Fluid-bed gasifiers

cold gas efficiencies of 80-85% should be reached [120,121]. The upper water-cooled wall is made of a special ceramic-matrbc-composite material and is followed by a full water quench to 370 °C at the bottom. A test plant is in operation at the Gas Technology Institute in Des Plaines, Illinois, since 2011 and a 300 t/d dry solids extrusion pump is tested since 2012 at the Energy and Environmental Research Center in Grand Forks, North Dakota [122,123]. 

Because gasifying agents serve not only as gasification reactant but also as fluidization blast, control is complex and start-up, shutdown, and load change remain in certain limits. 

Fluidization occurs in stationary (segregation of bed and free board), turbulent (circulating fluid bed), and forced dense-phase (transport reactor) mode. All systems require hot gas cyclones either to recycle entrained particles back into the fluid bed or just to separate them from the product gas. 

Equilibrium models with systematic temperature-dependent deviations (see Section 5.6.1) are a well-known measure to lump all effects in a fluid-bed in simple correlations. More sophisticated models can be a nearly ideal continuously stirred tank reactor because of the good mixing inside the bed. For transport reactors, plug flow models are suitable. 

The available technologies are discussed in the following section with focus on high-temperature Winkler (HTW) technology. 

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The agglomerating fluid-bed gasifier can be blown by either air or oxygen at 2.8 MPa (406 psia) pressure. Pressurized operation has sev-... 

In the process, excess coke is converted to a low-heating value gas in a fluid bed gasifier with steam and air. The air is supplied to the gasifier to maintain temperatures of 830-1000°C (1525-1830°F), but this is insufficient to bum all of the coke. Under these reducing conditions, the sulfur in the coke is converted to hydrogen sulfide, which can be scrubbed from the gas prior to combustion. 

While the evaluations of cost and plant size discussed in the preceding paragraphs have been devoted to fixed bed,systems, the conclusions are valid for all coal gasification techniques. Estimates of fluid bed gasifiers have also been prepared. (2) Unfortunately, insufficient data are available to substantiate the operability and actual productivity which must form the basis for any cost estimate. Using these tentative costs, we find that costs for LBG or MBG might be below 3.00/million Btu. For entrained flow systems, still under development, costs in the below 3.00/million Btu range are estimated. However, actual implementation of these advanced systems or even state-of-the-art... 

The advanced pressurized fluid-bed gasifiers currently ready for demonstration have internal diameters of around 3.6 m (12 ft) and are 20 to 28 m (66 to 92 ft) in height. Production of more than 100,000 mVh of gas (at gasifier temperature and pressure) is achieved by the advanced gasifiers, equivalent to 33,260 kg/h of bituminous coal (air blown). 

This process converts crushed coal, oxygen, and steam at 820 to 1000°C and near-atmospheric pressure in a fluid-bed gasifier. After passage of the raw gas through a waste heat recovery section, flyash is removed by cyclones, wet scrubbers, and electrostatic precipitators. Further processing, depending on end use, yields a gas suitable as synthesis gas or pipeline gas. 

Tampella Corporation is commercializing the U-GAS process, which was developed by the Institute of Gas Technology. Tampella has constructed a 10-MW, integrated U-GAS-combined cycle power plant in Finland that uses coal, peat, and wood wastes as feedstocks. U-GAS incorporates a single-stage, fluid-bed gasifier in which coal reacts with steam and air at 950 to 1090°C at pressures... 

At least five industrial-scale biomass gasifiers were available commercially from U.S. manufacturers in the 1990s. A two-stage stirred-bed gasifier is available from Producers Rice Mill Energy. The company built three gasifiers of 11 to 18 t/day capacity in Malaysia for rice hull feedstocks. Sur-Lite Corporation built small-scale, fluid-bed gasifiers of up to 10 GJ/h capacity for cotton-gin... 

RDF Gasification in an Atmospheric, Air-Blown, Circulating Fluid-Bed Gasifier... 

Table II. Conditions of Operation and Results of Tests Using Pretreated Coals in the Fluid Bed Gasifier at 2.5 atm.

In an attempt to overcome these difficulties, the system has been revised as shown in Figure 2. Now the coal is fed into the top of a 10-ft. long, 6-in. diameter pipe and falls freely through an upward flow of gas from the 3-in. diameter fluid bed gasifier. As it drops, the coal is carbonized and devolatilized, and by the time it enters the fluid bed it should be noncaking. Preliminary tests of the revised system indicate that it is operable, and the methane yield is higher owing to countercurrent flows. 

Figure 2. Free-fall reactor and fluid bed gasifier system Conclusions...

The gasification technologies are normally classified into 3 groups moving bed gasifiers fluid bed gasifiers and entrained flow gasifiers. The main characteristics are summarised in Table 1.16. Typical raw gas compositions are shown in Table 1.17. 

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