Method of biomass gasification

This invention generally relates to bio-fuels, and in particular to a method of carrying out biomass gasification.

Biomass is receiving increased attention as a renewable energy source, in this era of emphasis on energy security and increasing environmental concerns.

Multiple ways of utilization of biomass are being tried. Traditionally, the most common method of using the biomass has been to burn it, to provide heat energy. This heat energy is then used directly, or used to produce steam.

It has been demonstrated that gasification of biomass is a better option than burning it. Gasification produces a clean burning gas that can be easily utilized. The product gas can be used as a feedstock for production of value added fuels and chemicals. The burning of this gas in internal combustion engines or turbines offers higher efficiencies, as compared to steam devices.

Historically, gasification technology was used to convert coal into a combustible gas. This gas was in some places also distributed as coal gas or town gas. Biomass was traditionally used for direct firing, or was carbonized to convert it into charcoal. Charcoal was then further used as fuel directly, or was gasified to produce a combustible gas. Direct gasification of biomass has also been attempted with different gasifier configurations, in an attempt to produce a combustible gas from the biomass. Although gasification of biomass is a desired option, there are many difficulties encountered, due to the inherent nature of biomass. Moreover, there are operational difficulties associated with the process.

Biomass encompasses a wide spectrum of material. Within different types of biomass, on the elemental level, there is a wide variation in carbon, hydrogen, and oxygen content. Also, there are other components such as moisture and ash, and the content of these components varies widely in different feedstocks. These components do not add to the calorific value of the feedstock. Thus, high ash content, high moisture content, and high oxygen content, implicitly suggest feedstocks with lower calorific value as compared to coal—the most common solid fuel. There are many differences between coal and biomass as a feedstock for gasification. As an example, due to the higher moisture content often present in biomass, a considerable amount of energy is required for drying the biomass.

Biomass also has a larger amount of total volatile matter, as compared to coal. During gasification, the volatile matter is released at relatively lower temperatures. A large part of this volatile matter is responsible for the tar formation during gasification of biomass. Tar levels in the product gas also depend on the gasifier configuration. In an updraft fixed bed gasifier that operates between about 300° C. and about 1000° C., the product gas contains up to 35000 ppm tar. In a downdraft configuration, the tar level in product gas is comparatively low, but still contains about 500 and about 1000 ppm tar. Depending on the gasifier configuration, tars tend to deposit on the walls of gasification equipment, or get transported downstream with the product gas. The tar deposits create the need for frequent maintenance, and may also reduce the operating life of the gasification equipment. The carryover of tar can deposit on and block filters, pipes, valves and turbochargers, leading to a decrease in performance. In some instances, elaborate cleaning systems are required to address these problems.

Most of the biomass gasifiers commercially available today operate at or near atmospheric pressures. If these gasifiers are operated at elevated pressures, higher overall thermal efficiencies can be achieved. However pressurized biomass gasifiers have their own set of problems. One significant problem relates to the feeding of the biomass into the gasifier. The characteristics of biomass, such as low density, varying particle size, and varying moisture content, lead to problems like clumping and interlocking of particles of different sizes.

Handling and transportation of solid biomass feedstock may involve transportation through bins or bags. Frequently, mechanical transportation such as pneumatic transport, or mechanical transport such as belt conveyer, chain conveyer, screw conveyer, bucket elevator, or vibrating pans, are employed. Such transport systems are usually adequate under atmospheric pressure conditions. However if the gasification of biomass is carried out at an elevated pressure, it may be necessary to operate the biomass feeding system at a pressure higher than the gasifier itself. This may require use of systems such as lock hoppers or piston feeders. Many practical difficulties have been faced in maintaining such feeding systems at high pressures. Lock hopper systems do not have large turndown capability, and require the use of a considerable quantity of inert pressurizing gas for the lock hoppers or piston feeders.

Hydraulic conveying of solid particles has been successfully deployed for coal gasification at higher pressures. This typically involves forming a slurry of coal with water, and feeding the same to the gasifier. Slurry feeding has an advantage in that the solid suspension can be directly pumped into the gasifier. However, water slurries involve energy penalties of evaporating the water in the gasifier. A coal gasification unit is able to sustain such energy penalties due to the inherent, high energy content of coal. However, if a water slurry is used for feeding biomass feedstock to a pressurized biomass gasifier, it may not sustain the gasification, due to the lower energy content of biomass feedstock.

Thus, there is a need to develop new methods for efficiently gasifying biomass—especially at elevated pressure—while minimizing problems like tar formation.

Briefly, some embodiments of the present invention provide for a method of biomass gasification wherein the biomass feedstock is combined with a light hydrocarbon composition to form a slurry, and the slurry is then fed to a gasifier to produce a fuel gas.

According to some embodiments of the present invention, a method for the gasification of biomass comprises the steps of combining a biomass feedstock with water to form a slurry, feeding the slurry to a gasifier to produce a fuel gas, and injecting a light hydrocarbon into the gasifier, to generate gasification temperatures greater than about 900° C., by partial or complete combustion of the light hydrocarbon.

According to some other embodiments of the present invention, a method for the gasification of biomass comprises the steps of gasifying a biomass feedstock in a gasifier to produce a product gas, feeding the product gas to a reformer section, injecting a light hydrocarbon into the reformer section, and generating temperatures of greater than about 1000° C., by partial or complete combustion of the light hydrocarbon, in the reformer section.