Entrained-Flow Gasifier

In entrained-flow gasifiers, the speed of flow (air or oxygen) is fast, resulting in entraining the coal particles from the injection location to the exit. It can be designed as either upflow or down-flow system. The coal and oxidant are fed either from top or bottom of the gasifier. Gasifiers of this type typically operate at very high temperatures to melt coal ash into inert slag. The fine coal feed and high operating temperature allow the gasification reaction to occur at a very high rate (the typical residence time is on the order of few seconds), with high carbon conversion efficiencies (98-99.5%). The tar, oil, phenols, and other liquids produced from devolatization of coal inside the gasifier are decomposed into hydrogen (H2), carbon monoxide

(CO) and small amounts of light hydrocarbon gases. Entrained-flow gasifiers have the ability to handle practically any coal feedstock and produce a clean, tar-free, syngas. The fine coal feed can be fed to the gasifier in either a dry or slurry form. The former uses a lock hopper system, while

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the latter relies on the use of high-pressure slurry pumps. The slurry feed is a simpler operation, but it introduces water into the reactor which needs to be evaporated. The result of this additional water is a product syngas with higher H2 to CO ratio, but with a lower gasifier thermal

efficiency. The feed preparation system is designed along with other process design alternatives, for a particular application. Figure 1.7 presents an illustration of an entrained-flow gasifier.

Figure 1.7 Schematic of an entrained-flow gasifier (Holt, 2004)

Entrained-flow gasifiers typically exhibit the following characteristics: • Fuel flexibility, can accept a variety of solid feedstocks

• Large oxidant requirements

• Can either be oxygen or air blown, but most commercial plants are oxygen blown • Uniform temperature within the reactor

• Slagging operation

• Short reactor residence time

• Higher carbon conversion, but lower coal gas efficiency

• High level of sensible heat in product gas; heat recovery is required to improve efficiency

• Environmentally most benign; produced syngas consists of mainly H2, CO and carbon

dioxide (CO2) with trace amount of other contaminates which can be removed

downstream of the reactor

14 Examples of entrained-flow gasifiers:

(1) Shell Coal Gasification Process (SCGP)

Shell’s coal gasification technology uses a dry-feed, pressurized, entrained-flow, slagging gasifier that operates on a wide variety of feedstocks. Figure 1.8 shows the schematic of Shell gasifier. Dried, pulverized coal is fed to the gasifier through pressurized lock hoppers using a transport gas (syngas or nitrogen). Preheated 95% pure oxygen and steam (as a moderator) are mixed and fed to the injector. The coal reacts with oxygen at typical conditions of 2,700-2,900°F and 350-600 psi to produce syngas consisting of only small amounts of carbon dioxide (CO2) and no hydrocarbon liquids or gases. Theraw syngas leaves the gasifier at a

relatively high temperature (typically 2,500-2,700°F) and is sent on to a syngas cooler for heat recovery, generating high-pressure superheated steam. The syngas cooler typically consists of three sections—superheat, convection, and economizer. To protect the syngas cooler from fouling, corrosion, and erosion, the hot raw syngas leaving the gasifier is first quenched with cooler, recycled product gas to convert any entrained molten fly slag to a hardened solid material prior to entering the syngas cooler. The bulk of the fly ash contained in the raw syngas leaving the syngas cooler is removed from the gas using commercial filter equipment or cyclones. Any remaining fly ash is captured downstream with a wet scrubber. The syngas cooler is an integral part of Shell gasifier technology.

The Shell technology uses a refractory-lined reactor vessel, equipped with an inner membrane wall consisting of circulating water/steam-filled tubes. During operation, ash is converted into molten slag. The molten slag, cooled by the membrane wall, vitrifies to form a protective layer against slag erosion of the refractory. This specific reactor design contrasts most rival gasification processes, which have brick refractory walls inside the gasifier that are gradually eroded by the hot slag and must be routinely replaced. Shell’s gasifier membrane wall is said to have a 20-year life. Inner reactor wall temperature is controlled by circulating water through the membrane wall, producing steam. Produced slag flows down the reactor into a water bath, where it solidifies and is removed through a lock hopper as slurry.

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Figure 1.8 Schematic of the Shell gasifier

(2) General Electric Gasifier (Previously Texaco gasifier)

GE gasification uses a single-stage, downward-feed, entrained-flow refractory-lined reactor to produce synthesis gas (syngas) from a coal/water slurry (~ 65% in wt) and oxygen (> 95% pure). The slurried feedstock is pumped to a custom-designed injector mounted at the top of the gasifier. The coal reacts exothermically with oxygen at high temperature (~ 2,200 to 2,700°F) and pressure (>300 psi) to form syngas and slag. No hydrocarbon liquids are produced in these high temperature conditions. The syngas produced contains mostly hydrogen (H2) and carbon monoxide (CO). The raw syngas leaving the gasifier can be cooled by a radiant

and/or convective heat exchanger and/or by a direct quench system, where water or cool recycled gas is injected into the hot raw syngas. The radiant cooling design uses a soot-tolerant radiant syngas cooler that generates high-pressure steam. Slag is quenched in a water pool located at the bottom of the reactor vessel, and removed through a lock hopper. This design maximizes heat recovery as well as CO production. The syngas is further cooled after leaving the gasifier by a water scrubber to recover the fine particulate matter and char for recycle to the gasifier, before the gas is sent on to downstream processing.

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A direct quench system uses an exit gas water quench. Hot gas exiting the gasifier is contacted directly with water via a quench ring; it is then immersed in water in the lower portion of the gasifier vessel. The cooled, saturated syngas is then sent to a scrubber for soot and particulate removal. The quench design is less efficient, but also less costly, and it is commonly used when a higher hydrogen to CO ratio syngas is required.

Figure 1.9 Schematic of the General Electric gasifier

(3) Conoco-Phillips (E-Gas) Gasifier

The E-Gas coal gasifier is a pressurized, upflow, slurry-feed, entrained slagging design with a unique two-stage operation. Wet crushers produce slurries from the raw feed coal. Coal slurry concentrations typically range from 50 to 70% depending on the inherent moisture and quality of the feed coal. About 75% of the total slurry feed is fed to the first (or bottom) stage of the gasifier through mixer nozzles, along with 95% pure oxygen. This stage involves highly exothermic oxidation reactions (i.e., combustion) and operates at typically 2,600°F and 400 psig. These conditions do not allow the formation of hydrocarbon gases and liquids. Ash in the coal melts and exits through a taphole at the bottom of the gasifier into a water quench, forming

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an inert vitreous slag product. The hot syngas from the first stage then enters the second (top) stage where the remaining slurry feed is injected. Endothermic gasification and devolatization reactions take place at 1,900°F, resulting in the formation of some hydrocarbons in the product gas and the production of char, which are both recycled to the first stage where they are readily gasified. The product gas exits the gasifier at 1,900°F and is cooled in a fire-tube cooler to 1,100°F, generating saturated steam. Particulates and chlorides are removed from the cooled syngas in a wet scrubber and char is recycled to the gasifier. The syngas is then treated for removal of carbonyl sulfide (COS) and hydrogen sulfide (H2S) before being used for energy

conversion or other applications.

Figure 1.10 Schematic of the Conoco-Phillips (E-Gas) gasifier (4) PRENFLO (PREssurized ENtrained-FLOw) Gasifier

PRENFLO, which operates at the pressure above 40 bar, is a further development of the Kopper-Totzek process developed in the 1940's. PRENFLO was developed by Uhde, which later merged with Krupp Koppers. PRENFLO is a one-stage, high pressure, dry-fed, oxygen- blown, slagging gasifier. The gas temperature inside a PRENFLO gasifier can exceed 2000°C

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(3600°F) and uses a membrane wall. PRENFLO gasifiers are used in the world's largest solid- feedstock-based IGCC power plant in Puertolanno, Spain.

Figure 1.11 shows an illustration of a PSG (PRENFLO with Steam Generation) gasifier. Coal is injected together with oxygen and steam through several injectors in the lower part of the gasifier. Raw syngas is then sent through the waste heat boiler to cool down and produce steam. The exit gas temperature is 1350-1600°C (~2450-2900°F). In a PDQ (PRENFLOW with Direct Quench), illustrated in Figure 1.11b, coal and oxygen/steam are injected in the upper part of the gasifier. Raw syngas is quenched by water in the direct quench section in the lower part of the gasifier. The syngas is cooled down to 200-250°C (~390-480°F).

(a) PSG (b) PDQ

Figure 1.11 (a) PRENFLO with Steam Generation (PSG) and (b) PRENFLO with Direct Quench (PDQ)

19 (5) Siemens Gasifier

The Siemens gasifier, shown in figure 1.12, is a dry-feed, pressurized, entrained- flow reactor, which can be supplied with either a refractory lining for low ash feedstocks or with a cooling screen in the gasification section of the gasifier. The cooling screen consists of a gas- tight membrane wall structure that is studded and refractory-lined with a thin layer of silicon carbide for protection. The molten slag formed in the gasifier chamber cools and solidifies as it contacts the cooling screen, forming a compact slag layer, protecting it from further damage by the flowing slag. Once a slag layer is formed over the cooling screen, subsequent hot slag flows down the reactor chamber into the quench section of the gasifier where it solidifies upon contact with water from a ring of quench nozzles and is removed through a lock hopper. Feedstocks with an ash content of greater than two percent by weight are preferred when using the cooling screen design; in this scenario the gasifier can achieve carbon conversion rates higher than 99%. Siemens gasifier is suited for coals from anthracite to lignite, as well as biomass, petcoke, and residual oil.

20 (6) MHI (Mitsubishi Heavy Industries)Gasifier

The MHI gasifier is a pressurized, dry-feed, upflow, entrained-flow slagging reactor with a unique two-stage operation. The current focus of the effort is on air-blown (or enriched air blown) IGCC application. R&D activities are being carried out to develop an oxygen-blown system for coal to fuels and chemicals applications.

Figure 1.13 shows a simplified drawing of the MHI gasifier. The reactor consists of two sections (or stages): a lower combustor and an upper reductor. Dry milled coal is fed at two separate points into the gasifier with a portion being fed into the combustor together with air (or enriched air) where it is burned to produce carbon monoxide (CO) and carbon dioxide (CO2), plus water vapor. The temperature generated at the combustor is sufficiently high to melt the coal ash. The molten slag falls to the bottom of the gasifier where it is quenched in a water bath and then removed using a lock hopper system. The gas produced in the combustor rises to the reductor where the remaining coal is added, without any additional air. At the reductor stage, heat provided by the hot combustor gas is used to drive the endothermic gasification reactions. The reductor is operated at a lower temperature than the combustor. Any molten ash carried over by the upward gas is solidified. The syngas produced exits the reductor through a syngas cooler generating steam. A cyclone is used downstream of the sygnas cooler to collect the char and recycle it to the combustor section to increase the overall carbon conversion efficiency. The raw syngas leaving the reductor section of the gasifier is typically at 2,200°F, high enough in temperature that very little hydrocarbon gases and liquids are formed.

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(7) ECUST (East China University of Science and Technology) Gasifier

In the early 1990s, the Institute of Clean Coal Technology (ICCT) at the ECUST began its development of coal gasification technology in partnership with the Yankuang Coal Mine Group. The ECUST gasifier can accommodate either coal-water slurry feeding, or dry-feeding (via N2 or CO2) applications. The entrained flow gasifier incorporates opposed multi-burner

(OMB) technology, and a water-quench in a down-flow configuration. Operating conditions are dependent upon dry/wet feed, as well as the end product. The gasifier temperature and pressures fall in the range of 1,300 to 1,400°C, and 1 to 3 MPa. Figure 1.14 shows schematic of ECUST gasifier.

Figure 1.14 Schematic of the ECUST gasifier

(8) HCERI (Huaneng Clean Energy Research Institute) Gasifier

The Huaneng Clean Energy Research Institute (HCERI), formerly the Thermal Power Research Institute (TPRI), has developed gasification technology that is being used in numerous gasification facilities throughout China. HCERI gasification technology is a two-stage dry-feed and water-cooled gasifier, as shown in figure 1.15. The first stage of the gasifier reacts 80 to 85% of the coal feed with pure oxygen and steam. The steam and the remaining 15 to 20% of the feed coal are fed into the second stage, which operates at about 1400 to 1500°C. The temperature of the outlet syngas is decreased to 900°C due to the second stage's endothermic reaction - helping the slag particles to solidify, as well as improving the gasifier's thermal efficiency. The gasification technology can also be applied to other feedstocks, such as petcoke,

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and low quality coals with high sulfur content. Commercially available sulfur capture equipment can effectively remove up to 99.9% of the sulfur from a gasification gas stream, ensuring the plant's environmental compliance.

2nd stage (coal inlet)

Figure 1.15 Schematic of the HCERI gasifier

(9) EAGLE (Energy Application for Gas, Liquid, and Electricity) Gasifier

EAGLE, an acronym that stands for Energy Application for Gas, Liquid, and Electricity, is a project funded by the Electric Power Development Company of Japan, in collaboration with Japan’s New Energy and Industrial Technology Development Organization (NEDO). The EAGLE gasifier is a two-stage, pressurized, upflow, oxygen-blown, entrained-flow gasifier with the bottom stage operating in the slagging mode, with a second non-slagging stage on top to increase overall gasification efficiency. Figure 1.16 (a) shows a schematic of the gasifier. Being a two-stage reactor, the EAGLE gasifier is similar to the E-Gas™ gasifier and MHI’s gasifier. An unique feature of the EAGLE gasifier is its tangential feed injection and burner system which allows a spiral flow pattern to be developed along the inter-reactor wall between the upper and the lower reactor stage. This flow pattern is claimed to (1) create a longer residence time for the coal particles, and thus increase the overall gasification efficiency, and (2) help facilitate slag removal as the spiral flow pattern creates a pressure differential between the wall

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and the center of the gasifier which help with drawing the slag toward the bottom of the gasifier for discharge. The EAGLE gasifier uses a pneumatic system for dry coal fine feeding, using either nitrogen or recycled gas. Its reactor interior is protected by a water-cooled membrane wall; both features are similar to the Shell and Siemens designs.

Figure 1.16 (b) shows a simplified drawing of the overall EAGLE gasifier vessel, shown with a radiant cooler on top of the gasifier reactor. The gasifier operates the first (bottom) stage at high-temperature slagging conditions with only part of the coal feed, but a relatively larger amount of oxygen.The remaining coal and oxygen are added to the second (top) stage, where the hot gas drives the endothermic gasification reactions. The relative amount of coal/oxygen feed distribution into each stage depends on the nature of the coal. The distribution is optimized for high gasification efficiency versus stable slag discharge. The second stage is non-slagging. The particulate matter in the syngas contains unreacted char and dry ash. They are removed from the raw syngas downstream of the syngas cooler and recycled to the first stage. In this manner, almost all the ash in the system is removed as slag.

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Figure 1.16 (b) Schematic of the Overall EAGLE Gasifier Vessel