Higman Chris Gasification (Газификация угля)

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Gasification

furnace is insufficient to maintain the reaction. Although this is only rarely a limitation

for plants gasifying refinery residues, gasification of low-sulfur coal can easily

produce an acid gas with such a low H

S concentration. Although it is possible to

increase this concentration with some AGR systems, using oxygen instead of air as

the oxidant in the Claus furnace often offers a more economic approach, especially

since the oxygen demand for the Claus unit is only a fraction of that required for the

gasifier itself. In addition to saving the cost of concentration within the AGR, the

cost of the Claus unit itself is lower. The chief determinant for sizing the equipment

in a Claus plant is the volume of gas throughput. Elimination of all or most of the

nitrogen involved with air-blowing reduces the gas volume by between 30 and 60%,

depending on the acid gas quality. The equipment is accordingly smaller.

There are a number of suppliers of oxygen-blown Claus technology, such as

Lurgi, BOC, APCI/Goar. All use special burners to handle the oxygen. One example

is the Lurgi OxyClaus burner, which is shown in Figure 8-14. The design of this

multipurpose burner provides for operation on air, enriched air or pure oxygen. The

burner has a series of acid gas burners arranged concentrically around a central

burner muffle. Each acid gas burner consists of three coannular lances, with oxygen

being injected through the inner lance, acid gas through the intermediate one, and air

through the outer ring.

One aspect of processing acid gas in a Claus plant, which deserves special

mention, is the problems that can occur, when excessive amounts of ammonia or

HCN are present in the gas. These components must be oxidized fully to molecular

PROCESS AIR

OXYGEN

S ACID GAS

PILOT

FUEL GAS

STEAM

SWS GAS

COMBUSTION

AIR

OXYGEN

Figure 8-14. Lurgi OxyClaus Burner (Source: Knab, Neumann, and Nehb 1994)

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321

nitrogen as otherwise ammonium salts are formed downstream and can plug the liquid

sulfur lines. Ensuring complete oxidation is essentially a matter of ensuring that these

components are combusted stoichiometrically at a sufficiently high temperature,

eventhough the H

S combustion is sub-stoichometric.

The main path of ammonia and HCN in a gasification unit is into the process

condensate, from which they are removed in a sour water stripper (SWS). A feature

of the Lurgi OxyClaus burner is the separate nozzle for the sour water stripper off-gas,

which allows this gas to be combusted with a different stoichiometry and therefore

a higher temperature than the bulk acid gas from the AGR unit.

8.4.2 Tail Gas Treatment

Environmental regulations require a high degree of sulfur removal in Claus units.

Typical are the German regulations (TA Luft), which require 97% sulfur recovery for

plants up to 20t/dS, 98% for plants between 20 and 50t/d, and 99.5% for larger plants.

The thermodynamics of the Claus reactions do not allow one to reach these yields. It is

therefore necessary to incorporate some form of tail gas treatment to achieve these yields.

Superclaus

One approach developed by Comprimo is the inclusion of a selective catalytic

oxidation step after the second Claus stage, which goes by the name of Superclaus.

Hydrogenation and Absorption

There is an important class of tail gas treatment processes typified by Shell’s SCOT

(Shell Claus Off Gas Treating) process, although there are now a number of similar

processes on the market. The principle arrangement of all these processes is similar

and is portrayed in Figure 8-15. The tail gas is hydrogenated in a catalytic reactor, so

that all sulfur species are converted to H

S. The gas is then quenched with water and

fed to an amine scrubbing unit to remove the H

S. Since the proportion of H

S in the

gas at this point of the plant is low, it is important for the economics to choose a

selective washing system, typically MDEA. The sulfur-rich gas from the regenerator

is recycled to the Claus furnace. Alternatively, the regeneration can be integrated

with that of a main H

S amine wash.

Other systems using hydrogenation and subsequent H

S removal are Beavon,

which used a Stretford wash for H

S removal (but now also uses MDEA), and others

which use, for example, Flexsorb.

Subdewpoint Processes

An alternative to hydrogenation and absorption is to continue making use of the

Claus reaction, but to do this at lower temperatures, at which the sulfur condenses

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Gasification

out in the catalyst bed. Such processes include Amoco’s cold bed adsorption (CBA)

process and the SNEA-Lurgi Sulfreen process.

The Sulfreen process depicted in Figure 8-16 serves as a typical example of this

class of tail gas treatment. The process is a cyclic one, which uses one reactor in the

adsorption mode while the second is being regenerated. For large plants it can be

advantageous to include a third reactor. During the adsorption step, tail gas is passed

over the catalyst at a temperature of 120–140°C. The low temperature enables the

Claus reaction to proceed further towards the production of sulfur, which is

adsorbed on the catalyst. In the regeneration mode, hot tail gas is used to desorb the

sulfur, which is then recovered in a steam raising sulfur condenser. The heat for the

regeneration loop can be supplied by a separate fired heater (as shown) or by heat

recovery from the thermal incinerator. Overall sulfur recoveries of 99.0 to 99.5%

can be achieved with a standard Sulfreen unit, depending on the Claus feed gas quality.

Additional enhancements can be incorporated to achieve higher recoveries, such as

incorporating an upstream hydrolysis (Hydrosulfreen, 99.5–99.7%) or a down-

stream direct oxidation (Carbosulfreen > 99.8%) (Lell 1993).

Figure 8-15. Typical Tail Gas Treatment Plant

CLAUS

UNIT

WASTE WATER

TO STRIPPER

CLEAN

TAIL GAS

ACID GAS

BFW

STEAM

SEPARATOR

ABSORBER

REGENERATOR

QUENCH

COOLER

COMBUSTOR

HYDROGENATION

REACTOR

TAIL GAS FEED

AIR

PURIFIED TAIL GAS

INCINERATOR

AND STACK

HEATER

AIRFG

BLOWER

SULFUR

CONDENSER

REGENERATION LOOP

SULFREEN REACTORS

Figure 8-16. Sulfreen Tail Gas Process (Source: Lell 1993)

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323

8.4.3 Integration in an IGCC

In contrast to most chemical applications for gasification, the presence of CO

IGCC syngas is not only no problem but even beneficial, in that it increases the

mass flow through the gas turbine and therefore contributes to the energy output.

This fact allows the integration of the tail gas treatment of the Claus unit into the

main AGR system in a manner that eliminates the SRU tail gas as an emission

completely. The principle is shown in Figure 8-17. The tail gas from the Claus plant,

which contains both H

S and SO

, is hydrogenated and recycled back to the AGR,

where the sulfur is again removed from the syngas. Clearly, for the economics of

this system it is necessary to keep the recycle compression costs low. This puts a

premium on the selectivity of the AGR, which in the IGCC scenario is in any case

desirable, so as to keep the CO

in the recycle low. Furthermore, one needs to use an

oxygen-blown Claus unit to avoid large quantities of nitrogen placing an unneces-

sary load on the recycle compressor. On the other hand, since the Claus tail gas is no

longer an emission as such, there is no need to include more than one catalytic stage

in the Claus unit itself. This type of integration has proved itself both in coal appli-

cations (Puertollano using an MDEA AGR) and for oil gasification (Sarlux using

Selexol).

8.4.4 Sulfuric Acid

An alternative outlet for sulfur to the production of merchant sulfur is sulfuric acid.

This is particularly interesting in locations where there is significant production of

phosphate fertilizers, such as Florida, where the Polk IGCC unit processes its acid

gas to sulfuric acid rather than sulfur. Other examples can be found in ammonia

SINGLE STAGE

CLAUS

HYDROGENATION /

QUENCH

RAW SYNGAS

SELECTIVE

AGR

SULFUR

CLAUS

TAIL GAS

S RECYCLE

CLEAN FUEL GAS

TO TURBINE

Figure 8-17. Integration of Claus Unit in an IGCC

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Gasification

plants, which have formed the core of complex fertilizer production sites in Portugal

and China, for example.

Wet Catalysis

Processing H

S to sulfuric acid differs from conventional acid plants (whether

using sulfur or metallurgical off gases as feed) in that water is already present in the

converter. This water reacts immediately with the SO

formed there and therefore

demands a totally different approach to the recovery of liquid acid compared with a

standard double absorption unit.

The H

S component of the acid gas is first combusted with excess oxygen to form

and water (see Figure 8-18). The excess is selected to produce a O

/SO

ratio in

the hot gas of between 1.1 and 1.3. The hot combustion gases are cooled in a waste-

heat boiler to a converter inlet temperature of 440°C. The SO

and oxygen react

over a vanadium catalyst according to the reaction

+½ O

←→ SO

(8-6)

and the water reacts with SO

as follows

O ←→ H

(8-7)

The converter uses interbed quenching with cold air to control the temperature

increase arising from these exothermic reactions. Depending on the feed gas quality,

three or four catalyst beds are included.

Since the acid is already present in the gas phase leaving the converter, this is

recovered by condensing at about 60°C. In a classic wet catalysis plant, the product

is a 78% acid, below the 80–90% range that is critical for corrosion. The condensation

is achieved by recycling this product acid to the condensation tower in a manner

designed to minimize formation of acid aerosols. A candle filter is provided to

remove any aerosols that are formed. If there is insufficient water in the gas leaving

PROCESS WATER

ACID PUMP

TANK

COOLERS

CONDENSATION

TOWER

QUENCH COOLED

CONVERTER

WASTE HEAT

BOILER

COMBUSTION

FURNACE

BLOWER

AIR

PRODUCT ACID

TAIL GAS

CANDLE

FILTER

Figure 8-18. Wet Catalysis Sulfuric Acid Plant (With permission: Lurgi)

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325

the converter, then make-up water is added to keep the acid strength at 78%—out of

the corrosive range.

Advanced wet catalysis processes such as Concat (Lurgi) or WSA (Topsøe) are

able to produce 93–98% strength acid, even with very weak feed gases and excess

water. Concat achieves this, for example, by using a two-stage condensation section,

the first of which operates at 180–230°C so that a minimum of free water is present

in the condensate, which is already about 93% H

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Chapter 9

Economics,

Environmental, and Safety

Issues

9.1 ECONOMICS

The economics of every major capital investment are individual to the project

concerned. Given the broad range of applications and feedstocks, this is especially

so for gasification. Nonetheless a number of trends can be identified.

Gasification is generally a capital-intensive technology, which has, however,

the capability of working with cheaper or more difficult feedstocks than many alter-

natives.

Ammonia. The capital intensity of gasification is clearly visible in the data in Table 9-1

for a 1800 t/d ammonia plant located in Northwest Europe based on different tech-

nologies and feedstocks. The capital estimates and feed rates are those of Appl

(1999).

This data can be presented in a manner that allows comparison of production

costs with varying feedstock pricing for the different feedstocks, as in Figure 9-1.

Based on these data ammonia production by gasification of heavy residue becomes

competitive with U.S.$2.50/MMBTU natural gas, if the residue is valued at about

U.S.$20/t. The natural gas price must rise to over U.S.$4/MMBTU before coal or

petcoke can become a competitive feedstock.

Methanol. Higman made a similar study for methanol comparing natural gas and

vacuum residue feed (1995). The results are in principle similar (Figure 9-2).

His conclusions are that under today’s economic conditions, it would not be

competitive to manufacture methanol by gasifying locally available resid compared

with importing methanol produced from cheap natural gas in a remote location. “If,

however, strategic or other considerations demand that production be located in one