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| Fig. 1: Spatial distribution of biomass resources from all sources shown in the mature-market medium scenario (unit: Dry short tons per SQMI). [1] Purple shades indicate adequate spatial density to support a facility of at least 725,000 dry tons per year within a 50-mile radius (i.e., at least 100 dry tons per square mile). (Courtesy of the DOE.) |
Biomass refers to organic materials derived from plants, animals, or waste that can be used as renewable energy sources. Biofuel is a form of liquid or gaseous fuels derived from biomass, such as ethanol or biodiesel. In 2023, the energy compensated by green energy is approximately 5% of total U.S. primary energy consumption. [1] Biofuel accounts for 53% of biomass energy use. [2] The U.S. Department of Energy (DOE) "Billion-Ton 2023" report found the U.S. could triple biomass production to more than 1 billion tons/year under sustainable practices. [3] Fig.1 shows the spatial distribution of biomass resources from all sources over the U.S.
The energy density of biomass is 2.0 × 107 Joules/kg and is lower compared to coal (~3.0 × 107 Joules/kg) and gasoline (~4.2 × 107 Joules/kg). Efficiency of biomass plants can be up to 35% (compared to natural gas ~50%). [4] Many current biomass-to-electricity plants using direct combustion of wood/biomass in steam turbines show efficiencies of about 22~45% of the fuel's lower heating value (LHV). Here, I take 35% efficiency (moisture should be considered but here taken the average). Stream and stirling engine are not sensitive to moisture and no moisture content should be taken into account when stream engine is used. Some more modern plants achieve around 30~35% electrical efficiency (LHV basis) for high-quality wood chips in biomass combined heat and power (CHP) (~80% typically) mode with good steam conditions (e.g., ~540°C). The energy efficiency may be summarized as
| Electricity-only (LHV basis): | Eelectric = 0.35 × 2.0 × 107 = 7.0 × 106 J/kg | |
|---|---|---|
| CHP (electric + useful heat): | ECHP = 0.8 × 2.0 × 107 = 1.6 × 107 J/kg | |
| Energy Difference: | ΔE = ECHP − Eelectric = (1.6 − 0.7) × 107 J/kg = 9 × 106 J/kg |
If a CHP plant processes 1 ton (1000 kg) of biomass weo obtaind
If we use gasoline to compensate the energy loss, the required mass m of gasoline is
| m | = | 9 × 109 J 4.2 × 107 (J/kg) |
= | 214 kg |
The gasoline is much more efficient than biomass unless the technologies of extracting biomass energy advance to raise the efficiency.
The relatively low efficiency of biomass power generation compared to fossil fuel technologies results from both thermodynamic limitations and practical engineering challenges. Biomass combustion generally occurs at lower temperatures and pressures than modern coal or natural gas systems, restricting the maximum achievable thermal efficiency. Additionally, the high moisture content of many biomass feedstocks, often around 40~50% for fresh wood, reduces the effective heating value and forces more energy to be spent on drying before combustion can occur. [2] The heterogeneity of biomass materials, including variations in density and composition, also contributes to inconsistent combustion behavior and reduced energy conversion rates. Most biomass power plants are relatively small, typically under 20 MW, and rely on older steam-cycle technologies that cannot match the efficiencies of large-scale gas turbine systems. [5] Further energy losses arise in upstream processes such as harvesting, processing, and transportation of the biomass itself. These cumulative factors explain why biomass-to-electricity conversion efficiencies often remain in the 20~45% range, while combined-cycle natural gas plants can exceed 50%. Despite these drawbacks, the use of combined heat and power (CHP) systems offers a way to recover waste heat and raise overall efficiency to around 75~85%, helping biomass become a more viable renewable energy option when used in integrated and well-managed systems. [6,7]
Biomass and biofuels offer a practical pathway toward reducing dependence on fossil fuels, but their broader adoption is still limited by energy efficiency and production challenges. The relatively low energy density of biomass, around 2.0 x 107 J/kg compared to gasoline's 4.6 x 107 J/kg, means more material must be processed to yield the same amount of usable energy, which affects both cost and logistics. Although combined heat and power (CHP) systems can significantly increase overall efficiency to nearly 80%, most stand-alone biomass power plants still operate at around 20~45% efficiency, far below that of modern natural gas systems. According to the U.S. Department of Energy, sustainable expansion of biomass production could reach over one billion tons per year, potentially offsetting a larger fraction of fossil energy if supported by improved conversion technologies and policy incentives. [1] Data from the U.S. Energy Information Administration further indicate that biofuels already account for more than half of total biomass energy use nationwide. [1] These figures suggest that while the physics of biomass combustion impose limits, steady advances in feedstock management, processing efficiency, and heat recovery could make biomass and biofuels an increasingly important part of a balanced, low-carbon energy strategy for the future.
© Wen-Shin Lu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "2023 Billion-Ton Report: An Assessment of U.S. Renewable Carbon Resources," U.S. Department of Energy, March 2024.
[2] "Monthly Energy Review, October 2025," U.S. Energy Information Administration, DOE/EIA-0035(2025/10), October 2025, Table 10.1.
[3] A. Demirbas, "Competitive Liquid Biofuels from Biomass," Appl. Energy 88, 17 (2011).
[4] "Catalog of CHP Technologies," U.S.Environmental Protection Agency, September 2017.
[5] "Regulatory Impact Analysis: Renewable Fuel Standard Program," U.S. Environmental Protection Agency, EPA-420-R-10-006, February 2010.
[6] E. Worrell, M. Corsten, and C. Galitsky, "Energy Efficiency Improvement and Cost Saving Opportunities for Petroleum Refineries," U.S. Environmental Protection Agency, EPA 430-R-15-002, February 2015.
[7] J. Sadhukhan, K. S. Ng, and E. M. Hernandez, Biorefineries and Chemical Processes: Design, Integration and Sustainability Analysis (Wiley, 2014).