Gasification is a thermo-chemical process in which carbonaceous (carbon-rich) feedstocks such as coal, petro-coke or biomass are converted into a gas consisting of hydrogen and carbon monoxide (and lesser amounts of carbon dioxide and other trace gases) under oxygen depleted, high pressure, high-heat and/or steam conditions. The resulting gaseous compound is called Syngas. Carried out under proper conditions, gasification is an efficient energy extracting process that can return double benefits as a waste stream disposal system.
Gasification has been around for decades. Scottish engineer William Murdoch gets credit for developing the basic process. In the late 1790s, using coal as a feedstock, he produced syngas in sufficient quantity to light his home. Eventually, cities in Europe and America began using syngas — or “town gas” as it was known then — to light city streets and homes. Eventually, natural gas and electricity generated from coal-burning power plants replaced town gas as the preferred source of heat and light.
Today, with a global climate crisis looming on the horizon and power-hungry nations on the hunt for alternative energy sources, gasification is making a comeback. The Gasification Technologies Council expects world gasification capacity to grow by more than 70 percent by 2015. Much of that growth will occur in Asia, driven by rapid development in China and India. But the United States is embracing gasification, as well.
Types of Gasification
In direct gasification, the thermal energy required for the reaction is produced by initial partial combustion of feedstock with air or oxygen supplied into the reactor. Due to the high content of nitrogen in the air, in the case of air-blown gasification the resulting product gas contains a lot of nitrogen, which reduces the calorific value2 of the product gas. On the contrary, if oxygen is used, the resulting product gas contains less nitrogen and thus has a higher calorific value, however the drawback of the oxygen-blown gasification is that it is much more expensive than the air-blown process, due to high cost of oxygen production.Oxygen blown gasification produces gas rich in hydrogen and CO and therefore is of interest for applications with higher added value such as FT Diesel or DME production, whereas it is not directly relevant for power and heat generation at present.
In indirect gasification the heat required for the gasification process is supplied by additional process, which takes place outside the main gasifier (the heat can be supplied for example by gases or bed material circulated through the heat exchanger or by combustion of char or ash from the main reactor). Steam is the most commonly used indirect gasification agent, because it is easy and cheap to be produced and increases the hydrogen content and consequently the calorific value of the combustible gas.
Indirect gasifiers are grouped as char- and gas indirect gasifiers, depending on the type of internal energy source. An indirect gasifier using gas as a heat source (gas indirect gasifier) is a fluidized bed gasifier, equipped with heat exchange tubes. Part of product gas is burned with air as oxidizing agent in a pulse combustor. The resulting heat is used for gasifying the fuel that is fed into the reactor. An indirect gasifier using char as a heat source (char indirect gasifier) consists of two separate reactors: a CFB steam gasifier that converts fuel to product gas and a CFB combustor that burns residual char to provide heat, which is needed to gasify the fuel. Silica sand, which commonly is the bed material, is circulated between the two reactors to enable heat transfer. In such a way, all gaseous and condensable products are reserved for gas production. This process is also known as “fast fluidized process” and generates high product gas yields.
What is Biomass Gasification?
“Biomass” generally refers to any organic plant material that when processed through gasification can be converted to biomass energy, such as heat, syngas, biofuels, and chemicals. Biomass gasification offers a renewable resource that provides a wealth of benefits for our nation’s economy and fuel supply.