Gasification and Emissions

Extracting the energy of biomass through combustion or incineration, if uncontrolled, leads to emission into the atmosphere of large amounts of Nitrogen Oxides, Sulfur Oxides, Carbon Monoxide, as well as ash and other particulate matter. A number of mechanisms have been devised to eliminate or abate those emissions. However, they are notorious for being expensive and impose large parasitic loads on the power generation system.

On the other hand, gasification in general is inherently cleaner and the BCT system is cleaner than average. The emission of nitrogen oxides, sulfur oxides and particulate matter from the BCT system syngas is summarized in Table 3. These emissions are below that of the 2005 EPA New Source Performance Standards (NSPS) for Power Plants (EPA, 2005) summarized in Table 2 below.

Although BioConversion Technology, LLC has not measured the emissions from a reciprocating engine/generator using their syngas as a fuel, it is expected that the emissions of these three pollutants should not be greater than those values listed in Table 3 for syngas that was flared.

Table 2
2005 EPA New Source Performance
Standards (NSPS) for Power Plants 1

1 U.S. EPA (2005)

Table 3
The Emission Level of Nitrogen Oxides Sulfur Oxides
and Particulate Matter from Flared BCT Syngas

The results above result from processing approximately 7.5 tons per day of wood waste, a fairly typical biomass feedstock. If the gasifier were operated at that rate on a 24/7 basis it would result in the emission of only 1/2 ton per year of NOx.

The USEPA has recognized that emissions from modern gasifiers are far below Clean Air Act Standards. Emissions of every measured solid and gaseous pollutant from the gasification process are substantially lower (and in some cases, by orders of magnitude) than emissions from traditional incinerator combustion plants. Of particular importance is the absence of hazardous emissions of dioxins and furans that derive primarily from plastics and excess oxygen in the material stream.

Compared to Current Gasifier Designs

The benefits of the BCT Gasifier are premised on a number of realizations. First, it was determined that, all things considered, an entrained flow design could substantially reduce the amount of the ash. However, unlike other entrained flow designs, or designs which use air or oxygen to propel the feedstock through the direct fired reaction zone, the BCT system uses the produced syngas and process steam to propel the feedstock through the segregated steam reforming reactor. Among other benefits, this technique raises the caloric value of the syngas by not diluting the product syngas with nitrogen or carbon dioxide as is the case with an air blown gasifier. Nor does it require a costly separate supply of oxygen or the elevated temperatures and run away reaction issues associated with the use of an oxygen blown gasifier. The design and operation of the gasifier could thus be greatly simplified.

Devolatization Reactor and Process

Second, it was observed that purer syngas without significant amounts of carbon dioxide can be produced by controlling the oxidation of the feed material. More particularly, the BCT Gasifier employs a separate devolatization reactor and process which slowly raises the temperature of the feed material above 450°F (the temperature at which combustion will occur) not before a substantial portion of the oxygen has reacted with more reactive material in the feed such as hydrocarbons and the like. Once the available oxygen has been reacted at below combustion temperature, the feed material temperature is raised to a higher temperature, for example 650°F, prior to combination with super heated steam (1500°F) and a subsequent rise in temperature to react with the carbonaceous feed material to produce carbon monoxide and hydrogen, i.e., syngas. This reforming step produces a consistent and clean syngas composition. Additionally, this approach had the salutary effect of avoiding flash pyrolysis of the volatile organic compounds released from the feedstock while reforming them into useful product gas.

Two important benefits of this multi-stage approach as reported in the research literature are that

1. It results in very high conversion efficiency, and
   
2. The tar content in the produced gas is extremely low.

Unfortunately, the uncontrolled combustion reaction has several undesirable results. Primarily it produces carbon dioxide (CO 2) which must be removed. Further, it produces large amounts of ash and further creates slag. Basically, it raises the temperature of the feed material to above the melting point of the ash, forming slag. Slag is unusable and frequently requires expensive disposal measures.

Gasification And Steam Reforming

The gasification agent is super heated steam. It is well known, that gasification with steam results in higher reaction rates than gasification with CO2. If steam is the primary gasification agent or moderator, the temperature in the gasifier can be lower. There are several other advantages related to steam reforming gasification.

• Lower temperatures: Reforming temperatures are lower within a controlled environment.


• Lower soot production: This is partly due to lower maximum temperatures and partly due to a higher steam carbon ratio which reduces the soot production.


• Lower emissions: The gas composition is different when steam is added as a gasification agent. The H 2 content is increased and the CO 2 content is lowered. This results in a faster combustion and lower emissions of SOx and NOx When the syngas is burned.

Quench Water Cleaning; Ash Characteristics

Testing has shown that problematic metals and other compounds associated with feedstocks like coal, MSW, construction waste, poultry litter, etc., including material like mercury and chromium report to either the quench water or the ash. A unique feature of the BCT system is that it employs a closed-loop, zero emissions system by converting all such harmful compounds in the quench water to inert, non reactive, harmless oxides. Clean water is recirculated to the steam generator and gas cleaning circuits.