Syngas is the short name for a gasification product known as synthesis gas. It s a mixture of hydrogen, carbon monoxide, and carbon dioxide that used as an intermediate in processing synthetic petroleum and as a potential intermediate in the conversion of certain biomass in fuel. To understand what syngas is, one must first understand what gasification is.
Gasification is a thermochemical process that converts organic carbon into carbon monoxide, hydrogen, and carbon dioxide. The process is carried out at high temperatures (>700 C) in a controlled environment. Usually, a specific mixture of oxygen or steam is injected into the reaction chamber. The key to this process in the combustion does not occur.
The point of gasification to convert carbon compounds into syngas, which tends to burn more efficiently than the original fuel because it burns at higher temperatures. Syngas can be burned directly or it can be used to produce ethanol and hydrogen. It can also be processed into other synthetic fuels through a process known as the Fischer-Tropsch process. Biomass can be converted to biofuel via gasification.
The gasification of biomass has historically resulted in low yields. Recently, the University of Minnesota developed a metal catalyst, which reduces the reaction time for biomass by a factor of 100. It also allows the process to be carried out “autothermically,” which means no exogenous heat need be applied. The catalyst therefore greatly improves the efficiency of biomass gasification.
Uses of Syngas
The primary use of syngas is in the production of other fuels, namely methanol, and diesel fuel. In industrial settins such as steel milling and petroleum refining, large amounts of waste gas are produced. Rather than vent these toxic gases into the atmosphere, they are captured and used to produce syngas. Doing so not only benefits the environment, but the products derived can be sold or used in cogeneration facilities, both of which can help to make plants more profitable.
The production of diesel fuel relies on the Fischer-Tropsch process, which is a series of chemical reactions that converts carbon monoxide and hydrogen into liquid hydrocarbon. In most cases, methane from land fills acts as the feedstock for producing diesel fuel. This is technically considered biodiesel because it is not derived from fossil fuels.
A novel use of syngas is to directly power hydrogen fuel cells. Hydrogen is simply captured from the gas and refined for use in fuel cells. Of course, this process tends to defeat the “zero emissions” aim of fuel cells and so is not widely used outside of research settings.
Syngas can also be used for the production of:
- Carbon Monoxide
- Carbon Dioxide
- Minerals and Solids
Whether the above products can be derived from synthesis gas depends upon the original feedstock. Though synthesis gas need only contain hydrogen and carbon monoxide, it frequently contains other components as well.
Syngas can be burned directly in internal combustion engines, but, if it is to be used directly, is often burned in an integrated gasification combined cycle where heat is captured for electricity, but waste heat for space or water heating.
Microbial fermentation of syngas can be used to develop fuels and chemicals. Most notably, syngas fermentation can produce:
- Acetic Acid
- Butyric Acid
The benefit of fermentation is the it is simpler and takes place a lower temperatures than chemical conversion. Oddly enough, biologic fermentation can also tolerate high levels of sulfur, making it ideal for use in steel factories and power plants that burn coal. In general, the process is simpler because it does not require careful control of reaction conditions or specific quantities of CO and hydrogen. The biggest disadvantage is that it is low throughput, which means that it takes a long time.
1. Stock + Air → Low Energy Syngas → Burned in IC engines or used to generate heat/power.
2. Stock + Steam → Medium Energy Gas → IC engines or used to heat/power generation.
3. Stock + Oxygen → Medium Energy Gas → Pipeline energy or chemicals like methanol, ammonia & gasoline.
4. Stock + Heat → Char or Oil → Oil used to produce pyrolysis oil
Ultimately, the stock provided is less important than the process it is subjected to so long as the stock has a high carbon content. Thus, the reactions above work as well with biofuel stock as they do with fossil fuel stock. Biofuels like Switchgrass and other lignocellulose feedstock are highly amenable to syngas production.
Biomass and Syngas
The basic biomass gasification reaction is as follows:
C6H12O6 + O2 + H2O → CO + CO2 + H2 + other compounds
(other compounds result from contaminants like sulfur, nitrogen, etc.)
It is estimated that as much as 1.2 billion dry tones of biomass could be available for conversion to syngas by 2050. This would result in about 21 quadrillion BTU/year of energy, which is well above the 16 quadrillion BTU/year used in transport and roughly 21% of the total 98 billion BTU of energy used each year in the United States.
Now, biomass can be converted via biological mechanisms as well, so what are the advantages of using thermochemical conversion via syngas to convert biomass? First, gasification is fast, taking minutes compared to days or weeks for biochemical conversion. Second, gasification is able to extract more energy from the biomass and this is its primary advantage. The reason it can extract more energy is that it can convert carbon stored in lignin (the tough part of plants often found in stems and trunks) into usable products. At this point, gasification of biomass (other than crops like corn and soybean) is more efficient than biochemical conversion. In particular, the gasification of waste such as corn stalks, corn cobs, and other agricultural byproducts is highly efficient and ultimately can improve the energy yield per acre of first generation biofuels.