Higman Chris Gasification (Газификация угля)

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140

Gasification

5.4.3 Lurgi’s Multipurpose Gasification Process (MPG)

Lurgi has maintained a leading position in coal gasification since the 1930s, but for

many years worked as contractor and licensing agent for the Shell SGP process for

partial oxidation of liquids and gases. In 1998 Lurgi announced that it would now be

marketing its own technology under the name of LurgiSVZ multipurpose gasifica-

tion (MPG; Figures 5-31 and 5-32). This technology had been in existence since

1969 at what is today SVZ Schwarze Pumpe (Hirschfelder, Buttker, and Steiner

1997). It was developed originally out of a Lurgi moving-bed gasifier to process tars

produced in the other twenty-three Lurgi gasifiers at the location, which produced

town gas from lignite.

Recently the start-up after the revamp of an existing 60 bar 16 t/h asphalt feed

reactor has been reported (Erdmann, Liebner, and Schlichting 2002).

Process Description

The gasification reactor is a refractory-lined vessel with a top-mounted burner. The

burner has a multiple-nozzle design that allows it to accept separate feed streams of

otherwise incompatible materials.

QUENCH WATER

MPG REACTOR

VENTURI

SCRUBBER

MP-STEAM

SLURRY

COLLECTOR

/STEAM

FEEDSTOCK

C.W.

QUENCH

ALTERNATIVE ROUTE

TO RAW GAS SHIFT

BFW

MP-BOILER

COOLER

C.W.

RAW

SYNGAS

WASTE

WATER

METALS/

ASH

METALS ASH

REMOVAL SYSTEM

(MARS)

REFRACTORY

Figure 5-31.

Figure 5-31.Figure 5-31.

Figure 5-31. Lurgi MPG Process (Quench Configuration) (Source: Liebner 1998)

Gasification Processes

141

Waste Heat Recovery

MPG is offered with two alternative syngas cooling configurations, quench and heat

recovery. The criteria for selection of the cooling configuration are listed in Table 5-14.

QUENCH

WATER

MPG REACTOR

/STEAM

FEEDSTOCK

SOOT

SEPARATOR

HP-STEAM

SOOT

SLURRY

RAW

SYNGAS

WASH

QUENCH

HEAT

RECOVERY

BOILER

BFW

C.W.

WASTE

WATER

METALS/

ASH

METALS ASH

REMOVAL SYSTEM

(MARS)

RETURN

WATER

REFRACTORY

SCRUBBER

Figure 5-32.

Figure 5-32.Figure 5-32.

Figure 5-32. Lurgi MPG Process (Syngas Cooler Configuration) (Source: Liebner 1998)

Table 5-14

Selection Criteria for Quench versus Heat-Recovery Configuration

Quench

Configuration

Heat Recovery

Configuration

Feedstocks: Gas, residue,

wastes (sludges, coal,

coke), extreme ash,

and/or salt contents

Highest flexibility Limited by possible salt

precipitation

Product range: Syngas

+CO, H

, CO)

Fastest (cheapest)

route to H

Syngas at high temperature

–CO equilibrium

Energy utilization MP steam available

Trade-off efficiency

versus cost

HP-steam, heat recovery

at highest efficiency for

IGCC possible

Investment cost Lowest cost for

gasification unit

Boiler (i.e., a high

efficiency) at extra cost

Source: Liebner 1998

142

Gasification

Carbon Management

The carbon is washed out of the gas with a conventional water wash. Lurgi’s carbon

management process for MPG, the metals ash removal system (MARS), is a filtra-

tion-multiple hearth furnace process. The flowsheet is very similar to that of Shell’s

SARU described in Section 5.4.2. Differences are a matter of detail in equipment

design and selection. Lurgi uses its own proprietary design of multiple-hearth

furnace, which already had a long track record in the vanadium industry before find-

ing application in the field of residue gasification. Lurgi also propagates the use of

belt filtration. This has the advantage of being a continuous process with easier

operation and maintenance. In order to achieve the same dewatering performance as

a membrane filter press, flocculants are required.

Process Performance

A particular feature of MPG is its multinozzle burner, allowing a wide range of

feedstocks. The Table 5-15 lists operational ranges and maximum concentrations of

base components and contaminants as experienced with MPG.

Typical product gas quality is in Table 5-16.

5.4.4 New Developments

Despite various improvements over the years, it is generally recognized that hand-

ling of the soot produced by partial oxidation of heavy residues places a consider-

able financial burden on the overall process. This has caused operating companies

and others to investigate alternatives. In the 1970s one operating company was

already using a toluene extraction process to recover the soot as saleable carbon

black. A number of other companies made similar attempts, but the economics of

these processes, together with the variable product quality depending on feed quality

to the gasifier, have prevented commercialization beyond single demonstration plants.

Nonetheless, two process, both based on filtration of soot slurry and subsequent

treatment of the filter cake, have been reported on in recent years and may form the

basis for further development.

Norsk Hydro Vanadium Recovery Process

Norsk Hydro developed its own process for its 65 t/h feed heavy oil gasification

plant in Brunbüttel. This plant has been in operation now for several years and has

definitely provided operating benefits compared with the original 1975-designed

pelletizing plant (Maule and Kohnke 1999).

Gasification Processes

143

The Norsk Hydro VR (vanadium recovery) process (see Figure 5-33) is based on

filtration of soot slurry and combustion of the filter cake. In this process the filter

cake is first dried and pulverized before being burned in a special cyclone combus-

tor in which part of the vanadium is combusted to a liquid V

that is then scraped

from the combustion chamber floor. The heat of combustion is used to generate

steam, which in general is sufficient to provide the necessary heat for the drying

stage. Fly ash from the combustion stage, which also contains V

, is collected in a

bag filter and combined with that collected from the combustion chamber.

Table 5-15

MPG Feedstock Flexibility (Liquids and Slurries)

Actual Operating Ranges and

Maximum Concentrations

Component “Normal” Feeds Waste Feeds

C, wt% 65–90 90

H, wt% 9–14 14

S, wt% 6 6

Cl, wt% 2 8

LHV, MJ/kg 35–42 5–330

Toluene

insolubles, wt% 6 45

Ash, wt% 3 25

Water, wt% 2 5–100

Trace components (selection only)

Al, ppmv 600 70,000

Ag, ppmv 5 10

Ba, ppmv 500 2000

Ca, ppmv 3000 170,000

Cu, ppmv 200 800

Fe, ppmv 2000 40,000

Hg, ppmv 10 25

Na, ppmv 1200 8000

Ni, ppmv 50 500

Pb, ppmv 200 10,000

V, ppmv 10 100

Zn, ppmv 1200 10,000

PCBs, ppmv 200 600

PAK, ppmv 20,000 40,000

Source: Liebner 1998

144

Gasification

In the published flow diagram, which is more complex than what is shown in

Figure 5-33, there is no specific provision for sulfur recovery from the flue gas.

Depending on the circumstances, this may have to be included. A fuller description

of the process is available in the literature.

In the meantime, Texaco has acquired the rights to this process, and it remains to

be seen just what further development the process will experience in the hands of

Table 5-16

MPG Product Gases (IGCC Application)

Quench Mode

(Coal Oil)

Heat-Recovery Mode

(Heavy Residue)

Raw Gas Raw Gas

Clean Gas after

Desulfurization

, mol% 4.00 3.24 3.26

CO, mol% 53.03 48.25 48.63

, mol% 40.80 46.02 46.39

, mol% 0.15 0.20 0.20

, mol% 0.85 0.65 0.66

Ar, mol% 1.15 0.85 0.86

S, mol% 0.02 0.79 ≤10 ppmv

Total, mol% 100 100 100

HHV, MJ/Nm

11.96 12.24 12.13

LHV, MJ/Nm

11.15 11.31 11.22

Note: Gasification with oxygen (95%v) at about 30 bar

Source: Liebner 1998

FILTER

WASTE WATER

STRIPPER

VANADIUM

ASH

BAG FILTER

BOILER

SOOT

COMBUSTOR

MILL

FLUID BED

DRYER

STEAM

SOOT

SLURRY

RETURN WATER TO POX

WASTE

WATER

PRODUCT

>30% V

FLUE GAS

BFW

Figure 5-33.

Figure 5-33.Figure 5-33.

Figure 5-33. Norsk Hydro VR Process

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145

a licensor. It is worth noting that Krupp Uhde reported a similar development under the

name of CASH (Keller et al. 1997). No commercial application is known, however.

Soot Gasification

A totally new approach to handling filter cake is under development at the Engler-

Bunte-Institut (EBI) of Universität Karlsruhe (Figure 5-34) (Higman 2002). The

development of the filtration-based processes was driven by the recognition that the

behavior of the vanadium in the ash is crucial to the oxidizing treatment of the filter

cake. In particular, the MHF concepts operate at a low temperature specifically to

prevent exceeding the melting temperature of the V

formed, which is about

700°C. This low operating temperature, for all its benefits, has the disadvantage of a

low reaction rate, and thus high residence times and large equipment.

Soot gasification retains the vanadium in the trioxide state and, as with the main

gasifier, can operate at high temperatures without creating liquid vanadium pentox-

ide. The gasification is optimized to achieve maximum carbon conversion, whereby

a higher level of CO

is tolerated than in most gasification processes, that is, mini-

mizing the residual carbon in the ash is more important than H

+CO yield. The

process exploits the existing gasification infrastructure by using oxygen and can

thus produce a low-pressure synthesis gas with a fuel value. This is in contrast to the

large waste gas flow of a multiple hearth furnace, which contains carbon monoxide

and requires incineration.

The process line-up includes a typical filtration step followed by fluid-bed drying

and milling of the filter cake to <500 µm. Gasification takes place with oxygen and

steam in an atmospheric entrained-flow reactor. The ash is removed from the product

gas in a dry candle filter and meets the requirements of the metallurgical industry.

This process, which is still under development, has the potential to reduce the costs

of carbon management significantly. Further possibilities include the development

of a pressurized version, that could handle soot filtered dry directly out of the main

FILTER

WASTE WATER

STRIPPER

VANADIUM

ASH

FILTER

SOOT

GASIFIER

MILL

DRYER

SOOT

SLURRY

RETURN WATER TO POX

WASTE

WATER

PRODUCT

>30% V

FUEL GAS

Figure 5-34.

Figure 5-34.Figure 5-34.

Figure 5-34. EBI Soot Gasification Process (Source: Higman 2002)

146

Gasification

syngas stream. This would decrease equipment size further, reduce the cost of the

wash water circuit, and offer an economic possibility to recycle the fuel gas into the

main stream, thus increasing the syngas yield. Clearly, developments in the field of

oil gasification are by no means at an end.

5.4.5 Process Safety

Process Automation

Partial oxidation is a process that requires careful control and monitoring so as to

ensure that accidental temperature runaways are avoided, especially during transient

conditions such as start-up and shutdown. Process licensors have developed sophis-

ticated control systems together with integrated automated start-up and shutdown

procedures, which play an important role in safe operation.

Butzert described the most important features of such systems in 1976. Each

licensor includes details specific to his own process and experience. The fundamentals

described by Butzert are still valid today, although the radical change in instrument-

ation and control hardware since then has allowed the incorporation of many addi-

tional functions. For instance, de Graaf and colleagues (1998; 2000) report on

advanced features of the system in Pernis incorporating reactor preheat, panel restart

after a spurious trip, and gas transfer to the turbines controlled so that flaring of

sulfur-containing gas could be eliminated. Further refinements are reported by

Weigner and colleagues (2002). Plants in a predominately power production applica-

tion include a load following function to comply with the demands of the grid.

Reactor Shell Monitoring

Another safety aspect typical of all partial oxidation processes is the necessity of

controlling and monitoring the reactor shell temperature in case of damage to the

refractory lining. Early plants had to rely on a multiplicity of point thermocouples

around the reactor, usually chosen to correspond with potential weak points in the

refractory brickwork. This was often complemented by the use of thermosensitive

paint and regular thermography. Later, special coaxial cables with a temperature-

sensitive resistance between core and sheath became available. With these coils a

continuity of coverage became possible far in advance of the discrete point measure-

ments previously used. On the other hand, location of the hot spot from these

measurements was only possible with a grid of such coils that noted which two were

showing the high temperature.

Fiber-optic systems are now available that provide a combination of continuity and

localization. With appropriate software this can be converted to a screen visualization

identifying hot spots on the control panel screen (see Figure 5-35) (Nicholls 2001).

This fiber-optic system has been used on oxygen-fired secondary reformers, and

recently two Texaco gasifiers have been equipped in this manner.

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147

5.5 BIOMASS GASIFICATION

When looking at biomass conversion it is instructive to look at coal conversion, as

there are many similarities. This is not so surprising since biomass is nothing but

young coal. For coal gasification the minimum temperature required is about 900°C

as is demonstrated in the old water-gas process in which the temperature during the

steam run was allowed to drop from the maximum of 1300°C to 900°C. About the

same maximum temperature of 800–900°C is required to gasify the most refractory

part of almost any biomass. In other words, the temperature required for the complete

thermal gasification of biomass is of the same order of magnitude as for coal. This high

temperature in combination with the impurities, whether sulphur or ash components,

is why indirectly heated coal and biomass gasification processes in which external heat

has to be transferred via a metal surface have not yet achieved any commercial success.

On the other hand there are a number of significant differences between coal

gasification and biomass gasification, which are directly attributable to the nature of

the feedstock. Firstly there is the quality of biomass ash, which has a comparatively

low melting point but in the molten state is very aggressive. Secondly, there is the

generally high reactivity (see Figure 3-3) of biomass. Furthermore, particularly with

vegetable biomass, there is its fibrous characteristic. Finally, there is the fact that, par-

ticularly in the lower temperature range biomass gasification has a very high tar make.

Although an entrained-flow process might have an apparent attraction in being

able to generate a clean, tar-free gas as required for chemical applications, and the

low melting point of the ash would keep the oxidant demand low, the aggressive

quality of the molten slag speaks against such a solution, whether using a refractory

Figure 5-35.

Figure 5-35.Figure 5-35.

Figure 5-35. Screen Shot of Fiber-Optic Reactor Shell Temperature Monitor

(Source: Nicholls 2001)

148

Gasification

or a cooling membrane for containment protection. Furthermore the short residence

times of entrained-flow reactors require a small particle size, to ensure full gasification

of the char. No method of size reduction has yet been found, which will perform

satisfactorily on fibrous biomass.

A number of fixed-bed processes have been applied to lump wood, but they are

limited to this material. They would not work on straw, miscanthus or other materials

generally considered for large-scale biomass production unless these were previously

bricketted. Furthermore in a counter-flow gasifier, the gas would is heavily laden with

tar. The alternative of co-current flow could reduce the tar problem substantially,

but the necessity to maintain good control over the blast distribution in the bed

restricts this solution to units of very small size.

With this background it is probably not surprising that most processes for

biomass gasification use fluid beds and aim at finding a solution to the tar problem

outside the gasifier. In co-firing applications where the syngas is fired in an associated

large-scale fossil fuel boiler, the problem can be circumvented by maintaining the gas at

a temperature above the dewpoint of the tar. This has the added advantage of bring-

ing the heating value of the tars and the sensible heat of the hot gas into the boiler.

There are many biomass processes at various stages of development. Summaries

are given in, for example, Kwant (2001) and Ciferno and Marano (2002). The selec-

tion chosen here represents generally those that have reached some degree of

commercialization.

5.5.1 Fluid-Bed Processes

Lurgi Circulating Fluid-Bed Process

The Lurgi CFB process is described in Section 5.2. Plants operating on biomass and

or waste include those in Rüdersdorf in Germany (500 t/d waste) and Geertruiden-

berg in The Netherlands (400 t/d waste wood). In the latter plant the hot gas leaving

the cyclone at a temperature of about 500°C is directly co-fired in a 600 MW

coal

boiler. In Rüdersdorf, the gas is fired in a cement kiln (Greil et al. 2002).

Foster Wheeler Circulating Fluid-Bed Process

The Foster Wheeler (originally Ahlstrom) CFB process was developed to process

waste biomass from the pulp and paper industry. The first unit was built in 1983,

and the gas was used to replace oil firing of a lime kiln at a paper mill. Three further

units were built in Sweden and Portugal for similar applications. The size range is

between 17 and 35 MW

The largest unit to date has a capacity of 40–70 MW

(depending on fuel) and

operates co-firing the gas as a supplement fuel in an existing coal-fired boiler in

Lahti, Finland (Anttikoski 2002). The feed is primarily biomass, but various refuse

derived fuels are also used.

Gasification Processes

149

All these units operate at atmospheric pressure. In a different development,

Foster Wheeler has also developed a pressurized version that formed the basis for

the 6 MW

biomass IGCC at Värnamo in Sweden (see Figure 5-36). The gasifier

operates at 20 bar and has a capacity of 18 MW

The gasifier feed is pressurized in lock hoppers and a screw feeder is used for the

transport from the high-pressure charge bin. Gasification takes place at 950–1000°C.

Primary ash removal is via lock hoppers at the bottom of the gasifier. Fine particulate

removal takes place in a hot gas filter with ceramic candles (later replaced by metal

candles) at 350–400°C, at which temperature the gas enters the combustion chamber

of the gas turbine. Tar production from the gasifier is reported to be less than 5 g/Nm

dry gas. The demonstration program was completed in 1999 after over 8500 hours

operation. Technically, it has been a success. The economics are competitive vis-à-vis

other biomass systems, but are still dependant on a general biomass-to-power support.

(Sydkraft 1998).

The TPS Process

The TPS process of TPS Termiska Processer AB is an atmospheric CFB that was,

like the Foster Wheeler process described above, developed in the mid-1980s to

FUEL

INPUT

GASIFIER

SYNGAS

COOLER

HOT GAS

FILTER

GAS

TURBINE

STEAM

TURBINE

DISTRICT

HEATING

HRSG

STACK

BOOSTER

COMPRESSOR

Figure 5-36.

Figure 5-36.Figure 5-36.

Figure 5-36. Flow Diagram of Värnamo IGCC (Source: Sydkraft 1998)