Types of Gasification

A summary of how gasifiers are categorized as well as their comparative advantages and disadvantages.

Low-Temperature Gasifiers

Low-temperature gasifiers lack feedstock flexibility and generally operate at a lower efficiency than a FastOx gasifiers. These systems also require feedstock sorting, and are limited in their ability to scale.


Downdraft Gasifiers

In downdraft gasifiers, gases flow down in the same direction as the feedstock. Downdraft units are typically easy to control at lower temperatures (ranging from 788-1,200°C/1,450-2,200°F) and produce a syngas that is low in tar and particulate matter. Downdraft gasifiers have smaller capacity levels and cannot handle more than 20% moisture (Roos, 2010).


Pros Cons
  • Small capacity size
  • Portable
  • Modular


  •   Feedstocks need to be sorted
  •   Toxic char or ash is produced and must be landfilled
  •   Size limitations
  •   Low moisture compatibility




Fluidized Bed Gasifiers

Fluidized bed gasifiers are usually “composed of granular solids such as sand, limestone, dolomite, or alumina” (Roos, 2010). These units maintain temperatures between 540-980°C (1,000-1,800°F). (EP, FBG) Waste, air, and granular solids mix and react within the bed of the gasifier, with gases and remaining solids that are later separated in a cyclone. This process offers some feedstock flexibility and is especially effective for biomass applications. Fluidized bed gasifiers are limited in their capacity and are less efficient than other gasifiers (Roos, 2010).

Pros Cons
  • Well suited for biomass
  • Some feedstock flexibility


  • Feedstock sorting is necessary
  • Process typically uses only one form of feedstock
  • Produces ash as a by-product needing disposal
  • Organic materials require drying
  • Low moisture compatibility
  • Unable to process large amount of metals or glass
  • Limited efficiency





High-Temperature Gasifiers

High-temperature gasifiers have higher waste flexibility and more complete conversion. Previously, the only high-temperature gasification technology actively in use was plasma gasification. Sierra Energy’s patented FastOx technology provides the only high-temperature gasification alternative to plasma gasifiers.

Plasma Gasifiers

Plasma gasifiers are a young technological development aimed to increase efficiencies by increasing temperatures over conventional gasifiers. They operate by passing pressurized gas through a high-voltage electric arc. This arc creates high temperatures (up to 4,425°C/8,000°F) that break down waste at the molecular level.
Plasma gasifiers produce syngas and slag, and are similar to FastOx gasifiers in their feedstock flexibility and in the range of materials they can process. However, their capital and operating costs are prohibitive and they use 40-50% of the energy they produce, known as the parasitic load, to run the system.
The high capital and operational costs are due in part to the method used to reach their high temperatures. Electrodes at the base of the gasifier produce an electrical arc running between them that is the source of energy for the process and provided the heat needed to reach the high temperatures. However, the energy required to produce this arc requires a large amount of electrical input.

Pros Cons
  •   Low emissions
  •   Well proven
  •   Reduced sorting needs
  •   Little to no landfill disposal
  •   Produces useful materials


  •   Low efficiency
  •   Plasma gasifiers require a large capital investment and are expensive operate
  •   Higher-than-average parasitic loads
  •   Complicated design and processes
  •   Large footprints
  •   Multiple units necessary for large capacities




FastOx Gasification

Developed to directly address the limitations of its predecessors, FastOx gasification combines the high-temperatures of plasma gasification with the streamlined simplicity of low-temperature gasifiers.
FastOx gasification is a fixed bed gasification system that breaks down feedstock at the molecular level by injecting highly concentrated streams of oxygen and steam at rapid rates.
The injections result in extremely high temperatures (around 2,200°C/4,000°F) without capital intensive plasma-arcs or difficult-to-operate molten baths. The FastOx gasifier has a cold gas efficiency of 66–79% and a parasitic load of 16–20%, which maximizes the output and minimizes natural resource inputs. FastOx gasification drives off 90% of the components in waste (the organic fraction), directly reforming it to clean and high quality syngas. The syngas is an intermediate fuel used to produce valuable energy products such as electricity, diesel, hydrogen and more. The high temperature of FastOx gasifiers allows the remaining 10% of the components in the waste (the inorganic fraction and metallic compounds) to be melted and recovered as recycled metal and inert stone that can be used or sold as construction aggregate.