Gas Production Process – Overview
Gasification is a thermochemical process, the result of the correct interpretation and application of numerous scientific principles requiring a certain order of execution and accompanied by predictable and measurable results.
The fixed bed, downdraft linear process exemplifies the above statement and underscores the simplicity of the technology required for the gasification process. Below is an explanation of the process:
Beginning with a “start-up” charge of charcoal and kindling in the combustion zone of the gasifier, gasification proceeds as follows:
A controlled flow of intake air is drawn laterally into the gasifier and a momentary flame enters to ignite the start-up charge and combustion begins.
Feedstock is slowly introduced. Heat radiating upward from the gasifier drives moisture from the feedstock at 100 C. and as charcoal beneath is consumed, the feedstock sinks closer to the source of heat. At this point, the temperature is 250-450 C. and volatile substances distilled from the feedstock may ignite, raising temperature further and rapidly. At 600 C. the charred feedstock ignites, rapidly increasing the temperature.
Very high temperatures are achieved in the system at this level when air entering the combustion zone contacts and reacts with incandescent char to produce carbon monoxide, a fuel gas, which diffuses back into the gasified spaces between the char pieces where it oxidizes and burns to form carbon dioxide, a flue gas. As the carbon dioxide increases in the gasified spaces, a counterbalancing decrease in the free oxygen occurs in the same spaces and concentrations of both these gases approach equilibrium. The carbon monoxide produced in the initial charcoal/air oxygen reaction quickly falls to a low concentration at which it is held in dynamic stasis.
Simultaneously, the steam from the feedstock has joined with the air also to react with the incandescent char where it decomposes to carbon monoxide and hydrogen, which together, also diffuse back into the gasified spaces between the char pieces where they too are oxidized to form carbon dioxide and steam respectively. The process temperature during the above reaction is 800°C to 1,100°C.
At this time the heat requirements of the process balance the heat created and the temperature is at its maximum, approximately 1,100°C. to 1,250°C. and the oxygen is nearly depleted.
Below this level, much of the heat involved in the process of “Oxidation and Combustion” is consumed in the process of reduction.
At this stage in the process, the carbon dioxide and the steam in the gas spaces diffuse to, and react with, the hot char in the Reduction Zone to produce carbon monoxide and hydrogen, the main fuel constituents of “Producer Gas’. These reactions consume both heat and char. As the process temperature continues to decrease, the rates of these reactions also decrease, until, when the temperature falls to 700°C-800°C. and/or the hot char supply is exhausted, the gasification process stops.
The main product gases; carbon monoxide and hydrogen can be piped away for use as fuel, producing flame temperatures of 2,100°C and 1,960°C respectively.
Although some stages of the process occur sequentially and others simultaneously, they are continuous and in equilibrium, while a feedstock and managed primary air are fed to the gasifier to sustain the process.
Except for the feedstock “drying” stage, the other gasification processes occur within the reaction vessel called the gasifier. Because of the high temperature plus the oxidizing and reducing atmospheres which must be developed and sustained within it, the gasifier incorporates special refractory materials, surface treatment and thermal insulating materials, selectively placed to withstand the rigor of the main gasification processes and equally distribute the high heat.
Please note: several generations of the 3i Gasification System have evolved since this document was prepared by Jack P. Humphries, P.E. Associated. The modifications applied are interior configuration changes and/or alterations, method of feedstock introduction and producer gas handling. Though many of the changes and modifications are proprietary, limited information is available upon request.