Buzz
The assistant director at Southern Illinois University's Coal Research Center stands beside a model gasification system that transforms small quantities of coal into syngas. View more images of sustainable science.
AP Photo/Steve JahnkeKey Insights
- Gasification transforms carbon-based substances into syngas (a blend mainly of hydrogen and carbon monoxide) through high-pressure, high-temperature reactions.
- This process promotes cleaner energy by facilitating the capture and storage of pollutants such as sulfur, mercury, and CO2.
- Syngas can be used for electricity generation, producing substitute natural gas, or creating other chemicals.
Some of the most promising and attention-grabbing energy alternatives are not as new as they may seem. Concepts like windmills and waterwheels, which have existed for centuries, are being enhanced today with innovations such as advanced turbine designs. These improvements are revolutionizing these age-old devices into modern technologies capable of addressing global energy demands.
There is another age-old method – one you might not be familiar with – that is gaining traction and could join wind and hydropower as a clean, renewable energy source. This method is known as gasification, a series of chemical reactions that uses limited oxygen to transform carbon-based feedstock into a synthetic gas, or syngas.
It might seem like combustion, but it’s different. Combustion relies on abundant oxygen to produce heat and light through burning. Gasification, on the other hand, uses only a small amount of oxygen, combined with steam, and is subjected to high pressure. This triggers reactions that create a gaseous mixture mainly consisting of carbon monoxide and hydrogen. This syngas can be directly burned or serve as a starting point for creating fertilizers, pure hydrogen, methane, or liquid fuels for transportation.
Surprisingly, gasification has been around for quite some time. Scottish engineer William Murdoch is credited with developing the basic process. In the late 1790s, using coal as a feedstock, he produced enough syngas to light his home. Eventually, cities in Europe and America adopted syngas, or 'town gas,' to illuminate streets and homes. Over time, natural gas and electricity from coal-burning plants replaced town gas as the preferred energy source for lighting and heating.
In today’s world, as the global climate crisis looms and nations seek alternative energy solutions, gasification is experiencing a resurgence. The Gasification Technologies Council anticipates a more than 70 percent increase in global gasification capacity by 2015. A large portion of this growth will be driven by the rapid development in Asia, particularly China and India. However, the United States is also adopting gasification technologies.
Let’s take a deeper dive into how this process functions. We’ll begin with coal gasification, the most common form of gasification.
Coal Gasification
In 2005, a Tiverton, R.I., resident stands beside a container labeled "Gas Plant Waste." The state believes that contamination in the area was caused by decades of waste disposal by the Fall River Gas Co. from coal gasification processes.
AP Photo/Stew MilneAt the core of a coal-powered plant is a boiler, where coal undergoes combustion to convert water into steam. The chemical equation for burning coal is as follows: C + O2 --> CO2. While coal isn’t pure carbon, it consists largely of carbon bound with various other elements. The combustion process involves carbon reacting with oxygen to produce carbon dioxide, a major contributor to global warming. Additionally, coal combustion releases sulfur oxides, nitrogen oxides, mercury, and naturally occurring radioactive substances.
In a gasification plant, the heart is not a boiler, but a gasifier—a cylindrical pressure vessel approximately 40 feet (12 meters) high and 13 feet (4 meters) wide. Feedstocks enter from the top, while steam and oxygen flow in from below. Almost any carbon-based material can be used as a feedstock, but in coal gasification, coal is the primary choice. A typical gasification facility may process 16,000 tons (14,515 metric tons) of lignite, a brownish coal, daily.
A gasifier operates at much higher temperatures and pressures than a coal boiler—reaching about 2,600 degrees Fahrenheit (1,427 degrees Celsius) and 1,000 pounds per square inch (6,895 kilopascals). These extreme conditions prompt a series of chemical reactions. First, partial oxidation of the coal's carbon generates heat, which fuels the subsequent gasification reactions. The first step is pyrolysis, where coal’s volatile components break down into various gases, leaving behind char, a charcoal-like material. Then, reduction reactions convert the remaining carbon in the char into a gaseous mixture known as syngas.
The main constituents of syngas are carbon monoxide and hydrogen. In a process called gas cleanup, the raw syngas passes through a cooling chamber that helps separate its various components. This purification process eliminates harmful impurities such as sulfur, mercury, and unreacted carbon. Even carbon dioxide can be extracted from the gas and either stored underground or used in the production of ammonia or methanol.
What remains is pure hydrogen and carbon monoxide, both of which can be burned cleanly in gas turbines to generate electricity. Alternatively, some power plants convert syngas into natural gas by passing the cleaned gas over a nickel catalyst. This reaction causes carbon monoxide and carbon dioxide to combine with hydrogen to form methane. Known as "substitute natural gas," it behaves like regular natural gas and can be used for electricity generation or heating homes and businesses.
If coal isn't available, gasification can still occur with another feedstock: wood.
While the electric power industry is only recently adopting gasification, it has been a standard in the chemical, refining, and fertilizer industries for decades. This is because hydrogen and carbon monoxide, the main components of syngas, are key ingredients for producing a variety of other products. Among the most important products derived from syngas are methanol, nitrogen-based fertilizers, and hydrogen for oil refining and transportation fuels. Even slag, a glassy byproduct of gasification, has practical uses in roofing materials and roadbed construction.
Wood Gasification
Coal gasification is often referred to as 'clean coal' due to its ability to produce electricity while minimizing the emission of toxins and carbon dioxide into the air. However, it remains reliant on nonrenewable fossil fuels, and its mining processes can still leave long-lasting environmental scars and harmful waste products. Biomass gasification, or more accurately, 'biomass conversion,' may serve as a more sustainable alternative. Biomass, derived from organic matter like trees, crops, and even waste materials, is a renewable energy source.
Biomass gasification operates in much the same way as coal gasification. In this process, a feedstock is introduced into a gasifier, which heats the organic material in an oxygen-deprived environment to generate syngas. The feedstocks typically fall into one of four categories:
- Agricultural residues are byproducts left after harvesting crops. These include straw from wheat, alfalfa, beans, and barley, as well as corn stover. The majority of this biomass comes from wheat straw and leftover corn materials.
- Energy crops are specifically cultivated for use as feedstocks. These include hybrid poplar and willow trees, as well as switchgrass, a fast-growing prairie grass native to North America.
- Forestry residues are any organic materials left behind following timber harvesting. This can include dead wood, as well as scraps produced during debarking and limb-removal processes.
The design of a gasifier is influenced by the feedstock selected. In biomass gasification, three common gasifier types are used: updraft, downdraft, and crossdraft. In an updraft gasifier, wood is introduced into the chamber from the top, where it settles onto a grate to form a fuel pile. Air is drawn from below the grate, flowing upwards through the fuel, while the syngas (or producer gas) is expelled from the top. Downdraft and crossdraft gasifiers may have different entry and exit points for the air and syngas.
The selection of both fuel and gasifier design has a significant impact on the composition of the resulting syngas. For instance, when wheat straw is processed in a downdraft gasifier, it produces the following composition:
- 17-19% hydrogen gas
- 14-17% carbon monoxide
- 11-14% carbon dioxide
- Almost no methane
When charcoal is used in a downdraft gasifier, it produces the following syngas composition:
- Carbon monoxide constitutes between 28 and 31 percent of the composition
- Hydrogen gas makes up about 5 to 10 percent
- Carbon dioxide is present in a range of 1 to 2 percent
- Methane is found in amounts between 1 to 2 percent
[source: Rajvanshi].
You’re now prepared to create your own wood gasifier. Keep going to explore the steps ahead.
Surprisingly, one of the key applications of wood gasification was to fuel internal combustion engines. Before 1940, vehicles powered by wood gasification could occasionally be seen, especially in Europe. When World War II broke out, fuel shortages led people to explore alternative energy sources. In Western Europe, the transportation sector relied on wood gasification to fuel vehicles and deliver essential goods to consumers. After the war, with the availability of gas and oil, the practice faded. However, a future oil crisis could renew interest in this old technology. It’s possible that future drivers might request a refill with wood instead of gasoline.
Building Your Own Gasifier
Could gasifiers like this one hold more promise for the future of energy production?Gasification offers the benefit of scalability. The Polk Power Station, located southeast of Tampa, spans 4,300 acres (1,740 hectares). It processes 100 tons (90.7 metric tons) of coal per hour, generating 250 million watts of electricity to power around 60,000 homes and businesses [source: Folger].
Gasification is not limited to large power plants. You can create a simple gasifier using everyday materials found at home. For example, a YouTube video demonstrates a homemade gasifier where a paint can acts as the pressure vessel, allowing gasification reactions to take place. The produced syngas moves through basic plumbing to a burner can, where it is ignited.
Another captivating video shows a small team assembling and operating a wood gasifier based on plans created by the U.S. Federal Emergency Management Agency (FEMA) and the Oak Ridge National Laboratory. These plans, developed in 1989, were intended for small-scale gasification in case of a petroleum crisis. The detailed instructions outline how to build, install, and use a downdraft biomass gasifier. The unit, made from a galvanized metal trash can, a small drum, plumbing fittings, and a stainless steel bowl, can be mounted on a vehicle to provide syngas for combustion. This setup allows the vehicle to run on wood chips or other biomass fuel.
If gasification interests you but you're not into DIY projects, you might want to purchase a ready-made gasification unit from a manufacturer. For instance, New Horizon Corporation offers gasification systems designed for home installations. These biomass boilers can heat buildings like homes and garages and are compatible with various fuels, such as seasoned wood, corn cobs, sawdust, wood chips, and pellets.
Regardless of the method, gasification is expected to play a crucial role as one of the leading energy alternatives in the upcoming decades. It offers the cleanest way to utilize coal, while also being highly effective with renewable energy sources like biomass. Moreover, since hydrogen is a major byproduct of gasification, this process serves as a gateway to producing large amounts of hydrogen for fuel cells and cleaner energy sources.
Continue reading for additional resources on the future of energy and green technology.
