ALTERNATIVE CONSTRUCTIONS AND GAS FLOW ARRANGEMENTS FOR IMPROVED FIXED BED GASIFIER CONTROL AND THERMAL PERFORMANCE
Publication Date: 2016-Oct-25
The IP.com Prior Art Database
A technique to control and suitably distribute, in an adjustable manner, a flow of reacting and heat exchanging gases through a moving porous bed of a fixed bed gasifier is proposed. The technique includes more than one inclined feedstock passages that allow solid fuel to converge to a single oxidation zone. Feedstock drying, heating and pyrolysis occur in every conduit that lead to a common reactor region where oxidation takes place. An embodiment of the technique includes a gasifier geometry and an L-turn shaped feedstock passage that allows moving the feedstock when transitioning to a common oxidation region. At the common oxidation region, there is an opportunity of feeding oxidant through injection orifices both at the top and/or on a periphery of a cylindrical oxidation zone. This thus makes possible a more uniform oxidant distribution within the oxidation zone cross section, maximizing overall tar exposition to high temperature and its consequent degradation. Such an efficient gasifier construction also minimizes a possibility of having tar rich pyrolysis gases bypassing oxidation process due to insufficient exposure to oxygen. Another embodiment of the technique includes a mechanism for maximizing fixed bed gasifier performance. One other embodiment of the technique includes a mechanism that employs multiple injection and suction tubes to enable effective distribution and collection of gases within gasifier porous bed.
The present disclosure relates generally to a fixed-bed gasifier for biomass gasification and more particularly to a technique for controlling and suitably distributing flow of reacting and heat exchanging gases through a moving porous bed of fixed bed gasifiers.
A fixed-bed gasifier, typically, a down draft gasifier, generally includes a gas injection port through which a feedstock, typically, biomass is fed into a reactor. The feedstock is fed from a top opening of the gas injection port along with some oxidants, frequently air. Figure 1 depicts a schematic representation of a traditional downdraft gasifier.
As charge of biomass required for gasification decreases, there is tremendous increase in temperature inside the gasifier. Consequently, the gasifier experiences drying, pyrolysis, oxidation of tar gases and, finally, reduction in combustion products. During pyrolysis, solid materials fed into the gasifier are thermos chemically broken into char and a variety of heavy molecular weight components that, at typical operating temperatures, are in gaseous form. After pyrolysis zone, the additional oxidant feed allows for incomplete, fuel rich, combustion of pyrolysis gases. Tar, which is released from the pyrolysis zone, is decomposed as a result of exposition to high temperatures in an oxidation region. Further, carbon dioxide and water formed in the oxidation region react in a reduction zone with a solid charcoal left from the pyrolysis zone. Reduction reactions are known to be endothermic. As a result, temperature of gaseous stream decreases as thermal energy is stored into the chemical bounds of forming carbon monoxide and hydrogen.
In case oxidation media, preferably air, is fed radially into the reactor as depicted in Figure 1, there exists a challenge to ensure sufficient penetration of the oxidation media into a descending stream of solid particles. In the event that formation of the oxidation zone is not uniform over an entire cross section, danger exists that pyrolysis vapors are discharged along with a producer gas. Tar gases are not problematic as such gaseous fuel is intended for prompt combustion in a boiler of oven as the gaseous fuel leaves the gasifier. In contrary, if producer gas is intended for use in an internal combustion engine, tar is required to be avoided. At lower temperatures, tar condensates deposits over engine mechanisms, thereby threatening reliability of a prime mover.
Conventional techniques suggest use of an additional feed of oxidant to the combustion zone through a central pipe. Feeding additional amount of oxidant helps to bring a more uniform distribution of higher temperature areas over reactor cross section, thereby, reducing tar concentration in the producer gas that leave the gasifier. Figure 2 depicts use of an additional oxidant feed through a central pipe as desc...