There is no question that the energy landscape in the United States is undergoing a metamorphosis towards energy carriers and energy conversion processes that possess a reduced anthropogenic carbon footprint. Because of its increased efficiency compared to internal combustion, the transportation sector is seeing the gradual electrification of passenger vehicles which is expected to dramatically grow in the next decade. In the electricity sector, coal is becoming less and less economically attractive and is being replaced largely by natural gas and an ever-increasing amount by solar and other renewable energy technologies that are becoming more and more cost competitive every day. On top of this, the threat of global climate change has brought considerable social, economic and political attention to the need for a significant reduction in CO2 emissions around the world. However, in the industrial sector, the least efficient sector in our economy after transportation, transformative innovations have lagged. The largest subset of the industrial sector from an energy perspective is the production of commodity chemicals – accounting for 5.5% of global energy consumption, which relies heavily on conventional natural gas and petrochemical processing.
As the main component of natural gas, methane is a well-established and widely available (at low cost with the recent discoveries of vast amounts of shale gas) feedstock for the production of chemicals, particularly small chain organics. Despite being widely deployed, its use has an intrinsic constraint that makes the process unavoidably expensive and environmentally concerning: methane is first converted to synthesis gas (“syngas”) through steam reforming, which is done at very high temperatures (~ 900C) and is very highly exothermic (DelH@900C = 227 kJ/mol). The need for this significant amount of high-quality heat is met through burning of additional methane, making this process extremely carbon inefficient (~1/2 of all methane becomes CO2, not a desired product). Therefore, many have called for the low temperature, direct transformation of methane to chemicals and fuels one of the “Holy Grails in Chemistry” – and advances in catalysis are central to achieving it.
On the petrochemical side, refining provides two primary classes of intermediate chemicals in addition to syngas: olefins (e.g. ethylene) and aromatics (e.g. benzene), which are converted in downstream or off-site processes to commodity and specialty chemicals. Both olefins and aromatics are typically obtained from steam cracking of hydrocarbon feedstocks. Steam cracking requires a very large energy input – where the latent heat of water needs to be overcome and the resulting vapor superheated to 850C; in fact, in a typical petrochemical plant, steam cracking alone is responsible for more than 1/3 of the consumed energy. The boiler requirements are met by burning natural gas and gas-phase refining products. Additionally, separations processes, pumps, compressors, etc. consume additional thermal and electrical energy. In the end, the combination of losses translates into very poor carbon efficiency for chemicals produced through petrochemical processing – where as little as 46% of the C atoms that enter the facility may become desired products, and the rest are lost to the atmosphere as CO2.
Therefore, there is a strong desire to replace these fossil fuel dependent processes with alternatives that have reduced energy consumption – particularly high temperature energy – as well as higher carbon efficiency and lower CO2 emissions. One of the most promising alternatives, which has been discussed for several years, is biomass. It has been estimated that if properly implemented biomass could account for 60% of all renewable energy use in only ~15 years. Biomass has the distinct advantage of being able to sequester expended CO2 at the rate it is evolved (unlike geological sources), assuming that it is replaced at the same rate it is consumed. Additionally, biomass can provide pathways to create complex organic molecules. Though there are many routes for the conversion of biological sources to fuels and chemicals (e.g. gasification), biomass fermentation (anaerobic digestion) is likely the most attractive due to its relatively low energy demand, low temperature operation, and product selectivity. Though there are many possible products that are possible from naturally-occurring and genetically modified species, four of the primary products from fermentation are: methane and CO2 gases, and ethanol and acetic acid liquids. Therefore, it is expected that these four molecules will be important feedstocks for next-generation processes and provide the building blocks to produce future fuels and chemicals, as illustrated in the figure below.
Professor William E. Mustain
Department of Chemical Engineering
Swearingen Engineering Center
University of South Carolina
301 Main St.
Columbia, SC 29208