Most people are familiar with renewable energy, derived from sources that are not finite and that naturally replenish themselves over a relatively short timescale. Solar and wind power are the two energy-generation technologies most often associated with the term “renewable.” Recently, new gaseous and liquid renewable fuels have prominently emerged as commercial products to replace conventional gasoline and diesel in transportation applications, serving as additional repositories of constantly replenished energy. These include 1) renewable diesel (RD) produced from a wide array of animal fats and vegetable oils; and 2) renewable natural gas (RNG) produced from the natural decay of organic matter that results from normal activities of modern life (e.g., management of wastewater, solid waste, agriculture, food and green waste). Alternatively, renewable fuel (usually gaseous) can be produced from surplus renewable power (electricity generated from the sun and wind when there is no demand), and used to store that energy until needed. All of these types of renewable energy contribute to a sustainable future, i.e., one in which society embraces the practices of reducing, reusing, recovering and recycling resources.
In their search for new ways to produce cleaner and sustainable forms of energy, scientists have been exploring and developing processes to produce yet another valuable renewable alternative to a common fossil fuel – propane. This fuel, also known as liquid petroleum gas (LPG), is commonly used by millions of U.S. households as an alternative to methane (natural gas) in cooking, water heating, and space conditioning. In addition, nearly 50 million customers use propane to fuel their outdoor grills. Propane has also long been used as a very good alternative vehicle fuel. It has good combustion characteristics, burns more cleanly than gasoline or diesel in vehicles, is easily stored on vehicles in low-pressure tanks at reasonable cost, and is easily obtained through a vast network of retailers or dispensed as “autogas” at vehicle fueling stations.
Like all fuels and technologies, propane must continue to improve its market and environmental performances, and important new strides are being made by the propane industry and its stakeholders. Advanced propane engine technology has now emerged that provides very low emissions of nitrogen oxides (NOx), the key precursor for formation of photochemical smog (ozone). In fact, one propane engine manufactured by Roush CleanTech was recently certified to meet California’s Optional Low-NOx Standard, at a NOx level 75 percent lower than the current mandatory federal standard for heavy-duty engines. This low-NOx propane engine technology can now power heavy-duty on-road vehicles like school and shuttle buses. Moreover, it can also be transferred to non-road applications, where propane is particularly favored as an alternative fuel. This includes forklifts and remote agricultural uses of propane engines to power water pumps, irrigation equipment and citrus crop heaters that protect from frost damage.
Renewable Propane Can Help Transform Our Transportation Sector
In addition to these engine-related improvements for propane-fueled products that reduce criteria pollutants, fuel-related improvements are need to reduce greenhouse gases (GHG) emissions and help transform America’s transportation sector. This is where biopropane can make a big difference. Conventional propane is a by-product of the production of oil and gas, as well as the oil refining process. When used in today’s mainstream propane vehicles, it can provide very significant reductions of NOx and other criteria pollutants (e.g., particulate matter). However, fossil propane provides only modest GHG reductions relative to fossil gasoline or diesel. Over the long term, California (and all of America) needs vehicles and other combustion-based commercial products that can simultaneously deliver near-zero-emission levels while operating on very-low-GHG renewable fuels.
Fortunately, the propane vehicle industry is stepping up to this tandem need for very clean propane engines – through the recent certification of low-NOx heavy-duty engine technology for propane vehicles – combined with gradual emergence of low-carbon-intensity renewable propane to fuel them.
How and Where is Renewable Propane Produced?
Numerous commercially proven processes can be used to produce renewable propane (aka “biopropane”), which is chemically nearly identical to the conventional version. For example, during the refining process to make renewable diesel from organic waste, about 10 percent of the off-gas production is renewable propane. This process is already being used to produce significant quantities of commercial-grade renewable propane in Europe and Asia. Specifically, biofuels developer Neste Corporation is capturing renewable propane as a bi-product during production of renewable diesel from vegetable oils and waste animal fats at its facility in Rotterdam. Neste has modified this process and plans on producing about 160,000 metric ton of renewable propane over a four-year period, which will be used for transportation applications in Europe. For its pioneering efforts in the development of this much-needed renewable alternative to conventional propane, and its leadership in the production of other renewable fuels, Neste received the prestigious 2016 Platts Global Energy Award in Industry Leadership – Biofuels.
Locally in California where it is most needed, AltAir Fuels in Paramount is using a similar process as Neste to coproduce renewable propane, while making commercial-grade renewable jet fuel. AltAir is now exploring the potential to capture this renewable propane off-gas and redirect it for use as an automotive fuel, which ideally would specifically power medium-duty vehicles equipped with newly certified low-NOx engines.
In California and around the world, other technologies to produce renewable propane are being developed that rely on microbial biosynthetic pathways. Scientists have genetically engineered the bacterium E-coli, introduced some common enzymes, and are using glucose as the feedstock to produce a form of renewable propane that can serve as a “drop-in substitute” for conventional propane made from fossil hydrocarbons.
Opportunities and Challenges
One barrier faced by the propane industry – and now being addressed – is how to make the production and end use of renewable propane more cost competitive. One key is to get renewable propane into California’s Low Carbon Fuel Standard program as a low-carbon-intensity fuel that can generate credits. This will help monetize the low-GHG value of renewable propane and offset the current higher cost of producing it, compared to conventional propane. This monetary value can help incentivize fuel producers, end users, and propane vehicle manufacturers to expand deployments in California of extremely low-emitting propane vehicles with very low GHG emissions.
While there are challenges to address, it’s clear that producing and using renewable propane in California could significantly help regulators address a variety of environmental and economic challenges. Agriculture is one of the state’s leading industries, producing more than $47 billion in revenue in 2015. Managing operational wastes from this vital sector of our economy simultaneously presents significant challenges and major opportunities. By-products from California’s diverse agricultural economy can be diverted to the production of renewable propane, then turned around and used as a substitute for fossil fuels in food production. Clearly, renewable propane can be one of the keys to help California develop a genuinely sustainable economy, where we routinely recover valuable clean energy by reusing materials previously regarded as waste.