The gasification process starts with a high temperature reactor, where dried biomass (<15% moisture content) is converted into synthesis gas. Gasification reactors resemble tube furnaces, with a fuel inlet at the top and a grating at the bottom. A primary difference between a furnace and a gasifier is the stratification of different layers, or zones inside the reactor. With a furnace, the combustion is at the bottom; near the grating; fuel is added to the top, with gases and smoke exhausting out the top.
With a gasifier, there is also a combustion, or oxidation zone, however it is positioned higher up in the tube, and there is another zone below, called the reduction zone, where the synthesis gas is produced. Basically, in the oxidation zone the fuel is initially burned same as in a furnace, with the hydrocarbons in the fuel converting to carbon dioxide (CO2) and water vapor (H2O). This reaction is exothermic and produces a large amount of energy as heat; but in the reduction zone below, the CO2 and H2O convert to carbon monoxide gas (CO) and hydrogen gas (H2).
These reactions are endothermic and need a lot of energy to take place; the oxidation zone directly above supplies the necessary energy to form these gases. These two gases are the main combustible ingredients of the synthesis gas. Other combustible hydrocarbon gases like methane (CH4) and others are also present from incomplete combustion.
These are called pyrolysis gases, and they increase the caloric value of the synthesis gas. There are typically small amounts of other more complex hydrocarbons (tars) also present in the gas due to incomplete combustion, along with the incombustible components; Nitrogen gas (N2), and typically some unreacted CO2 and H2O. So simply explained, a gasifier is a type of furnace arranged so that there is a reduction zone underneath the traditional oxidation zone. Another major difference between a furnace and a gasifier is that there is no exhaust of gases and smoke; all the volatiles emitted become synthesis gas, with no other exhaust emitted.
The Biogen Advantage
The Biogen gasification process is unique and superior because of a combination of unprecedented temperature control in the reactor with our exclusive closed loop tar recovery process and water catalyst reduction. These systems are combined to provide an environmentally friendly virtually no waste closed loop process when following the approved process.
We employ custom specialized systems that are utilized to clean synthesis gas virtually tar-free and particle free, and the filtered tars/particles are recycled as fuel in the reactor. The result is exceptionally clean synthesis gas that will run a wide variety of generator engines and boilers in a closed-loop process, and the ability to operate with the widest variety of biomass.
The Biogen energy process has:
- Higher efficiency than biomass processes for steam, power or CHP (boiler).
- Significantly less investment in CAPEX and OPEX
- Scalable process solution from 250KWe to 20MWe equivalents in gas production at a single site which allows for quick installation and upgrade over time
- Production of Biochar as a by-product for resell directly for soil enhancement or further processed to fully activated carbon for filtration
- Wide range of engine and burner compatibility due to the gas cleaning process
Biogen gasification systems achieve the unique ability to gasify difficult fuels successfully by a combination of specialized downdraft reactor design and exclusive closed-loop gas cooling and gas cleaning patented and patent pending systems. We use waste heat effectively in an exclusive closed-loop heat recovery biomass drying process.
Our systems also supply substantial thermal energy as a true combined heat and power, or CHP process. In addition, the Biogen automated control system with real time feedback is employed to control the process.
Biogen uses a highly efficient, specially engineered, moving bed, downdraft reactor design; it eliminates typical side air intakes, which minimizes reactor heat leakage and allows for unprecedented low temperature gasification. This is one of the key innovations that allows for a broad range of biomass to be efficiently gasified without slag problems.
Primary Cleaning / Cooling
Hot gas exiting the reactor passes through a series dual cyclone cleaner to remove particulate, and is then sprayed with coolant separating further ash particles and some tars. Multiple coolant systems are used in series to provide specific stages of cooling temperature and closed loop heat recovery. The tar is separated from the coolant at each stage and fed directly to a decanter where tars are fed back in to the reactor, and water is sent to the evaporation system or filter system, providing a closed loop process.
Our systems include capability for washout of the gas to remove chemical contamination through the addition of specialized scrubbers designed for the specific application. This is an optional feature allowing for the use of unconventional biomass.
From there the cooled and partially cleaned gas passes through additional heat exchangers further reducing gas temperature and condensing tar. The gas is then pass through a sparger process as a final wash prior to entry to a proprietary high voltage wet electrostatic filter system. Gas is electrically charged and any remaining tar particulates or sub-micron droplets in the gas are attracted to the electrodes.
Tar/coolant drips down off the vertical electrodes and is collected for consumption in the reactor and conversion back to energy. The resulting gas can be fed directly into a variety of combustion generators or low BTU gas burners for continuous operation.
Closed Loop Environmental Friendliness
The result of all this is a closed loop system. The by-product is charcoal and ash; this is collected from below the reactor grating and also from the hot gas cyclone system. This is very clean, almost pure semi activated charcoal and can be sold as a soil additive, made into charcoal briquettes, or further processing to fully activate the charcoal for filtration or purification applications.
Depending upon the biomass an additional water purification process may be required before discharge.
Closed Loop Heat Recovery / Drying CHP
Biogen systems for the generation of electricity supply gas to a combustion engine and run an electrical generator. The electrical generation systems typically are packaged in cargo containers and include all the necessary equipment to connect to the electric grid. In addition, the waste heat from the engines can be used to dry biomass and perform other valuable needs of the site.
To achieve efficient biomass drying, fan cooling air from the genset engine radiators is mixed with the hot engine exhaust and fed into a proprietary design biomass fuel dryer. The entire process runs continuously; fresh fuel is continuously fed into the dryer, and dry fuel is continuously fed from the dryer into the reactor. Gas is continuously generated, cooled, cleaned, and fed into the genset.
The gasification process, including conveyor feed, and cooling water supply pump is controlled by the Gasifier PAC. Various additional process equipment may be added by our custom programming engineers, or central plant command can be designed, quoted and provided on a case-by-case basis to further the balance of plant automation capabilities.
The use of Biochar as a soil additive draws on ancient agricultural practices put into use by the local population of the Amazonian region thousands of years ago. In this area of the world, soil with very different properties compared to typical forestry soils was discovered. Soil called “Terra Preta do Indios” with black color and alkaline pH, give rise to endemic microorganisms and are particularly rich, and nourishing, due to high organic matter content, as well as high cationic exchange capacity.
The main feature of these soils is the high volume of black carbon content, more than seventy times above normal, to a depth of eighty centimeters. The charcoal was introduced into the soil by the native population throughout the ages, the charcoal content further increased from thermo-chemical processes (pyrolysis, or in-complete combustion) of vegetable materials.
In the opinion of many leading researchers, the carbon fraction’s chemical and microbiological stability is due to its complex aromatic structure, which has been able to persist for centuries in the environment.
The structure is slowly oxidized over the years producing carboxylic groups able to increase the capacity to retain the nourishing, but at the same time allows carbon storage and immobilization in the soil and reduces atmospheric emissions. These characteristics remain for centuries and still now the black Amazon earths are much more fertile than surrounding areas, long after the native population left the areas.
Soil Biochar addition, if applied in a large scale, seems able to mitigate climate change and could be both an economical and environmentally friendly way to increase agricultural production.
Gasification creates a fine-grained, highly porous, pollutant free charcoal that significantly varies in its chemical and physical properties depending on starting material (woody or herbaceous feedstock, residues).
Biogen is currently collaborating actively with the scientific community and is involved in experimentation and demonstration activity in order to better understand the soil dynamics and the growing effects of Biochar amendment. The results thus far are very positive and encourage field application.
In this way it will be possible to obtain a threefold benefit without reducing emissions through renewable energy production resulting in carbon sequestration, soil fertility and agriculture production increase.
Biogen is further collaborating and developing further monetization of bio-char through further processing certain types of Biochar to complete the activated carbon process.
One of the most important factors is for the biomass fuel to be the correct dryness. Ordinary burning can support quite a bit of moisture content in the fuel, as the water vapor escapes upward with the smoke and heat. With gasification all the combustion products, steam, smoke, heat etc… move downward through the mass of fuel; everything in the fuel gets converted, with only some small amount of ash or charcoal left over. How much moisture can be tolerated depends on the fuel.
Some moisture is necessary, especially with coal for fuel, as there is little hydrogen contained in it, and the combustible end products with gasification are carbon monoxide, hydrogen and methane. So in the case of coal, all the carbon comes from the coal, all the hydrogen comes from water, and oxygen comes either from the air in the original combustion reaction, or from water.
With fresh organic fuel, the feedstock contains both carbon and hydrogen, so the amount of water must be kept low in order for the reaction to convert everything effectively. Too much moisture results in some water ending up unconverted to gas. This residual water vapor winds up combining with the tars in the hot gas and can plug up the grating and restrict the air intake, as well as foul the mechanical systems used to remove charcoal and ash from the reactor.
With fuel like wood chips, moisture content of 10-15% by weight or less works really well in gasification. Fresh cut trees are close to 45-50% moisture content. High moisture content in the biomass fuel also directly and dramatically reduces the calorific value of the fuel per unit weight (see table below). The result is that biomass drying is a crucial factor for both the gasification process as well as the efficiency of delivering biomass to the gasification plant.
Moisture Content vs Caloric Value for Wood Chip Biomass Fuel
- Moisture Content, % by Weight
- Lower Heating Value,BTU/lb.
Fuel Chip Size, Quality, and Energy Content affect the energy value of the Syngas. For example, in the case of wood chips the Biogen uses the G50 chip specification. It is extremely important in the design of the plant and feed Systems to meet correct specifications to maximize the energy output of the system. Biogen has tested numerous feed stocks and can provide the specifications directly to use for plant design.