The entrained flow reactor (EFR) is well-known in coal combustion processes whilst the expe­rience with biomass is limited. In entrained flow gasifiers, no inert material is present but a finely ground feedstock is required. The fuel particles are fed co-currently with the oxidant agent and subsequently are the particles entrained with the gas stream. The EFR gasifiers operate at temperatures of 1200-1500°C, depending on whether air or oxygen is employed. Examples of entrained bed gasifiers are seen in Figure 6.10.

The temperatures in the EFR gasifiers are much higher than those encountered in fluidized bed gasifiers; hence, the product gas does contain low concentrations of tars and condensable gases. The high temperatures allow for the thermal conversion of tar and also some of the methane, thus the composition of the product gas will be close to those indicated by the chemical equilibrium. This even though the residence times are very short only around 1 second.

The conversion in the entrained beds configurations effectively approaches 100% and exhibit, in theory, the highest capacity of all gasifiers. However, the high-temperature operation creates problems of construction materials selection and ash melting.

Figure 6.10.

After the fuel pre-treatment the feedstock enters the gasifier as a pulverized solid by a pneumatic feeding. The mixing between the fuel and the oxygen should be as good as possible in order to optimize the gasification. Depending on the type of fuel, soot may also be formed. To avoid the soot formation, addition of steam in a proportion of 0.1 kg steam per kg oxygen is often necessary. When coal is used as a fuel it is crushed into a fine powder (~50 ^m). This touches upon the main disadvantage with respect to biomass application, as fine milling of biomass is very costly. This drawback can be partially handled by pre-treating the biomass through e. g. torrefaction.

However, this is at present a relatively unproven technology. In addition, in order to reach these high temperatures more product gas has to be combusted, which will hamper overall efficiency and thus costs. Another possibility is to pyrolyze the biomass to pyrolysis oil to be able to feed the oil into the reactor. This is possible if the pyrolysis oil is directly introduced in the gasifier and not stored since the oxygen-rich pyrolysis most likely will polymerize when stored. The polymerized pyrolysis oil forms two phases where the most energy-rich phase is very viscous and not possible to pump.

Entrained flow gasifiers may be found as slagging and non-slagging. In the slagging entrained flow gasifier configuration, melted slag products, originating from the fuel, are condensed, usually on the reactor wall, as it is the coldest part of the gasifier. The melt will accumulate on the wall, forming a slag layer that protects the wall from the high temperature corrosive atmosphere of the gas. The liquid slag is removed from the bottom of the gasifier. In order to get a liquid slag with the right viscosity at the given temperature, so-called fluxing material must usually be added. In coal-based power plants, limestone or other Ca-rich materials are often added into the bed.

The non-slagging entrained flow gasifiers do not produce any slag due to small amounts of minerals/ashes in the fuel. A small amount of soot is often deliberately generated in order to get condensation surfaces in the gas bulk by nucleation to preventing fouling of the walls by the slag.

Entrained flow gasifiers may also be used for gasifying black liquor. An example is the operation at SCA Ortviken in Sweden with recovery of liquors from a Ca-sulfite plant during 1970 and 1986. The reactor had injection of black liquor together with oil in the upper center and air was introduced around it. The residence time was very low, just a few seconds, and the temperature around 725°C. The gas was cooled in a waste heat boiler and solids separated in a Venturi scrubber. Since black liquor consists of large amount of alkalis sever materials problem is to be expected at high temperature. AT SCA Ortviken, the material problems were handled in the reactor and proved that at least 725°C was practical for long-term operations using Hoganas bricks as insulation inside the reactor.