Gasification is a process by which either a solid or liquid carbonaceous material, containing mostly chemically bound carbon, hydrogen, oxygen, and a variety of inorganic and organic constituents, is reacted with air, oxygen, and/or steam. The reactions provide sufficient exothermic energy to produce a primary gaseous product containing mostly CO, H2, CO2, H2O(g), and light hydrocarbons laced with volatile and condensable organic and inorganic compounds.
Most of the inorganic constituents in the feedstock are either discharged as bottom ash or entrained with the raw product gas as fly-ash. Unless the raw gas is combusted immediately, it is cooled, filtered, and scrubbed with water or a process-derived liquid to remove condensables and any carry-over particles.
Alternatively, the raw gas can undergo either medium-temperature (350° C. to 400° C.) or high-temperature (up to gasifier exit temperatures) gas cleaning to provide a fuel gas that can be used in a variety of energy conversion devices, including internal combustion engines, gas turbines, and fuel cells.
Biomass when gasified with steam and/or oxygen will produce “synthesis gas,” rich in CO and H2, which in turn can be catalytically converted to produce high-value fuels and chemicals.
In contrast to coal, which is currently used in several commercial gasification processes, biomass is more reactive and can be effectively gasified at lower temperatures. However, unlike coal and petroleum, biomass resources are dispersed and heterogeneous in nature. Consequently, special solids handling and feeding systems have to be designed, taking into consideration the heterogeneous nature and the low bulk density of biomass. The fibrous nature of herbaceous feed stocks means they are more difficult to handle than woody biomass. Another frequently encountered problem is the low-ash fusion temperatures of certain biomass, particularly under reducing conditions, which require special care in the design and operation of biomass gasifiers.
In one attempt to provide a superior biomass gasification process, the SilvaGas gasification process was developed by Future Energy Resources Corporation FERCO, (presently SilvaGas Corporation) to provide a means to convert a range of solid biomass fuels into a medium calorific value gas that can be directly substituted for natural gas, or as an input for chemical synthesis applications. For gas turbine power applications, the use of biomass fuels from the SilvaGas process provides a means to achieve high overall power generation efficiencies without introducing additional greenhouse gases to the environment. By converting the biomass into this high energy density gaseous fuel, significantly higher power generation efficiencies were achieved relative to direct combustion based systems (approximately 40% power generation efficiency compared to a maximum of 25% with conventional biomass systems (HHV basis)).
Unlike other biomass gasification processes, the SilvaGas biomass gasification process is not based on starved air combustion, but rather rapidly heats raw biomass in an air-free environment to generate gas, and a solid residue char that is used as a heat source for the biomass heating. Significantly fewer emissions are produced in the process because not having oxygen in the gasifier makes it impossible to form dioxins if a chlorine containing feed, such as processed municipal solid waster or recycled paper pulp sludges, is used. In addition, cleaning the high energy density, medium heating value gaseous product is simplified because the gasifier product gas is much lower in volume than the gas from an “air blown” gasifier.
The process was specifically designed to take advantage of the unique properties of biomass, such as high reactivity, low ash, low sulfur, and high volatile matter. The reactivity of biomass is such that throughputs in excess of 14,600 kg/hr-m2 (3000 lb/hr-ft2) were achieved. In other gasification systems, throughput is generally limited to less than 500 kg/hr-m2 (100 lb/hr-ft2). Other competing biomass gasification processes were either developed originally for coal gasification or were heavily influenced by coal gasification technology and therefore do not take full advantage of the properties of biomass.
In the SilvaGas process, biomass is indirectly heated using a hot sand stream to produce a medium calorific value gas (approximately 17 to 19 KJ/Nm3). The process uses two circulating fluidized bed reactors as the primary process vessels. One circulating fluidized bed is the gasifier in which the biomass is heated and pyrolyzed to produce a product gas which conveys the sand and residual char from gasification out of the gasifier. After separation of the sand and char from the product gas, the sand and char flow into the circulating fluidized bed process combustor where the char is completely combusted to reheat the sand for return to the combustor.
While the SilvaGas process has provided numerous advantages over conventional biomass conversion systems, there is still room for improvement. Therefore, it can be seen that a need yet exists for an enhanced biomass gasification process that provides the same level of biomass carbon converted to the desired product gas process as was achieved in the conventional SilvaGas process, but at much lower gasifier temperatures. It is to such a process that the present invention is primarily directed.