For thermal gasification of the refractory components of biomass (those diffi­cult to gasify) such as lignin, the minimum temperature requirement is similar to that for coal (~900 °C) (Higman and van de Burgt, 2008, p. 147). Entrained- flow gasification of biomass is therefore rather limited and has not been seen on a commercial scale for the following reasons:

• The residence time in the reactor is very short. For the reactions to complete, the biomass particles must be finely ground. Being fibrous, biomass cannot be pulverized easily.

• Molten ash from biomass is highly aggressive because of its alkali com­pounds and can corrode the gasifier’s refractory or metal lining.

Given these shortcomings, entrained-flow gasifiers are not popular for biomass. However, there is at least one successful entrained-flow biomass gasifier, known as the Choren process.

Choren Process

The Choren entrained-flow biomass gasifier is comprised of three stages (Figure 6.18). The first stage receives biomass in a horizontal stirred-type low-temper­ature reactor for pregasification at 400 to 500 °C in a limited supply of air. This produces solid char and a tar-rich volatile product. The latter flows into the second chamber (stage 2), an entrained-flow combustor where oxygen and the product gas from the first stage are injected downward into the reactor. Com­bustion raises the temperature to 1300 to 1500 °C and completely cracks the tar. The hot combustion product flows into the third chamber (stage 3), where the char is gasified.

The solid char received from the first stage is pulverized and fed into the third stage of the Choren process. It is gasified in the hot gasification medium produced in the second stage. Endothermic gasification reactions reduce the

temperature to about 800 °C. Char and ash from the product gas are separated and recycled into the second-stage combustor. The ash melts at the high tem­perature in the combustor and is drained from the bottom. Now molten, the ash freezes, forming a layer on the membrane wall that protects the wall against the corrosive action of fresh molten biomass ash. The product gas is processed downstream for Fisher-Tropsch synthesis or other applications.