Beyond Recycling: Renewable Fuels Could Help Build Lower-Carbon Roads
For years, efforts to make road paving more sustainable have placed strong emphasis on recycling, and appropriately so. Recycled asphalt pavement (RAP) allows material removed from existing roads to be incorporated into new asphalt mixtures. This reduces the need for virgin aggregate and asphalt binder and can lower both costs and environmental impacts without compromising pavement performance. Our Oregon renewable-propane study(link to report), however, points to another strategy that may provide an even larger reduction in emissions: decarbonizing the energy used to produce asphalt at the plant.
The September 2022 proof-of-concept provided a useful real-world test. A mobile asphalt plant produced comparable paving material using conventional propane before and after a production shift using renewable propane. Weather conditions were similar, and the paving type and production tonnage were the same. Most importantly, the contractor reported no noticeable operational difference between conventional and renewable propane. That finding is significant because a carbon reduction strategy is much easier to implement when it does not require major changes to the existing production process.
The life-cycle assessment evaluated what are commonly called A1-A3 emissions: production and supply of the raw materials (A1), transportation of those materials to the asphalt plant (A2), and production of the asphalt mixture at the plant (A3). For the 20% RAP mixture representative of typical Oregon practice, replacing fossil propane with renewable propane produced from fats, oils and greases (FOG) reduced total A1-A3 global warming potential (GWP) by 24.8%. The modeled reduction increased to 30% when renewable propane produced from camelina, an oilseed crop that can be used to produce renewable fuels, was considered. For comparison, increasing recycled asphalt content from 20% to 40% while continuing to use fossil propane reduced A1-A3 GWP by 10.3%. In this specific analysis, changing the plant fuel therefore produced roughly 2.5 to 3 times the reduction achieved by doubling RAP content.
The difference becomes even more pronounced when we look specifically at A3, the asphalt-production stage. FOG-based renewable propane reduced A3 GWP by approximately 65% compared with fossil propane, while the modeled camelina pathway produced a reduction of approximately 79%. RAP should remain a core strategy for lower-carbon paving, but asphalt recycling should not become the ceiling for carbon reduction. Substantial additional reductions are possible by also addressing the energy used at the asphalt plant. In fact, the strongest outcome came from combining the two strategies: increasing RAP from 20% to 40% while also using camelina-based renewable propane reduced total A1–A3 GWP from 64.84 to 38.66 kg CO₂e per tonne of asphalt mixture, a 40.4% reduction. The strongest pathway, therefore, is to combine increased recycling with lower-carbon plant energy.
The 2022 pilot also revealed an important limitation: renewable fuels are not yet equally available everywhere. The renewable propane used in the Oregon demonstration originated in Geismar, Louisiana. It traveled 147 miles by truck to Mississippi, approximately 3,740 miles by rail to Eugene, Oregon, and then another 101 miles by truck to the asphalt plant in Crescent. The fuel still provided a substantial net emissions benefit, but such a long supply chain adds cost, logistical complexity and additional emissions. The 3,740-mile rail movement alone contributed approximately 1.95 g CO₂e/MJ to the renewable-propane pathway. This makes regional renewable-fuel production and distribution an important part of the equation.
The situation on the West Coast has improved since the pilot was conducted. Renewable-fuel production capacity has expanded substantially, particularly in California, although much of the current production is focused on renewable diesel and sustainable aviation fuel rather than renewable propane. Renewable propane therefore cannot yet be considered widely available throughout Oregon and California, but the broader regional renewable-fuel infrastructure is developing. As these supply chains continue to grow, lower-carbon fuels may increasingly be sourced closer to asphalt plants, reducing the type of cross-country transportation required for our original demonstration.
For stationary asphalt plants, another fuel may ultimately become even more relevant: renewable natural gas (RNG). Most stationary asphalt plants use natural gas rather than propane for heating, and our study therefore identified RNG as an important area for future evaluation. RNG is produced from sources such as landfills, wastewater facilities, agricultural waste and other organic materials and can, after appropriate processing, be introduced into the existing natural-gas system. Its share of the gas supply in Oregon and elsewhere on the West Coast remains relatively small but is growing, as of December 2025, RNG makes up about 6% of the gas SoCalGas delivers to its core customers, with a target to replace approximately 12% of traditional natural gas by 2030. California has also established significant biomethane procurement requirements for its major gas utilities, providing another indication that renewable gas is gradually becoming a larger part of the regional energy supply. In the long term, increasing the renewable portion of the existing natural-gas supply could provide a practical pathway for reducing emissions from many stationary asphalt plants without requiring every plant to establish a separate renewable-fuel supply chain.
Camelina may be one of the most interesting renewable-fuel feedstocks for the future. In our study, camelina-based renewable propane had a modeled carbon intensity, essentially the amount of greenhouse gas emissions associated with each unit of energy, of 11.4 g CO₂e/MJ, compared with 21.7 g CO₂e/MJ for FOG-based renewable propane and 83.2 g CO₂e/MJ for fossil propane. Approximately 40% of the camelina crop can be converted to oil, while the remaining material can potentially be used as animal meal, allowing more than one useful product to come from the crop. Interest in camelina has also moved beyond research. Major energy companies have identified camelina and similar winter oilseed crops as potential feedstocks for renewable diesel and sustainable aviation fuel, while agricultural programs have examined growing camelina in rotational systems, including in the Pacific Northwest. This does not mean camelina should be viewed as a universal solution, but it suggests that a feedstock that was relatively uncommon when our project began is becoming part of a much larger commercial discussion about renewable fuels.
One qualification is land use. Our analysis did not include potential land-use effects in the modeled camelina carbon intensity, and including them could increase its calculated environmental impact. These effects can be particularly important if a fuel crop displaces existing food or agricultural production. However, the concern may be reduced when camelina is grown as an additional rotational crop or on agricultural land that would otherwise remain fallow. The environmental benefit therefore depends not only on the crop itself, but also on where and how it is grown.
Cost remains another major obstacle. Renewable fuels can still be more expensive than their fossil alternatives, particularly fuels such as renewable propane that have relatively limited production and distribution networks. Existing federal and state programs already provide incentives for lower-carbon fuels, including clean-fuel programs in California and Oregon. However, many of these programs are primarily designed around transportation fuels rather than fuels burned at industrial facilities such as asphalt plants. This creates an opportunity for transportation agencies to consider incentives that directly reward reductions in asphalt-production emissions.
One potential approach is greater use of Environmental Product Declarations(EPDs), which are standardized reports that communicate the environmental impacts associated with producing a material. Agencies could use EPD information, low-carbon material specifications, or other procurement incentives to reward actual reductions in A1-A3 emissions. Such an approach would not need to prescribe one technology. A producer could receive credit for achieving lower emissions through higher RAP, renewable propane, renewable natural gas, another lower-carbon fuel, or an effective combination of strategies.
The larger lesson from the Oregon study is broader than renewable propane itself. Recycling remains an essential part of sustainable road construction, but it should not become the ceiling for how far the industry believes it can reduce carbon emissions. For the mixtures evaluated in this study, doubling RAP from 20% to 40% reduced GWP by about 10% when fossil propane was used, while changing the plant fuel reduced total A1-A3 GWP by approximately 25% with FOG-based renewable propane and 30% with the modeled camelina pathway. These results suggest that the next stage of lower-carbon road paving should move beyond recycling alone. The greatest opportunity may come from combining higher-performing recycled mixtures with progressively lower-carbon plant energy, shorter regional renewable-fuel supply chains, carefully selected feedstocks such as camelina, and policies that reward measurable reductions in environmental impact We are not there yet, but renewable fuels are becoming more available and practical, and the renewable-fuel industry is moving in that direction quickly.
A 25% to 30% reduction in total A1-A3 emissions simply from changing the plant fuel is not a marginal improvement. It suggests that the energy used to produce asphalt has the potential to become one of the road-building industry's most powerful tools for reducing carbon emissions.
