Higman Chris Gasification (Газификация угля)

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Gasification

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Chapter 5

Gasification Processes

In the practical realization of gasification processes a broad range of reactor types

has been and continues to be used. For most purposes these reactor types can be

grouped into one of three categories: moving-bed gasifiers, fluid-bed gasifiers, and

entrained-flow gasifiers. The gasifiers in each of these three categories share certain

characteristics that differentiate them from gasifiers in other categories. Some of

these characteristics are summarized in Table 5-1.

Moving-bed gasifiers (sometimes called fixed-bed gasifiers) are characterized by

a bed in which the coal moves slowly downward under gravity as it is gasified,

generally by a counter-current blast. In such a counter-current arrangement, the hot

synthesis gas from the gasification zone is used to preheat and pyrolyse the down-

ward flowing coal. With this process the oxygen consumption is very low but pyrol-

ysis products are present in the product synthesis gas. The outlet temperature of the

synthesis gas is generally low, even if high slagging temperatures are reached in the

heart of the bed. Moving-bed processes operate on lump coal. An excessive amount

of fines, particularly if the coal has strong caking properties, can block the passage

of the upflowing syngas.

Fluid-bed gasifiers offer extremely good mixing between feed and oxidant, which

promotes both heat and mass transfer. This ensures an even distribution of material

in the bed, and hence a certain amount of only partially reacted fuel is inevitably

removed with the ash. This places a limitation on the carbon conversion of fluid-bed

processes. The operation of fluid-bed gasifiers is generally restricted to temperatures

below the softening point of the ash, since ash slagging will disturb the fluidization

of the bed. Some attempts have been made to operate into the ash-softening zone to

promote a limited and controlled agglomeration of ash with the aim of increasing

carbon conversion. Sizing of the particles in the feed is critical; material that is too

fine will tend to become entrained in the syngas and leave the bed overhead. This is

usually partially captured in a cyclone and returned to the bed. The lower tempera-

ture operation of fluid-bed processes means that they are more suited for gasifying

reactive feedstocks, such as low-rank coals and biomass.

Entrained-flow gasifiers operate with feed and blast in co-current flow. The resi-

dence time in these processes is short (a few seconds). The feed is ground to a size

of 100 µm or less to promote mass transfer and allow transport in the gas. Given the

short residence time, high temperatures are required to ensure a good conversion,

Gasification

Table 5-1

Characteristics of Different Categories of Gasification Process

Category Moving-Bed Fluid-Bed Entrained-Flow

Ash conditions Dry ash Slagging Dry ash Agglomerating Slagging

Typical processes Lurgi BGL Winkler, HTW, CFB KRW, U-Gas Shell, Texaco,

E-Gas, Noell, KT

Feed characteristics

Size 6–50 mm 6–50 mm 6–10 mm 6–10 mm <100 µm

Acceptability of fines limited better than

dry ash

good better unlimited

Acceptability of

caking coal yes (with stirrer) yes possibly yes yes

Preferred coal rank any high low any any

Operating

characteristics

Outlet gas temperature low

(425–650°C)

low

(425–650°C)

moderate

(900–1050°C)

moderate

(900–1050°C)

high

(1250–1600°C)

Oxidant demand low low moderate moderate high

Steam demand high low moderate moderate low

Other characteristics hydrocarbons

in gas

hydrocarbons

in gas

lower carbon

conversion

lower carbon

conversion

pure gas, high

carbon conversion

Source: Adapted from Simbeck et al. 1993

Gasification Processes

and therefore all entrained-flow gasifiers operate in the slagging range. The high

temperature operation creates a high oxygen demand for this type of process. Entrained-

flow gasifiers do not have any specific technical limitations on the type of coal used,

although coals with a high moisture or ash content will drive the oxygen consump-

tion to levels where alternative processes may have an economic advantage.

There are one or two processes that do not fit into any of these three main categor-

ies. This includes in situ gasification of coal in the underground seam as well as

molten bath processes. These are discussed in Section 5.8.

One important point to note throughout all the above is the significance of the

slagging behavior of the ash. At temperatures above the ash softening point, the ash

becomes sticky and will agglomerate, causing blockage of beds or fouling of heat-

exchange equipment. Once above the slagging temperature, at which point the ash

has a fully liquid behavior with a low viscosity, it is possible again to remove it from

the system reliably. Thus, for all processes, there is a feedstock-specific “no-go”

temperature range between the softening and slagging temperatures of the ash.

5.1 MOVING-BED PROCESSES

Historically, moving-bed processes are the oldest processes, and two in particular,

the producer gas process and the water gas process, played an important role in the

early production of synthesis gas from coal and coke. The preferred feedstocks were

in general coke or anthracite, as otherwise the gas needs extensive cleaning to

remove tars from the gas. Both processes operate at atmospheric pressure.

Producer Gas

In the producer gas process, humidified air is blown upward through a deep bed of

coal or coke. The coal is fed from the top and moves slowly downwards as it is

consumed. Ash is drawn off at the bottom of the reactor from a rotating grate. The

air reacts with the coal, thereby producing a gas with a lower heating value of

6500 kJ/m

. The presence of about 50% nitrogen in the product gas is the main cause

for this low value. The advantage of the process was that it was continuous. The

main drawback is that because of its low heating value, producer gas cannot be

transported over long distances. When using low-rank feedstocks such as wood, the

heating value of the gas can be as low as 3500 kJ/m

. Gas producers continue to be

built in small numbers today where gas is required locally for firing industrial

furnaces, particularly in locations where coal is the only available energy form as in

parts of China (Wellman undated).

Water Gas

The water gas process is a discontinuous process in which steam reacts with red-hot

coke to form hydrogen and carbon monoxide. First, the coal or coke bed is heated by

Gasification

blowing air upward through the bed to temperatures of about 1300°C. Then the air

flow is stopped and steam is passed through the coal or coke bed, first upward and

then downward, thereby producing synthesis gas. The reason for the two different

directions of steam flow was to make optimum use of the heat in the bed, which is

required do drive the endothermic water gas reaction. When the temperature has

dropped to about 900°C, the steam “run” is stopped and the cycle is repeated. After

purification the syngas can be used for the synthesis of ammonia or methanol.

Before the advent of large-scale air separation plants, the water gas process was the

only means for making high-grade synthesis gas for chemical purposes. In order to

obtain a continuous gas flow, at least three water gas reactors are required.

5.1.1 The Lurgi Dry Ash Process

The patent for the Lurgi dry ash process, or “coal pressure gasification” as it was

originally known (“Vergasung von Braunkohle unter Druck mit Sauerstoff-Wasser-

dampf-Gemischen,” or “Gasification of browncoal under pressure with oxygen-steam

mixtures”), was granted in 1927. In 1931 Lurgi started to develop, a pressurized

version of existing atmospheric producer gas technology, using oxygen as blast,

initially for gasification of lignite (Lurgi 1970; Lurgi 1997). The development was

made in close cooperation with the Technical University in Berlin under the direc-

tion of Professor Rudolf Drawe, one of the fathers of modern gasification. The first

commercial application was built in 1936 (Rudolf 1984). In 1944 two large-scale

plants were in operation in Böhlen (Saxony) and Brüx (now Most, Czech Republic)

producing town gas. The position of the Lurgi dry ash process as the only pressure

gasification system for many years, together with continued development, for example,

in scale-up, automation, and operational optimization, have all contributed to the

commercial success of this technology.

Standardized reactor sizes are shown in Table 5-2.

These figures for throughput and gas production are nominal figures, which are

substantially influenced by the quality of coal processed. For instance, Sasol reports

Table 5-2

Sizes and Capacities of Lurgi Dry Ash Gasifiers

Type

Nominal

Diameter (m)

Coal Throughput

(t/h)

Gas Production (Dry)

(1000 Nm

/day)

MK III 3 20 1000

MK IV 4 40 1750

MK V 5 60 2750

. Source: Rudolf 1984

Gasification Processes

achieving 30% excess throughput compared with the original rated design (Erasmus

and van Nierop 2002).

Most commercial gasifiers operate at pressures of 25–30 bar. A demonstration

plant has been operated at pressures of 100 bar (see Section 5.1.3).

Process Description

The heart of the Lurgi process is in the reactor, in which the blast and syngas flow

upwards in counter-current to the coal feedstock (Figure 5-1).

Coal is loaded from an overhead bunker into a lock hopper that is isolated from

the reactor during loading, then closed, pressurized with syngas, and opened to the

reactor. The reactor is thus fed on a cyclic basis.

The reactor vessel itself is a double-walled pressure vessel in which the annular

space between the two walls is filled with boiling water. This provides intensive

cooling of the wall of the reaction space while simultaneously generating steam from

the heat lost through the reactor wall. The steam is generated at a pressure similar to

the gasification pressure, thus allowing a thin inner wall that enhances the cooling

effect.

JACKET STEAM

TAR RECYCLE

COAL

LOCK

COAL

DISTRIBUTOR

STEAM &

OXYGEN

ASH

WASH

COOLER

ASH

LOCK

GRATE

WATER

JACKET

Figure 5-1.

Figure 5-1.Figure 5-1.

Figure 5-1. Lurgi Dry Ash Gasifier (Source: Lurgi 1970)