AG

process

Based on the experiences from a 0.5 t d a y liquefaction unit at - 1 Bergbau Forschung GmbH, Essen, a 200 t d a y unit has been built at Bottrop. Start-up was planned for early in 1981. The process operates at 250-300 atm, uses an iron catalyst and will have a 6 _ 1 hydrogen consumption of 6%. A 6 X 10 t y r plant is planned for 1987 (Friedrich, Romey and Strobel, 1980; Langhoff et al, 1979; Peters, 1979). - 1

3.1.3

Developments in G r e a t Britain

NCB

supercritical

process

A 0.1 t d a y unit for supercritical extraction (e.g. with toluene) at 100-200 atm and 360°C has been operated since 1977. The extract - 1

Coal Conversion

Processes:

Gasification

and Liquefaction

83

is upgraded by hydrocracking after removal of the solvent. A 24 t day" unit is planned. NCB

solvent

extraction

process

A 0.75 t d a y unit for extraction of coal with a process-derived solvent at 10 atm and 400°C has been operated since 1977. The extract - 1 is hydrocracked at 200-300 atm and 470°C. A 24 t d a y unit is planned. The hydrogen consumption is 5%. Details are given by Urquhart, Martin and Whitehead (1979) and Topper (1980). - 1

3.1.4

Developments in Poland

The technology for recovering liquid fuel from coal developed in Poland is based on extraction of coal and hydrogénation of the extract. At the Carbochemistry Institute of the Central Mining - 1 Institute at Tychy-Wyry a 1.2 t d a y unit has been operated since 1977. A nickel/molybdenum catalyst and recycled oil as a donor solvent are used. The operating pressure is 240-250 atm. Research is currently being carried out of pyrolysis using fluidized-bed conditions on pyrolysis by using different kinds of heat carriers. At this Institute tests have been performed to hydrogenate tars (Hulisz et ai, 1979; Ihnatowicz, 1979). 3.1.5

Developments in the U . S . A .

Exxon

Donor

Solvent

A donor solvent process has been developed by the Exxon company at Baytown, Texas. The hydrogénation of the coal is performed by using a donor solvent and hydrogen at 50 atm and 430°C in a plug-flow reactor. No catalyst is employed in the coal hydrogénation step. The donor solvent is regenerated catalytically (nickel/molybdenum) by hydrogénation at 90 atm and 440°C. The - 1 process development has proceeded from a 0.5 t d a y unit, started -1 in 1972, to a 1 t d a y unit started in 1974, and finally a 250 t -1 d a y pilot plant which started up in June 1980 (Batchelor, 1979; Epperly and Tauntan, 1979). H-coal

The Hydrocarbon Research Company, Trenton, New Jersey, has been developing a process based on catalytical (cobalt/molybdenum)

84

Environmental

Implications

of Expanded

Coal

Utilization

hydrogénation of a slurry (mixed raw product and coal), in a fluidized bed at 440°C and 100 atm. The sulphur-resistant catalyst is fluidized by the slurry, and the reactor design makes it possible to change the catalyst continuously. The hydrogen consumption is 5%. - 1 -1 A 3 t d a y unit has been operated since 1969. A 600 t d a y unit was started in May 1980 in Catlettsburg, Kentucky (Batchelor, 1979; NAS, 1977; De Vaux and Dutkiewicz, 1979; Wölk, 1979; Stotler, MacArthur and Comolli, 1980). SRC I and SRC

II

Solvent-refined coal processes have been developed in two steps. SRC I operates at 70 atm and 425°C, with a process-derived solvent. The hydrogen consumption is 3% and the product is mainly a low-sulphur solid fuel. SRC II is a modification of SRC I to make it possible to produce a liquid fuel. The process operates under similar conditions but with a more refined solvent. The hydrogen consumption is 4%. None of the processes requires the addition of a catalyst, but on the other hand the processes operate well only with high-ash bituminous coals with a high content of volatiles. - 1 SRC I unit has been operated since 1973 in A 6 t day -1 Wilsonville, Alabama, and a plant scaled-up to 6000 t d a y was planned at Newman, Kentucky. The SRC II process has been tested -1 in a 30 t d a y unit since 1976 at Fort Lewis, Washington (a converted SRC I unit). This was planned to be scaled-up to 6000 t - 1 d a y . The full-scale unit will be located in Morgantown, West 1 Virginia. The start-up date for the two 6000 t day"" units was set for October 1984 (Urquhart, Martin and Whitehead, 1979; Koenig and Schmalzer, 1979; Moschitto, 1978).

Synth oil

The Synthoil process is based on the use of a fixed-bed catalytic hydrogénation. The U.S. Bureau of Mines started the development in - 1 1970 and in 1978, a 10 t d a y unit was started up. The operation has now been suspended (Nowacki, 1979; Rogers et al, 1978).

3.1.6

Developments in the U.S.S.R.

The Soviet Combustible Fuels Institute has been developing a process for coal liquefaction since 1960. A mixture of high-sulphur

Coal Conversion

Processes:

Gasification

and Liquefaction

85

crude oil, from which the heavy fraction has been removed, is used as solvent. The hydrogénation is performed at 100 atm and 430°C using an iron/molybdenum catalyst. A method for recovery of the molybdenum has also been developed. A further improvement of the technique and a scaled-up revision is planned (Kriczko and Julin, 1973; Kriczko, 1977). 3.1.7

F u t u r e development

The future development of the direct liquefaction technique will probably not lead to much-increased efficiency, but with increasing oil prices and oil shortage the processes will be more attractive financially. Full-scale plants are planned today in the U.S.A., the F.R.G. and South Africa. 3.2

Indirect

Liquefaction

The indirect route from coal to liquid fuels is at present the only one in full-scale operation. The technique involves total gasification of the coal to a raw gas according to the well-proven Lurgi or Koppers-Totzek methods. The raw gas is then refined to a synthesis gas which is catalytically converted to a liquid fuel. The conversion method is either the Fischer-Tropsch synthesis, yielding hydrocarbons, or to a lesser extent methanol synthesis. Methanol can be, in its turn, used as raw material for the Mobil process to give hydrocarbons. The objective is to produce a transportation fuel (or a chemical product). Methanol

Methanol synthesis is based on early work by the German company BASF on the reaction between carbon monoxide and hydrogen. In 1923 it was found that methanol synthesis is selective when a Z n O - C r 2 0 3 catalyst is used. A very high pressure, 200-400 atm, was necessary. During the 1960s ICI developed a Cu-ZnO catalyst that made it possible to reduce the pressure to 50-100 atm. Today the technique of making methanol from a synthesis gas is well developed and at least five processes are commercially available. The Mobil Oil Company has developed a zeolite catalyst that is capable of converting methanol to hydrocarbons with a very high efficiency. This process has been tested in a pilot plant and a full-scale plant is under construction in New Zealand.

86

Environmental

Implications Fischer-Tropsch

of Expanded

Coal

Utilization

process

The Fischer-Tropsch technology today exists in two versions: (1) the Arge process yielding straight-chain hydrocarbons suitable for diesel engines; (2) the Synthol process yielding branched and olefinic hydrocarbons suitable for Otto engines. Future

processes

Much world-wide research is now devoted to finding new catalysts for the conversion of synthesis gas to liquid fuels and other chemical products. French work on catalytic conversion to ethanol, for instance, appears promising. 3.3

Emission Sources of Liquefaction

Plants

In principle, the process of coal liquefaction involves chemical compounds which also occur in other carbochemical or petrochemical processes. It is well known that a number of these substances may be carcinogenic, teratogenic or mutagenic. The concentration of benzo(a)pyrene in coal liquefaction products can - 1 be between 50 and 60 μg g . The concentration in these coal products may be 10-100 times higher than that in cigarette smoke (Budden and Zieger, 1978). Various substances introduced during liquefaction processes (certain catalysts, inhibitors and antioxidants) are also health hazards. Before liquefaction of coal is practised widely on a commercial scale toxicological evaluation needs to be implemented. The emissions from a liquefaction plant may be more severe than from a gasification plant due to the higher complexity in the former. The presentation of a generalized liquefaction process in Figure 4.2 is very simplified and Figure 4.4 gives a more representative picture of the complexity. A liquefaction plant also contains a gasification plant. The continuous emissions to the atmosphere from a liquefaction plant will mainly come from the auxiliary facilities, the sulphur recovery section, the power and steam generation facilities and from the drying unit. Liquid emissions are present in waste-water streams. The effluent is rather similar to that from coke ovens. The effluents from the - 1 Polish 1.2 t d a y liquefaction unit represents 6-7% of the coal by

coal

coal s t o r a ge & preparati

on

hydrocarbon liquid

1

!

t

»

coal

product separation

LIQUEFACT NI O

area hydrogen

sour - gas

s o u r gas

liquid

char

:har

hi/ d r o tree ating •

1 1 1 1

1 1 coal, char, liquid o r p r o d u c t gas

hydrogen production by g a s i f i c a t i o n

i !

oxyçj^n

oxygen plant

acid gas removal

!

sulphur plant

—

hydrogen

( d o t t e d l i n e s indicate s t r e a m s a b s e n t in s o m e p r o c e s s e s )

power & steam generation

Auxiliary

F I G U R E 4.4

Flow

diagram

representing

liquid p r o d u c t s

raw water treatment

waste water treatment

cooling water

Facilities

a generalized

liquefaction

process

(DOE/HEW/EPA,

1978)

Coal Conversion Processes: Gasification and Liquefaction

oxygen 1

87

88

Environmental

Implications

Raw

of Expanded Coal

Utilization

materials

Coal preparation

22%

Extraction

Filtration

Low temperature| H carbonization

4 1 %

Filtrate distillation

Extract hydrogénation

Residue distillation

Product distillation

32%

5%

Products F I G U R E 4.5

Percentage discharge of effluents produced extraction liquefaction process

during

a

coal

weight. The relative magnitude of different sources is shown in Figure 4.5. Analyses for the different streams have been published (Chmielowski and Labuzek, 1979). Very large quantities of solid wastes are generated in a full-scale liquefaction plant. Up to thousands of tonnes per day of ash w^ill be -1 deposited from a 6000 t d a y unit using coal with an ash content of 5-40%. A considerable amount of spent catalyst also has to be taken care of, especially if the process uses a once-through catalyst. The Ç-coal process has a catalyst consumption of 1 t 200 t~ of

Coal Conversion

Processes:

Gasification

and Liquefaction

89

coal, for example, even though catalyst recycling takes place. Solid wastes will also be generated from the raw and waste-water treatment. 4

Control Aspects

Coal conversion techniques do not represent a very large industry 6 l today. However, large plants, converting 8-10 X 10 t y r ~ , are planned and in the U.S.A. it may be that the industry will involve 500 000 people by the year 2000 (DOE/HEW/EPA, 1979). The methods to be used are already developed and, from the environmental and health protection point of view, it is desirable that the emission control aspects are anticipated. To some extent new protection procedures and routines have to be developed and much can be gained from experience in the petrochemical and nuclear industries. From a technical point of view, almost any environmental or health problem arising in the conversion industry can be solved. But a great deal depends on how much the industry can afford to pay, and the regulations and standards governments decide to enforce. There are a number of possible control techniques that can be applied to potential gaseous, liquid and solid emissions. 4.1

Gaseous

Emissions

Gasification and liquefaction of coal is able to reduce or eliminate unwanted emissions like nitrogen and sulphur. Conditions in the thermochemical process are highly reducing and sulphur is converted to substances like H 2 S , COS and mercaptans, and nitrogen to N H 3 . These gases are produced at a high pressure and may be easily removed. This is often necessary also for subsequent methanol or methane synthesis as the sulphur content in methanol synthesis gas has to be less than 0.1 ppm to avoid catalyst inactivation. The following account deals mainly with sulphur but there are effective methods for control of other substances also. For example, activated carbon can be used to remove most hydrocarbon fractions. 4.1.1

In-process sulphur r e m o v a l and control

Sulphur in the raw product gas from a gasification or a hydrogénation step can be removed almost completely in two stages. The first (Table 4.6) brings down the sulphur content to a few ppm and the second (zinc oxide) further reduces the content below 0.1 ppm.

90

Environmental T A B L E 4.6 Processes streams (Cavanaugh,

Implications

of Expanded

Coal

Utilization

for sulphur removal from raw or synthesis gas Corbett and Page, 1977; Gas Processing Handbook, 1979)

Alkazid

MEA

Amisol

MDEA

Benfield

Purisol

DGA

Rectisol

Estasolvan

Selexol

F l u o r solvent

SNPA-DEA

Giammarco-Vetrocoke

Sulfinol

The first step yields a sour gas which has to be cleaned by use of a Claus or a Stretford unit, producing elemental sulphur and a tail gas. The tail gas from the Stretford unit is fairly clean and contains less than 8 ppm H 2 S and less than 75 ppm of COS. The tail gas from the Claus unit, however, needs more treatment and one of the processes shown in Table 4.7 can be used to further reduce the sulphur emission. In total the sulphur leaves the conversion process as elemental sulphur, zinc sulphide and small amounts of H 2 S and S 0 2 (Cavanaugh, Corbett and Page, 1977; Gas Processing Handbook, 1979).

T A B L E 4 . 7 Processes for cleaning tail gas from a Glaus (Cavanaugh, Corbett and Page, 1977; Gas Processing Handbook,

unit 1979)

A m m o n i u m bisulphite/ a m m o n i u m thiosulphite

Gives less than 900 p p m S 0 2 in off-gas

Beavon

Gives less than 10 p p m H 2 S in off-gas

BSR/Selectox

Gives s u l p h u r removal of 9 8 - 9 9 % w h e n used w i t h a C l a u s unit

Clear Air

Gives less than 50 p p m S 0 2 in off-gas

IFP C l a u s p o l 1500

Gives less than 1500 p p m S 0 2 in off-gas

SCOT

Gives less than 500 p p m H 2 S in off-gas

Sulfreen

Gives s u l p h u r removal of 9 9 % w h e n used w i t h a C l a u s unit

W-L S 0

2

recovery

Gives less than 100 p p m S 0 2 in off-gas

Coal Conversion

Processes: 4.1.2

Gasification

and Liquefaction

91

Flue gas control

Flue gases contain sulphur and nitrogen, as oxides, which can be removed by scrubbing with alkaline solutions or by contacting with solid calcium carbonate, dolomite or manganese dioxide, as is practised for power or heat generation from coal or oil (Hoffman, 1978). 4.2

Liquid

Emissions

The primary liquid emissions from coal conversion processes are often heavily contaminated by ammonia, sulphides and other inorganics, as well as by phenolic and various other organic compounds. The chemical oxygen demand (COD) and the biochemical oxygen demand (BOD) are often very high. Stutsman (1976) reports that the effluents from the Synthane gasification process and from the SRC liquefaction process can have BOD values 1 as high as 30 000 mg Γ and COD values up to 30-40 000 mg - 1 l . The heavy metal content is usually relatively low. Waste-water cleaning may have to be done in several stages: separation of particulate and emulsion phases; neutralization; removal of dissolved inorganics; removal of dissolved organics by adsorption or extraction; removal of organics by biochemical oxidation; a last step of ion exchange methods to remove inorganic salts. 4.2.1

Phase separation ( C a v a n a u g h , C o r b e t t and Page, 1977)

Flocculation-Flotation

By this widely used method the BOD and COD may be reduced by 80%, suspended solids by 75% and free oils by 97%. Oil-water

separators

The use of this simple technique, utilizing the difference in density between the oil and the water, can reduce the oil content by 60-99% and the solids by 10-50%. Filtration

When sand is used as the filter medium the reduction may be 36% for BOD, 25-44% for COD, 52-83% for suspended oils and 70-75% for suspended solids.

92

Environmental

Implications

4.2.3

Coal

Utilization

Neutralization

4.2.2

The use of inorganic waste-waters is effective Saturation with C 0 2 has alternative (Chmielowski

of Expanded

acids for neutralization of the alkaline but causes problems with corrosion. been shown to be a more advantageous and Labuzek, 1979).

Removal of dissolved inorganics ( C a v a n a u g h , C o r b e t t and Page, 1977)

Acid gas stripping can be used to remove acid gases like C 0 2 and H 2 S, as well as some other substances, from the water. A removal efficiency of 99% for H 2 S, 90% for N H 3 and 20-90% for phenol is reported. A special version (WWT) can be used to reduce the concentration of N H 3 below 50 ppm and of H 2 S below 10 ppm.

4.2.4

Removal of dissolved organics by extraction or adsorption ( C a v a n a u g h , C o r b e t t and Page, 1977)

Extraction-Phenosolvari

This widely used technology is based on liquid-liquid extraction with butyl acetate, isopropyl ether or a light aromatic oil. The efficiency of removing phenols is high. The following figures are given: removal of monohydric phenols 9 9 . 5 % ; polyhydric phenols 60% ; other organics 15%. The method is used at the SASOL plant in South Africa and an efficiency of 95% for total organics removal is reported. Adsorption

Activated coal or synthetic polymers can be used as adsorbents. Removal efficiency for phenol is 99% and for COD, 8 1 % .

T A B L E 4.8 Removal efficiencies (percentages) of the activated and aerated lagoon biological oxidation processes (Cavanaugh, and Page, 1977) Sulphides

sludge Corbett

BOD

COD

Phenols

Activated sludge

97- -100

88-90

60-85

95-99

Aerated lagoon

95- -100

75-95

60-85

90-99

Coal Conversion

Processes:

Gasification

and Liquefaction

93

1 2 5 0 kg coal 8% ash

I 1 0 0 kg Τ ash 79 7kg

mechanical separation

6 0 k g ash

20 3kgash

19 7 k g ash

electrostatic precipitation w i t h gas conditioning

18 6 kg a s h

0-3kg

1 1 kg ash

Ο 8kgash

F I G U R E 4.6 Measured values for particulate removal systems large coal-fired power plant (Hoffman, 1978)

4.2.5

for a

Removal of organics by biological oxidation

This microbial treatment is sensitive to pH levels in the feed. A proper pretreatment of the effluent is important (EPA, 1975). It should be ensured that the concentration of N H 3 , H 2 S and extractable substances is as low as possible in order to avoid inactivation of the bacteria. A number of processes exist and the removal efficiencies for two of them are given in Table 4.8. A number of substances derived from coal processing (cresols) are, however, characterized by a considerable resistance to the action of micro-organisms. 4.3

Solid

Emissions

The solid wastes from coal conversion processes are derived from coal ash, spent catalyst and solid sludges. Only part of the ash is emitted to the atmosphere when coal is used as fuel for steam or heat generation. Figure 4.6 shows the effectiveness of three different particulate removal systems. The problem is rather to do with the solid waste when it is collected, not so much how to collect it. Spent catalysts can sometimes be recovered because of the high content of these valuable metals. Sixty-five million tonnes of coal ash per year are produced by electric utilities in the U.S.A. (OECD, 1980). This ash is disposed of either in ponds or as landfill. Fly ash can be mixed with cement. This can be, however, only a partial solution to disposal as in the U.S.A., 6 - 1 for example, cement production is 80 Χ 10 t y r and the fly ash 6 production is 45 Χ 10 t. The maximum ash : clinker ratio is 1 : 5.

94 5

Environmental

Implications

of Expanded

Coal

Utilization

References

ANWER, J . and BÖGNER, F. (1976). Kohlevergasung i m Fluidatbett unter D r u c k . Brennstoff-Wärme-Kraft, 28, 57-60. ARNDT,

E.,

FISCHER,

R.,

FRÖHLICH,

W.,

JÜNTGEN,

H.,

TEGGEFS,

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