Gasification

Gasification is the conversion of a solid or liquid organic compound in a gas phase and a solid phase. The gas phase, usually called 'syngas' or 'producer gas', has a high heating power and can be used for power generation or biofuel production. The solid phase, called char, includes the organic unconverted fraction and the inert material present in the treated feedstock. The syngas produced is a gas mixture of carbon monoxide (CO), hydrogen (H₂), methane (CH₄) and carbon dioxide (CO₂) as well as light hydrocarbons, such as ethane and propane, and heavier hydrocarbons, such as tars. Undesirable gases, such as sulphidric (H₂S) and chloridric acid (HCl), or inert gases, such as nitrogen (N₂), can be present in the syngas. Conversion of organic material is achieved by exposing the feedstock to high temperatures, typically 700°C - 1100°C in the presence of a gasifying agent. The gasifying agents used are air, oxygen, steam or a mixture thereof.

Gasification of organic material offers several advantages. The produced syngas can be more efficient than direct combustion of the original feedstock, and can be used for multiple applications, such as heat and electricity generation, including high temperature heat for industry, for mechanical energy, as transport fuel, as raw material for chemicals, and when cleaned and upgraded to near pure methane, can be injected into the grid.

Feedstock

Origin and composition

Usually, gasifiers use wood and other lignocellulosic biomass. It can also be designed to convert the dry organic fraction of municipal solid waste (MSW). Depending on the nature of the organic material, the presence of the moisture content generally varies from 5% to 35%.

Pre-treatment

• Sizing (e.g., chipping, grinding)
• Drying

Process and technologies

Gasification process

The principal reactions of the gasification are endothermic and the necessary energy for their occurrence is granted by the oxidation of part of the organic material, through an allo-thermal or an auto-thermal phase. In the auto-thermal process, the gasifier is internally heated through partial combustion, while in the allo-thermal process the energy required fro the gasification is supplied externally. Considering the auto-thermal system, gasification can be seen as a sequence of several stages. The main steps of the gasification process are:

1. Oxidation (exothermic stage)
2. Drying (endothermic stage)
3. Pyrolysis (endothermic stage)
4. Reduction (endothermic stage)

Oxidation

The partial oxidation of the feedstock is necessary to obtain the thermal energy required for the endothermic stages of the process in order to maintain the operative temperature at the required value. The oxidation phase is carried out in near oxygen free conditions in order to oxidize only part of the feedstock. The main reaction that take place during the oxidation phase are the following:

${\ce {C + O2 -> CO2}}$ ΔH = -349 kJ/mol (Char combustion)

${\ce {C + 1/2O2-> CO}}$ ΔH = -111 kJ/mol (Partial oxidation)

${\ce {H2 + 1/2O2 -> H2O}}$ ΔH = -242 kJ/mol (Hydrogen combustion)

The main product of this phase is the thermal energy, while the combustion product is a gas mixture of CO, CO₂ and water.

Drying

During the drying phase, the moisture contained in the feedstock is evaporated. The amount of heat required is proportinal to the feedstock moisture content. Generally, the heat required derives from the other stages of the process.

Pyrolysis

Pyrolysis is the thermochemical decomposition of organic compounds. In particular, the cracking of chemical bonds takes place with the formation of three different fractions: a solid, a liquid and a gaseous fraction. The pyrolysis reaction takes place with a temperature in the range of 250-700°C. It is an endothermic reaction and, as in the drying step, the heat required comes from the oxidation process. The pyrolysis of organic material can be schematized with the following overall reaction:

${\ce {Biomass <=> H2 + CO + CO2 + CH4 + H2O + Tar + Char}}$

When the feedstock is made of biomass, since cellulose is its main component, in this reaction the biomass can be indicated with the chemical formula of cellulose (i.e., C₆H₁₀O₆)

Reduction

The reduction step involves the reaction between the products of the preceding stages of pyrolysis and oxidation, which results in the formation of the final syngas. The main reactions occurring in the reduction step are as follows:

${\ce {C + CO2 +<-> 2 CO}}$ (Boudouard reaction)

${\ce {C + H2O <-> CO +H2}}$ (Reforming of the char)

${\ce {CO + H2O <-> CO2 + H2}}$ (Water gas shift reaction)

${\ce {C + 2H2 <-> CH4}}$ (Methanation)

Gasification technologies

The reactors to gasify a pre-treated feedstock, called gasifiers, essentially differ from one another for mode of contact between the feed material and the gasifying agent, mode and rate of heat transfer, and residence time of the fed material into the reaction zone. Different technological solutions can be implemented to obtain different configurations. Principally, the mode of contact may be in counter-current, co-current, or cross flow, and the heat can be transferred from the outside or directly in the reactor using a combustion agent. The residence time can be in the order of hours (static gasifiers) or minutes (fluidized bed gasifiers). The main reactors used in the biomass gasification process are as follows:

1. Entrained flow reactor
2. Fixed bed reactor, either updraft (counter-current) or downdraft (co-current)
3. Fluidized bed reactor, either bubbling fluidized bed or circulating fluidized bed
4. Rotary kiln reactor
5. Plasma reactor