GASIFIER Models

The Gasifier’s versatile two-stage (gasification/thermal oxidation) structure creates producer gas from wet biomass through gentle updraft gasification in the first stage followed by a thermal oxidization process (ignition and burning) of the producer gas under vigorous cyclonic conditions to ensure complete combustion in a second stage. The 1010°C (1,850°F) products of combustion from a 105 GJ/h (100 million Btu/h)

Gasifier are routed to a waste heat recovery unit that can be used to produce steam for a condensing steam turbine electricity generating set or, alternatively, heat thermal transfer fluids for a turbine generator. Net power supplied to the electricity grid by a single 105 GJ/h Gasifier unit is approximately 5.5 MWe.

Compared to other gasification technologies the Gasifier’s parasitic load for operating the plant is minimal, thus making it an efficient means to generate electricity.

The versatility of the Gasifier allows it to be operated in tropical or temperate climates provided the moisture content of the biomass is between 35% and 60%. A 105 GJ/h Gasifier could utilize the following MSW loads as fuel.

For example:

• 287 tonnes/day of MSW at 35% moisture content; or
• 358 tonnes/day of MSW at 45% moisture content; or
• 476 tonnes/day of MSW at 55% moisture content.

The need for a minimum of 35% moisture content means that energy from the process does not need to be parasitized for drying the waste prior to processing, as is required in other gasification technologies on the 3 market, meaning more energy is available for generating electricity, thereby resulting in a higher output efficiency.

Assuming an average of 1.5 kg of MSW generated per capita, a single Gasifier unit can convert the organic fraction of municipal solid waste into heat for communities ranging in population size from 184,000 to 305,000.

The Gasifier is the cleanest wet biomass gasifier/thermal oxidizer currently available in the market.

Two Gasifiers were build previously Solway, Minnesota, USA and another two in Bahau, Malaysia which was selling electricity to the Malaysian grid from a generation capacity of over 10.5 MW while also generating revenue in the form of carbon credits from CDM.

Theory of GASIFIER

The Gasifier is a two-stage gasification unit that converts wet biomass (such as wood waste or MSW) into a combustible “producer gas” in its very large, first stage chamber of gentle, updraft gasification and subsequent conversion of this gas into heat through vigorous, double vortex thermal oxidation in its second stage.

Components and processes on an Gasifier project as illustrated in Figure 2 include:

• A Material Recycling Facility (MRF) that converts MSW into RDF (Refuse-Derived Fuel) to ensure that the Gasifier feedstock is the organic fraction and is less than 150 mm (for a 100 MMBTU unit) and 75 mm (for a 30 MMBTU unit) sizes. State of the art separation equipment or local labour will sort, size and remove ferrous and non-ferrous metals, plastics, glass and paper for recycling for added revenue, whenever economically feasible;

• A refuse storage facility to store MSW or other types of feedstock;

• A fuel-feed conveyor and mechanically operated fuel metering bin to feed MSW from the refuse storage facility into the Gasifier gasifier/thermo-oxidizer;

• The Gasifier gasifier/thermo-oxidizer that converts MSW into energy. In its first stage, MSW is gasified in an oxygen-starved atmosphere using a combination of primary, secondary and tertiary combustion air with damper-controlled fan banks. The resulting producer gas then rises to the second stage where it is completely oxidized in a refractory-lined secondary oxidization chamber at 1010°C;

• A refractory lined duct retains the products of combustion through a one-second residence time to ensure complete combustion before the hot products of combustion enter the heat recovery generator (HSRG);

• Pollution controls will ensure Gasifier emissions meet the most stringent emission standards prior to their release into the atmosphere. Pollution controls could include a bag house for particulate matter, multicyclone, selective catalytic reduction unit (SCR), activated carbon injection (ACI), and dry electrostatic precipitator (ESP);

• A heat recovery steam generator recovers the 1010°C products of combustion. This would be a waste heat boiler to produce steam;

• High pressure steam can then be converted to electricity by passing through a stream turbine driven electricity generator. The products of combustion from one 105 GJ/hr Gasifier will result in the generation of 6.5 MWe of green electric power; or

• Alternatively, where there is low water availability, the products of combustion can be transferred to gas turbine by converting the sys-gas to usage bio-diesel

Comparative Advantages of the Gasifier Technology

Competing gasification units use updraft, downdraft, plasma, fluidized bed, mass incineration or aerobic digestion technologies. Regardless of the specific technology details, several fundamental differences in approach distinguish the Gasifier from competitors’ products:

• Other systems strive to create a synthetic gas to fuel some sort of separate engine. This requires supplementary cleaning up of the producer gas and gas handling systems that require use external energy and cause complications in the engines. Gasifier are pure heat machines that supply heat for direct use or to drive an electricity generating turbine cycle;

• Other systems must use carefully controlled feedstock with small particle sizes and low moisture content to sustain their reactions. This requires elaborate and expensive feedstock screening, pre-treatment, and sometimes drying equipment that may use external energy or significantly parasitze produced energy. The Gasifier accepts waste streams with particles sizes from 6 mm (¼ inch) to 100 mm (4 inch) and with a moisture content of 35% to 65%, provided it has been screened to remove metals, 1

This includes < 1 ppm for CO, 15 ppm for NOx, < 100 mg/m3 for particulate matter, no traceable toxins. Further emissions reductions, if necessary, can be realized with the addition of a selective catalytic reactor to eliminate NOx by converting it to N2 and O2, and a “bag house” to capture particulate matter.; 4 plastics, and non-combustible materials. It uses the waste stream itself to provide energy for the gasification process;

• Other systems consume fuel very quickly (in the order of seconds at most) to sustain their reactions. This leaves almost no time to adjust to differences in electricity or fuel demand or fuel composition. The Gasifier has a fuel residence time of about an hour, a lower first stage burner temperature of 850°C (to avoid vaporizing sodium and chlorides that would otherwise precipitate on the walls of the gasifier and the ash grate) and can accommodate 15-second step changes in demand, allowing plenty of time to adjust for differences in fuel composition.

The consequence of the slower reaction rate of the Gasifier is that it tends to be much larger than other machines of equivalent energy output;

• The conversion of MSW to energy by the Gasifier is highly efficient in comparison to other technologies. The parasitic load or the proportion of energy used by the MSWTE plant to power its operations is in the order of only 16%. We understand plasma uses upwards of 40% of its own generation to power the plasma flame that converts the MSW to heat.

Compared to other waste-to-energy systems, the Gasifier produces minimal emissions, therefore it does not require expensive downstream flue gas treatment technologies.

• The low CO emission indicates complete combustion, attributable to the Gasifier’s double vortex combustion chamber.
• The low NOx, basically “fuel” NOx from nitrogen in the fuel indicates the efficacy of the low temperature and “staged combustion” employed in the Gasifier.
• The Gasifier discharge temperature is too low to generate “thermal” NOx. Much higher levels of NOx (50 to 150 ppmv) are common when burning fossil fuels and therefore are also generated in mass incineration plants.
• These higher NOx levels of “thermal” NOx are due to combustion temperatures typically in the 1400°C to 1500°C range. At such temperatures, the nitrogen in the combustion air (4 parts by weight of nitrogen are brought in with each part of oxygen) dissociates and readily combines with oxygen to form NOx. It also reacts with VOCs (volatile organic compounds) such as automobile exhaust, and forms ground level ozone.

It is important to note that the Gasifier’s products of combustion are, typically, routed through waste-heat boilers or waste-heat heat exchangers. In so doing, the sensible heat extracted from the products of combustion lowers the waste-heat device’s stack temperature down to, typically, 177°C.

A bag house can then be used to lower the amount of particulate released to atmosphere by a factor of 5, or down to 0.01 grains/dscf, 20 mg/Nm3 or 0.02 lb/MM Btu. Dioxins and Furans Dioxins form when chlorine atoms from natural biomass, PVC (polyvinyl chlorine) and plastics dislodge hydrogen atoms along the cellulose molecule and take their place. In the furnace of a typical utility boiler or mass incinerator, MSW burns on a grate at the bottom of the “black” (from a radiation perspective) waterwalllined furnace.

Any dioxins or furans that might have formed in the Gasifier’s first stage are completely consumed in the Gasifier’s second stage of vigorous, cyclonic combustion. No black waterwalls are present to chill the 7 reactions taking place in its inner vortex of flaming gases. The second stage is also lined with hot, glowing refractory material where no dioxins or furans could be formed.