Most of the gasifiers developed since 1950 are of the entrained flow type. The advantages of using entrained flow gasifiers lie in their flexibility in handling any type of coal as feedstock to produce clean, tar-free product gas. With the development of the Integrated Gasification Combined Cycle (IGCC) as a prospect technology for overcoming greenhouse gas emission issues and being more efficient, use of entrained bed gasifiers will further increase in the future for power generation. Entrained bed gasifiers operating at high pressure can supply the product gas at high pressure to the IGCC system without additional compression.

In the entrained flow gasifier, a dry pulverized solid is gasified with oxygen (much less frequently, air) in co-current flow. The gasification reactions take place in a dense cloud of very fine particles (typically <100 pm). The much smaller biomass particles mean that the fuel must be pulverized, which requires somewhat more energy than for the other types of gasifiers. Entrained flow gasifiers operate at high temperature (1300-1500°C) and high pressure (20-50 bar), and thus high throughputs can be achieved (Drift et al., 2004). The high temperatures also mean that tar and methane are not present in the product gas. Thermal efficiency is, however, somewhat lower, as the gas must be cooled before it can be cleaned with existing technology. By far the greatest energy consumption related to entrained bed gasification is in the production of oxygen used for the gasification.

There are two types of entrained bed gasifiers: slagging and non-slagging. One differs from the other by the way in which the ash is removed from the system. If the ash is removed in molten form, then it is of the slagging type. If the ash is removed in solid form, then it is of the non-slagging type. To ensure the proper operation of the slagging type, the flow of molten ash should be 6% of the fuel flow. The non-slagging type is mostly favored if the ash content of the fuel is below 1% (Drift et al., 2004).

To feed the fuel at higher pressure, the size of particles needs to be very small. This limits the use of biomass as fuel, as it is fibrous in nature and very difficult to cut into smaller sizes. Also lower bulk density and low heating value reduces its suitability as fuel for entrained bed gasification. To use biomass as fuel, a larger amount of carrier gas is required. This means higher energy for compression of the gas and also a product gas with a poor heating value due to dilution with the carrier gas. In the case of pneumatic feeding, the power penalty is high. For instance, pressurizing biomass up to 40 bars using pneumatic feeding consumes

°.°25 kW^Wth wood reducing efficiency by °.°4 kWsyngas^Wth wood (Drift et aU

2004).

Figure 16.10 shows different types of the entrained bed gasifier. The Siemens EGB gasifier consists of a top fired reactor. The reactants are introduced into the reactor through the single centrally mounted burner. This process has some special advantages: it provides axis symmetrical construction, reducing equipment costs, flow of the reactant is from a single burner, thus reducing the number of points to be controlled, and, lastly, the product gas and slag flow in the same direction, reducing any potential blockage in a slag trap (Higman and Burgt, 2008).

Koppers-Totzek atmospheric process is the first entrained flow slagging gasifier operated in atmospheric pressure. This process has been commercially built mainly for the ammonia manufacturing process. It consists of two-side mounted burners, where a mixture of coal and oxygen are injected. The gas leaving at the top at a temperature around 1500°C is quenched with water first. The reactor has a steam jacket to protect the reactor shell from the high temperature (Higman and Burgt, 2008).

The E-Gas gasifier is a two-stage coal/water slurry feed entrained flow slagging gasifier. It is designed to use sub-bituminous coal. The coal slurry is fed in at the non-slagging stage, where the upward flowing gas gives heat to it, thus the gas exits at a lower temperature. The gas is then passed through a fired tube boiler and is filtered in a hot candle filter. The char is separated out at the hot candle filter and is again taken back to the slagging zone of the gasifier. The slag is quenched in a water bath at the bottom of the slagging reactor (Higman and Burgt, 2003).

The British Gas/Lurgi proposes a novel coal gasifier. It is a dry fed, pressurized, fixed bed slagging gasifier. Oxygen and steam are introduced into the gasifier vessel through sidewall-mounted tuyeres (lances) at the elevation, where combustion and slag formation occur. The coal mixture (coarse coal, fines, briquettes, and flux), which is introduced at the top of the gasifier via a lock hopper system, gradually descends through several process zones. Coal at the top of the bed is dried and devolatilized. The descending coal is transformed into char and then passes into the gasification (reaction) zone. Below this zone, any remaining carbon is oxidized, and the ash content of the coal is liquefied, forming slag. Slag is withdrawn from the slag pool by means of an opening in the hearth plate at the bottom of the gasifier vessel (Phillips).

The Hitachi gasifier is an oxygen blown entrained gasifier where the pulverized coal is fed at two stages. At the upper stage, two burners are arranged tangentially to feed the pulverized coal spirally into the gasifier. This gives a swirl motion to the coal, thus increasing the residence time. Oxygen in excess is supplied at the lower zone to melt the slag. In the upper stage, reaction occurs at relatively lower temperatures in the presence of less oxygen. Thus, the coal particles get de-volatized and the char formed moves down to be reacted with high temperature gas.

BFW

Oxygen

(a) Koppers-Totzek EBG
(b) BLG slagging EBG

Siemens EBG

(d) MHI air blown EBG

16.10 Different types of entrained bed gasifier (a) and (c), Higman and Burgt (2008); (b), Basu (2006); (d), Higman and Burgt (2008); (e), EPRI/ Advanced Coal Generation).

Table 16.4 Comparison of different types of entrained bed gasifier (De Souza, 2004)

The Shell Coal Gasification Process can gasify any type of coal that can be pulverized to the right size and pneumatically transported. Buggenum in The Netherlands was the first IGCC plant built using SCGP with a capacity of 2000 tons/day (see Table 16.4).