Turning Trash Into Fuel? Study Finds Much of Municipal Waste's Energy Escapes Solid and Liquid Recovery

Discarded tires and municipal waste may contain valuable carbon, chemical energy and recoverable materials, but transforming them into usable resources is far more complicated than simply heating them into new products. New research reveals that two seemingly promising waste streams can produce radically different recovery outcomes under identical processing conditions, raising questions about how waste-to-energy technologies should be evaluated, financed and integrated into circular economies.

The study, titled "From Waste to Resource: Carbon, Nitrogen, and Energy Routing in End-of-Life Tires and MSW-Derived Refuse Fuel," was published in the journal Nitrogen. Conducted by El Jirie Baticados, Eunice Arzadon, Amirhossein Mahdaviarab and Sergio Capareda of Texas A&M University, the research examines how carbon, nitrogen and chemical energy move through pyrolysis, a thermal process that converts waste into solid char, liquids and gases.

Its key finding exposes a critical weakness in conventional waste-recovery assessments: producing substantial quantities of material does not necessarily mean recovering substantial quantities of usable resources. End-of-life tires retained 86.2% of their original chemical energy in solid and liquid products, compared with just 44.6% for municipal waste-derived fuel, despite both undergoing the same treatment.

The Hidden Problem With Measuring Waste Recovery

Waste-to-resource technologies are frequently evaluated through conversion efficiency, product yields and heating values. Yet these indicators reveal relatively little about whether recovered products can replace conventional materials, serve as industrial fuels or satisfy environmental requirements.

The researchers argue that meaningful resource recovery requires tracking the original constituents of waste throughout processing. Carbon may remain embedded in reusable solids, become concentrated in energy-rich liquids or move into gases. Nitrogen can follow entirely different pathways, potentially affecting product quality, agricultural suitability and emissions management.

To investigate these differences, the team examined shredded end-of-life tires and refuse-derived fuel obtained from a material recovery facility in the Dominican Republic. The municipal waste-based material contained paper, food waste, wood, textiles, plastics and other combustible components, making it considerably more heterogeneous than the tire feedstock.

Both materials were processed at 500°C for 60 minutes using approximately 500 grams per experimental run. Three independent runs were conducted for each feedstock, allowing researchers to compare product distributions, chemical characteristics and elemental retention under consistent laboratory conditions.

The starting materials already displayed substantial differences. Tires contained more carbon and had a higher heating value of 34.75 megajoules per kilogram, compared with 22.85 megajoules for municipal waste-derived fuel. The latter contained approximately 7.2 times more nitrogen per kilogram of prepared material.

Such compositional differences proved decisive. Rather than responding similarly to thermal conversion, the two waste streams distributed their recoverable constituents across different products, creating distinct opportunities and technical barriers for downstream utilization.

Tires Preserve Energy, While Municipal Waste Sends More Into Gas

The most notable difference emerged in the distribution of solid, liquid and gaseous products. Tire pyrolysis yielded approximately 45.62% char, 38.15% liquid and 16.23% apparent gas by feedstock mass. Municipal waste-derived fuel produced 31.53% char, 30.06% liquid and a substantially larger apparent gas fraction of 38.40%.

Carbon retention followed similarly divergent patterns. Tires preserved 41.1% of their initial carbon in char and another 41.4% in liquid, achieving combined carbon recovery of 82.6% across these products. Municipal waste-derived fuel retained 53.2% in char but only 9.2% in liquid, resulting in combined recovery of 62.4%.

Energy recovery revealed an even wider gap. Tire-derived char and liquid contained approximately 29.95 megajoules per kilogram of original feedstock, equivalent to 86.2% of the initial chemical energy. Corresponding products from municipal waste retained only 10.19 megajoules per kilogram, or 44.6%.

These figures describe gross chemical-energy retention rather than net process efficiency. Heating the reactor, supplying nitrogen and operating associated equipment all require energy, but those demands were excluded from the calculations. Commercial energy performance therefore remains unresolved.

The researchers found that municipal waste processing shifted more carbon toward the estimated gaseous fraction. Recovering useful energy from these gases could consequently play a greater role in improving overall resource utilization than simply collecting solid and liquid products.

However, the gaseous fraction was calculated by mass difference instead of direct collection and weighing. It may include unrecovered condensates, deposits and handling losses, making the estimated gas and carbon distributions less certain than the measured solid and liquid recoveries.

Nitrogen added another layer of complexity. Municipal waste-derived fuel retained approximately 61.3% of its original nitrogen in char, compared with around 33% for tires. Tire nitrogen was distributed more evenly, with approximately 36% entering the liquid product.

The researchers caution that nitrogen retention does not establish nutrient recovery. Chemically stable nitrogen in char may have limited plant availability, while unmeasured nitrogen-containing gases could create environmental and operational concerns.

Valuable Products, But Serious Barriers to Commercial Use

The recovered liquids provide perhaps the clearest illustration of why product quantity cannot substitute for product quality. Tire pyrolysis generated hydrocarbon-rich oil with a heating value of 42.28 megajoules per kilogram, suggesting potential applications in fuel upgrading, industrial combustion and chemical recovery.

Municipal waste-derived condensate, despite representing approximately 30% of the initial feedstock mass, consisted overwhelmingly of water. Its aqueous fraction accounted for 95.73% of the recovered liquid, leaving an organic fraction equivalent to only 1.27% of the original waste material.

Although this small organic fraction possessed considerable energy content, the complete condensate had a heating value of just 3.77 megajoules per kilogram. Consequently, recovering useful fuel would require substantial separation and concentration before further processing could be considered.

Even the more promising tire-derived oil was not ready for unrestricted industrial use. Screening against representative gas-turbine fuel requirements found that its energy content and certain physical properties were encouraging, but sulfur, nitrogen and other quality characteristics indicated the need for upgrading.

Solid products presented different possibilities. Both chars contained substantial carbon concentrations, approximately 85.67% for tires and 88.28% for municipal waste-derived fuel, supporting consideration for recovered carbon materials and certain industrial applications.

Potential uses include asphalt modification, recovered carbon black, adsorbent production and solid fuels. However, carbon concentration alone cannot establish functional performance, product certification or commercial suitability. Additional testing would be required before assigning these materials to specific applications.

Tire-derived char contained appreciable ash and sulfur, while municipal waste-derived char showed considerable nitrogen enrichment. These characteristics could affect processing requirements, combustion emissions and the suitability of recovered materials for different markets.

Agricultural applications require particular caution. Although municipal waste-derived char contained 4.34% nitrogen, the researchers did not establish whether this nitrogen was available to plants. Tire-derived char also fails to qualify automatically as agricultural biochar under the certification framework examined.

The gas products could potentially support internal process heating, electricity generation or chemical synthesis after suitable conditioning. Nevertheless, direct measurements of gas flow, heating value and contaminants would be necessary to determine their actual energy contribution and equipment compatibility.

The Circular Economy Must Prove What Happens After Conversion

The study challenges the assumption that waste conversion automatically delivers circular-economy benefits. Recovering carbon into a solid product, producing liquid fuel and burning gas each involve different environmental consequences, even when all three reduce the amount of material requiring disposal.

Carbon incorporated into reusable materials may substitute for virgin resources, whereas combustion releases much of that carbon into the atmosphere. Because tire carbon is predominantly fossil-derived, retaining it in recovered products must not automatically be classified as atmospheric carbon removal.

Environmental performance also depends on contaminants and emissions. Nitrogen-bearing compounds may require control during thermal processing, while sulfur, ash and other constituents can constrain downstream combustion or material applications. Chlorine, another potentially important factor for fuel quality and corrosion, was not measured.

For waste-management systems considering pyrolysis, the findings support evaluating technologies according to specific feedstocks and intended end uses. A process suitable for recovering energy-rich tire oil may not generate comparable results when applied to heterogeneous municipal waste.

The Dominican Republic feedstocks offer a practical context for examining these differences, although the researchers emphasize that municipal waste composition can vary considerably between collection periods and locations. Results from one prepared batch cannot establish performance across other waste-management systems.

Important uncertainties also remain around scalability. Laboratory experiments cannot determine whether industrial facilities will achieve comparable recovery rates, maintain consistent product quality or operate with favorable energy and material balances.

Future investigations must therefore move beyond laboratory-scale conversion and preliminary product screening. Direct gas measurement, comprehensive nitrogen-species analysis, continuous-operation trials and detailed product qualification would help establish more reliable recovery pathways.

Life-cycle and techno-economic assessments are equally necessary to determine whether recovered materials and fuels offer advantages over alternative waste-management approaches. Such evaluations must account for processing requirements, potential substitution of conventional resources and the possibility of transferring environmental burdens between different stages.

The research ultimately reframes the purpose of waste-to-resource technology. Pyrolysis can redistribute carbon, nitrogen and chemical energy into potentially useful products, but redistribution alone is not circularity. Its practical value emerges only when those products meet technical requirements, reach beneficial applications and demonstrate advantages beyond waste diversion.

For policymakers, waste-processing companies and investors, the distinction is consequential: the success of waste recovery should be measured not by how much material leaves the reactor, but by how much usable value survives the entire recovery process.