Higman Chris Gasification (Газификация угля)

Подождите немного. Документ загружается.

Gasification

syngas scrubber (Schellberg and Peña 2002). Some typical properties of animal

biomass are given in Table 4-15.

4.3.2 Black Liquor

Black liquor is an integral part of the papermaking process and represents about half

the mass of the wood entering the pulping plant. It is the principal source of energy

for the pulping process. In addition to the lignin and hemicellulose from the pulp

wood, it also contains the cooking chemicals, which contain significant quantities of

sodium and sulfur, the recovery of which for recycling to the pulp digesters is essen-

tial for the economy of the process.

Table 4-13

Analysis of Typical Biomass

Proximate Analysis

Volatile matter, wt% maf >70

Ash, wt% ar 1.5

Moisture, wt% ar 20

Fixed carbon, wt% ar

Ultimate analysis

C, wt% 54.7

H, wt% 6.0

O, wt% 38.9

N, wt% 0.3

S, v 0.1

Table 4-14

Ash Components in Various Biomasses as Wt% of Total Ash

Straw Miscanthus Wood

CaO 6.5 7.5 37.3

MgO 3.0 2.5 8.5

O 1.3 0.2 3.0

O 23.7 12.8 8.6

Source: Klensch 2001

Feedstocks and Feedstock Characteristics

Total black liquor production worldwide is an energy resource of about 600 TWh/y

or 51 million toe/y. This represents about 0.5% of the total world energy consump-

tion. In a country such as Sweden, with a strong pulping sector, black liquor can be

as much as 6% of the national gross energy consumption. An average-sized mill

produces 1000 air-dried tons of pulp per day, generating some 1800 tDS/d (tons dry

solids per day) of black liquor. On an HHV basis this is about 300 MW.

In a typical pulping process (see Figure 4-6) wood chips are cooked with sodium

hydroxide and NaHS in a pulp digester where lignin and other organic material, which

contributes almost half the mass of the wood, is dissolved into the white liquor

Table 4-15

Properties of Animal Biomass

HHV Moisture Volatiles Ash

MJ/kg wt% wt% wt%

Meat and Bone Meal (SRM) 20.1 2.54 65.27 26.13

Meat and Bone Meal (HRM) 18.9 5.03 63.7 25.54

Meat and Bone Meal (Blood) 23.3 14.96 93.82 1.45

Meat and Bone Meal (MBM) 20.0–28.0

Poultry Litter 13.0–14.0 63 25 20

Source: Schellberg and Peña 2002; Arbon 2002; and Bajohr 2002

WOOD

CHIPS

PULP

DIGESTER

PULP

GAS

(OR STEAM)

GASIFIER

(OR BOILER)

BLACK

LIQUOR

EVAPORATOR

WHITE

LIQUOR

GREEN

LIQUOR

CAUSTICIZING

SMELT

WATER

Figure 4-6.

Figure 4-6.Figure 4-6.

Figure 4-6. Gasification of Black Liquor

Gasification

containing the caustic. The solids content of the weak black liquor leaving the

digester is about 15 wt%. This is increased to about 70 wt% in evaporators to leave a

combustible pumpable material that in traditional processes is completely combusted in

a special (black liquor) recovery or Tomlinson boiler. The inorganic material, mostly

NaS and NaCO

, leaves the boiler as a smelt that is recovered by causticizing for

reuse in the digester.

The use of black liquor gasification is a relatively new development. It is located

in the same position in the liquor circuit as the boiler (see Figure 4-6). In some

applications black liquor gasifiers have been built in parallel with existing boilers as

part of a de-bottlenecking project.

Typically, the strong (evaporated) black liquor has a viscosity of 70–120 cP and a

density of 1400 kg/m

. The heating value is about 13.5–14 MJ/kg (HHV).

A typical elemental analysis of the dry material in black liquor is shown in

Table 4-16.

4.3.3 Biomass Production

Cropping

One of the most important limitations of biomass as a widespread fuel is the space

required to grow it. Bridgwater (2002) estimates that “a sustainable crop of 10 dry

t/ha/y of woody biomass can be produced in Northern Europe rising to perhaps 15 or

maybe 20 t/ha/y in Southern Europe.” Assuming that 1000 dry t/y will generate

150–300 kW depending on conversion efficiency then an area of the order of magni-

tude of 100 km

or more would be required to support a 30–40 MW power station.

Table 4-16

Composition of Black Liquor (Dry)

Element wt%

C 36.40

H 3.50

O 34.30

N 0.14

S 4.80

Cl 0.24

Na 18.60

K 2.02

Total 100.00

Source: Marklund 2001

Feedstocks and Feedstock Characteristics

It is unlikely that all transportation fuels and organic chemicals can in the future be

produced from biomass. This would call for an immense acreage of biomass planta-

tions of a few million square kilometers. This seems unrealistic for various reasons:

• The sheer size of such projects.

• It is far from certain that large-scale plantations of more or less conventional crops

such as Eucalyptus, miscanthus, euphorbia, and other species are so green as the

advocates of these plantations claim it to be. The large-scale biomass production

in Minas Gerais in Brazil for making charcoal to be used in blast furnaces is a

well-known environmental problem. Fertilizers, herbicides, and pesticides will be

required; ash may have to be recycled in order to recycle alkali, phosphorus, and

so on. Irrigation may be a problem, and so may soil erosion. Moreover, monocul-

tures are very sensitive to pests. Last but not least, such developments are often

difficult to reconcile with the wish for more biodiversity.

• Biomass is easy to transport when it comes to gathering firewood for nearby

domestic use. However, it is expensive to transport in large quantities over any

appreciable distance. The energy density of biomass is one-tenth that of liquid

hydrocarbons, and further, it is a solid that cannot be pumped. In order to make

wood and other biomass more amenable to transport, it can be chipped, but then

the energy density will decrease even further. As a result, the energy required to

get bulk biomass to usually distant markets is appreciable and constitutes a size-

able percentage of the fuel market it wants to replace.

Biomass Waste

On the other hand, there are circumstances where large quantities of waste bio-

mass arise as a result of some other economic activity such as forestry, papermak-

ing, and sugar. Under these circumstances most of the costs associated with the

collection of the biomass and its transport to a central location is borne by the

main product. Thus in these areas somewhat larger biomass gasifiers (500–1000 t/d

corresponding 80–160 MW

) can and have been built. However, none of these are

of a capacity to be compared with world-scale plants based on fossil fuel

feedstocks of 200–1000 MW

4.3.4 Development Potential

Bio-Oil

The major drawback of biomass is that the energy density is an order of magnitude

lower than that of crude oil (Table 4-17). When it is further considered that biomass

for fuel is a difficult to handle solid (grain is an exception in that it almost flows like

water), this implies that fuel biomass can never be shipped economically over long

intercontinental distances.

Gasification

Therefore, when contemplating really large biomass production, the first conver-

sion that has to take place is to convert the biomass into a transportable product. The

best way to accomplish this is by flash pyrolysis, which is a fast pyrolysis at a high

temperature of 450–475°C. In this way the solid biomass is converted into a transport-

able liquid with an energy density of about 20 GJ/m

(see Table 4-17), which is similar

to that of methanol. The pyrolysis plants can be located anywhere, because they can be

economically built on a reasonably small scale. A stand-alone gasifier is only likely to

be economic for world-scale plants, and hence they always depend on biomass from

a variety of sources. The energy conversion of flash pyrolysis is now 75% but could

well increase to 80% in the future. The bio-oil product is a transportable material that

has as its only disadvantage that it is corrosive due to the presence of organic acids. A

big advantage is that it can be gasified with oxygen in the same commercial gasifiers

that can process heavy oil fractions. An added advantage of bio-oil is that it is easily

homogenized. Holt and van der Burgt (1997) proposed such a concept of decentralized

flash pyrolysis plants feeding a single large gasification installation. Research on

details of actual implementation is continuing (Henrich, Dinjus, and Meier 2002).

Table 4-18 lists some of the most important properties of bio-oils. Additional

data, in particular of some important organic compounds in bio-oil, is contained in

Henrich, Dijus, and Meier (2002). In terms of its suitability for gasification in a

standard oil gasifier, the most important aspects are the pH (material selection of the

feed train) and the alkaline ash content. This latter may favor the use of a radiation

screen instead of simple refractory lining.

Marine Cropping

A possible solution to large-scale biomass production in an environmentally

acceptable manner could be plantations of salt-water organisms in desert areas,

Table 4-17

Energy Densities of Various Fuels

Fuel Particle Density Bulk Density Energy Density

kg/m

GJ/m

bulk

product

Crude oil 855 35.8

Coal 1350 700 21

Natural gas (80 bar) 57 2.9

Biomass 450 230 3.7

Bio-oil 1200 20

Gasoline 760 35

Methanol 784 19

Feedstocks and Feedstock Characteristics

which are located close to the sea. The advantage of this approach is that problems

associated with fertilizers, irrigation, and so on are virtually absent. Moreover,

aquatic crops are probably easier and safer to harvest.

Such farms should of course be located where there is a lot of sunshine. These are

also locations where solar power is available and hence hydrogen can also be generated

by electrolysis of water. This in turn enables the conversion of the biomass into true

hydrocarbons for transport fuels or organic chemicals to be carried out close to the

biomass production, thus grossly diminishing the transport problems. In addition,

the option of first converting the biomass into bio-oil by flash pyrolysis could have

advantages.

For countries in the Middle East and North Africa, which are now heavily

dependent on crude oil exports for their income, this would suggest that they could

continue to supply the world with hydrocarbon fuels. But other countries, such as

Australia, also offer opportunities for such schemes.

Before considering biomass as a fuel it must be stressed that its primary function

in a renewables-based world is that it is the only source of concentrated carbon. This

aspect is often overlooked. It is important to remember, however, because when

biomass is our only make-up source for concentrated carbon, it must be made possible

to convert biomass into a material that can be an intermediate for all organic chemi-

cals and transport fuels. The solution is then to have a process available that can

convert biomass into synthesis gas efficiently. This can provide an alternative to

ethylene as the main source of organic bulk chemicals as well as of Fischer-Tropsch

products. This provides an opportunity for gasification (van der Burgt 1997).

Table 4-18

Typical Properties of Bio-Oil

Property Typical Values

Moisture content 20–30%

pH 2–3

Density 1200 kg/m

C 56 wt%

H 7 wt%

O 37 wt%

N 0.1 wt%

Ash 0–0.2 wt%

Viscosity (40°C,

20% H

O) 40–100 cP

Particulates < 0.3–1 wt%

Heating value 16–18 MJ/kg

Source: Meier 2002

Gasification

4.4 WASTES

“Waste” as a gasification feedstock covers a wide range of materials, both solid

and liquid. Some materials, which we include in this section, could as easily

have been included under biomass. Other chemical liquid wastes described here

could also have been included in Section 4.2. The selection of materials

described here is therefore to some extent arbitrary, but it has mainly been

guided by self-classification of specific projects and general tradition within the

gasification industry.

One of the most difficult aspects of waste as a feedstock, whether for gasification

or incineration, is its heterogeneous nature. In this connection the interconnected

properties of heating value and moisture content play an important part for both

solid and liquid feeds. For solids, particle size is another key parameter. The pre-

sence of a number of components in the waste such as sulfur, chlorides, or metals

can also vary considerably. The variations can be seasonal or even daily. Even with

well-defined industrial wastes, where several waste streams are fed together to the

gasifier, differing proportions of the individual streams can have an influence on

gasifier performance.

For these reasons feedstock preparation plays an important role in the design of

any waste gasification project. Equally important, this wide variation tends to make

each project unique.

The long-term supply of the feedstock is another aspect to be considered. For this

and other reasons it is often important that the refuse is processed with a support

feedstock such as coal or residual oil. Ideally, the economics of the plant should be

robust even in the absence of the refuse feedstock.

In the European Union there are about 500 million tons per year of waste that is

suitable for thermal treatment. Of this some 6% is classified as hazardous waste,

which includes clinical wastes, some wastes derived from agriculture, and chemical

industry wastes (Schwager and Whiting 2002).

4.4.1 Solid Waste

One of the difficulties with refuse is the variability of its chemical composition. For

a study comparing various refuse incineration and gasification processes, the State

Environmental Office of Nordrhein-Westfalen developed a “standard refuse”

composition, which is given in Table 4-19. The lower heating value was specified as

10 MJ/kg.

Löffler (1998) reports the composition of other types of refuse as specified for a

circulating fluid-bed gasifier in Rüdersdorf.

Care should be exercised, however, when evaluating such data, which is very

locality specific. In particular, local regulations on separation and recycling of

household waste including plastics can have a dramatic effect on the heating

value.

Feedstocks and Feedstock Characteristics

Feedstock Sizing. Typically, solid wastes in the “as-received” state have a highly

irregular and asymmetrical lump size. Both for reasons of transport into and in the

reactor, size reduction to a fine homogeneous material is required. Drying is in most

cases also a requirement to ensure a smooth-flowing pneumatic transport.

There are a number of different approaches to pre-preparation. Most of these

involve mechanical shredding and metals removal using magnetic and electric

devices. Much refuse cannot be ground to a sufficient degree of fineness to

permit slurry or dense phase fluid transport. Thus mechanical transport or dilute

fluidized transport are generally the only possibilities. The latter type of transport

cannot be economically performed under pressure, for then relatively large

amounts of transport gas would be required, which would render the process

uneconomical.

Görz (1998) has proposed an initial pyrolysis of the refuse simply to ensure a

material that can then be ground to a size suitable for an entrained-flow reactor.

Moving bed processes such as BGL have a different requirement from fluid-bed

and entrained-flow reactors. The feedstock is transported mechanically and introduced

into the reactor via lock hoppers (which allows pressure operation), but the reactor

bed can tolerate only a limited amount of fines without excessive pressure drop.

Table 4-19

“Standard Refuse” in Nordrhein-Westfalen, Germany

Composition Mass% Ash Composition kg/1000 kg

C 27.16 SiO

110

H 3.45 Al

O 18.39 CaO 31

N 0.30 Fe 30

S 0.20 Na

O 15.2

Cl 0.50 Fe

Moisture (H

O) 25.00 MgO 4.5

Ash 25.00 Al 4

Total 100.00 K

O 3

Zn 1.5

Pb 1.0

Cu 0.5

Cr 0.2

Ni 0.075

Cd 0.01

Hg 0.005

As 0.005

Source: Berghoff 1998

Gasification

Thus a lump size of 20–70 mm is desirable. Material preparation therefore includes

a pelletizing step in addition to initial shredding, metals removal and drying

(Hirschfelder, Buttker, and Steiner 1997).

Heating Value. As indicated in Tables 4-19 and 4-20, the heating value of waste

material can vary considerably. The drying required to facilitate pneumatic transport

will in many cases be sufficient to ensure satisfactory operation of the gasification

process. One should recognize, however, that wastes with excessive inert material

cannot be gasified without a support fuel. The lower limit is 7–8MJ/kg (Görz 1998).

4.4.2 Liquid Wastes

Organic Chemical Waste. Organic wastes from chemical production vary as widely

as the processes from which they originate. One published example is the feedstock

to a waste gasification plant at Seal Sands (United Kingdom), which processes

streams from caprolactam and acrylonitrile production. These waste streams include

mixtures of nitriles and amines as well as cyanide and ammonia compounds. The

differences in the characteristics of these streams, the flow rates of which can vary

independently from one another, can be observed in Table 4-21 (Schingnitz et al.

2000).

Other Liquid Wastes. Liquid wastes such as spent transformer oil can best be gasified

in oil gasifiers together with heavy residual oils. The advantage of this approach is

Table 4-20

Various Feeds for a CFB Refuse Gasifier

Lignite

Char Coal

Old

Wood Refuse Rubber

C, wt% 9.05 78.00 43.40 40.00 64.40

H, wt% 0.55 4.61 4.77 5.69 10.62

O, wt% 1.56 4.12 30.88 29.25 5.63

N, wt% 0.13 1.14 0.23 0.79 0.76

S, wt% 0.54 0.45 0.10 0.22 1.50

Cl , wt% 0.00 0.08 0.02 0.75 0.79

Ash, wt% 78.17 9.60 1.50 17.30 14.10

Moisture, wt% 10.00 2.00 19.10 6.00 2.20

Total , wt% 100.00 100.00 100.00 100.00 100.00

LHV, MJ/kg 3.3 31.4 15.4 15.9 34

Source: Löffler 1998

Feedstocks and Feedstock Characteristics

that use can be made of efficient large installations and that the concentrations of,

for example, chlorine remain moderate and acceptable in terms of corrosion.

Many attempts have been and are being made to dissolve polymers such as plastics

and rubber in oil and gasify them as a liquid rather than as a solid (e.g., Leuna-Werke

1995; Curran and Tyree 1998). The attraction of gasifying plastics as a liquid with

oxygen is that a synthesis gas quality gas is produced, which can either be used as a

fuel gas in a combined cycle or as a synthesis gas for methanol production, for

example. The main problems are technically to make the liquid sufficiently homoge-

neous that it can be gasified using residual oil burners and commercially to ensure a

long-term supply. Recycled plastic is generally too contaminated to allow a stand-alone

project possible. Offcut waste from manufacturing processes, however, does offer

both uncontaminated material and a better prospect for security of supply.

REFERENCES

Arbon, I. M. “Worldwide Use of Biomass in Power Generation and Combined Heat and Power

Schemes.” Proceedings of the Institution of Mechanical Engineers 216(A1) (2002):41–57.

Arnold, J., Bronicki, Y., Rettger, P., Hennekes, R., de Graaf, J. D., Hooper, M. “Integrated

Oil Development Using Orcrude Upgrading and Shell Gasification Process.” Paper pre-

sented at Gasification Technologies Conference, San Francisco, 2002.

ASTM, “Standard Classification of Coals by Rank,” ASTM Standard D 388–99, American

Society for Testing and Materials, Philadelphia, 1999.

Baader, A. “Über ‘Anomalien’ im Kälteverhalten der Öle.” Oel und Kohle 38 (1942):432–444.

Table 4-21

Waste Streams from Caprolactam and Acrylonitrile Production

Stream

1 2 3 4

C, wt% 62.5 36.5 18.9 25.3

H, wt% 7.4 8.5 9.4 6.6

N, wt% 23.8 19.8 7.1 11.0

S, mg/kg 90 <50 690 46, 500

Cl, mg/kg <50 <50 <50 125

Ash, wt% <0.05 <0.05 <0.05 0.68

Moisture, wt % 6.1 27.4 62.6 35.3

Heating value, MJ/kg 30.7 21.2 8.0 8.2

Source: Schingnitz et al. 2000