The first fluidized bed gasifier was developed by Fritz Winkler of Germany in 1921. It was later used for powering gas engines. From 1921, several companies were involved in making fluidized bed gasifiers which proved to be more efficient and competitive with other technology.

These types of gasifiers provide excellent gas-solid mixing. Fluidized bed gasifiers can be operated at lower temperatures — around 800-900°C — than fixed bed gasifiers. This directly affects NOx emission reduction. Also better fuel flexibility and efficiency in process carbon dioxide capture are some of the advantages of this type of gasifier.

A fluidized bed gasifier mainly consists of a bed of hot solid that is fluidized by the gasifying agent (air, oxygen, or steam). When the feedstock is fed into the hot bed, it undergoes gasification in the presence of a gasifying agent and the product gas leaves from the top of the gasifier. If the bed solid leaving the furnace is captured and again re-circulated into the gasifier, it is called a circulating fluidized bed (CFB) and if not then it is a bubbling fluidized bed (BFB). The bubbling bed gasifier is generally operated at a lower velocity (2-2.5 m/s) to ensure particles do not leave the reactor. The circulating fluidized bed is operated at higher velocity (3-5 m/s) and particles leaving the reactor are separated in a cyclone and fed back into reactor. CFB gasifiers are very suitable for large-scale syngas production (Tijmensen et al., 2002; Hamelinck et al., 2004; Wang et al., 2008; Zhang, 2009). CFB gasifiers are also considered to be rather fuel flexible and are most suitable for feedstocks with high volatile matter content and high char reactivity, such as biomass. Moreover, they offer short residence time, high productivity, low char/ tar contents, high cold gas energy efficiency and reduced ash-related problems (Wang et al, 2008). The gas produced by CFB gasifiers, operated at ~900°C contains, however, beside H2 and CO, considerable amounts of CO2, H2O, and

16.7

Temperature distribution along the height of the fluidized bed gasifier (Higman and Burgt, 2003).

hydrocarbons like CH4, C2H4, benzene, and tars. Thus, the product needs further treatment in a catalytic reformer to convert the hydrocarbons to H2 and CO.

Temperature distribution along the height of the fluidized bed gasifier is shown in Fig. 16.8. In the case of fluidized bed gasifiers, the temperature is more uniformly distributed.

Figure 16.9 shows different types of fluidized bed gasifiers that have been commercially developed. The Winkler gasifier, invented in 1920, was probably the first type of gasifier to use fluidization on an industrial scale to gasify pulverized coal and started with a capacity of 2000 m3/h of product gas. Foster Wheeler CFB is an air blown gasifier operating at atmospheric pressure. Depending on the fuel and the application needs, it operates at a temperature within the range of 800- 1000°C. The hot gas from the gasifier passes through a cyclone, which separates most of the solid particles associated with the gas and returns them to the bottom of the gasifier. In the twin reactor gasifier, the pyrolysis, gasification, and combustion take place in different reactors. In the combustion zone, the tar and gas produced during pyrolysis are combusted and heat the inert bed material. The bed material is then circulated into the gasifier and the pyrolysis reactor to supply heat. The char and heat carrier from the pyrolyser are taken into the gasifier. The gasification of char in the presence of steam produces the product gas. The residual char and the heat carriers from the gasifier are taken back into the combustor. This system was developed to overcome the problem of tar. The KBR transport gasifier is a hybrid gasifier having characteristics of both entrained bed gasifier and fluidized bed reactor. The KBR gasifier operates at considerably higher circulation rates, velocities (11-18 m/s), and densities than a conventional circulating fluidized bed. This results in higher throughput, better mixing, and higher mass and heat transfer rates. The solids that are transported are separated from the product gas in two stages and returned to the base of the riser. The gasifier operates at 900-1000°C and 11-18 MPa (Higman and Burgt, 2008). EBARA’s TwinRec

Process gasifier is used primarily to recover recyclable materials by removing their organic components through gasification and combustion (Steiner et al., 2002). Bharat Heavy Electrical Limited (BHEL) developed a pressurized fluid bed gasifier to take into account the higher ash conent of coal. Raw product gas from the cyclone is cycled and mixed with the feedstock in the drier zone. Again the feedstock is separated and cooled gas is taken for cleaning while the feedstock is supplied to the gasifier. BHEL is developing a 125 MWe IGCC demonstration plant at Auraiya in Uttar Pradesh, India (Higman and Burgt, 2008).