Stored feces and manure contain high quantities of water—80% and 66%, respectively—which is far too much for the biomass to burn well. When the raw material is dry, though, it creates such a high-producing energy source that it might be worth investing the energy to dry the piles and combust them in a furnace, said Luca Da Lio, in the University of Padova’s Department of Industrial Engineering, in Italy.

“Energetic balances and laboratory measurements demonstrate that the heat for the humidity extraction can be derived from the exhausts of horse manure combustion itself: The process is still thermodynamically convenient,” Da Lio said.

In other words, as soon as engineers fine-tune the practical details, the dry-before-burning process could become an efficient cycle, using manure-based fuel to dry the next batch of manure, he said.

Moisture and Heating Potential High, Regardless of Storage Method

Eager to find a way to benefit from the nearly 800 million metric tons of horse manure produced throughout the world annually, Da Lio and his fellow researchers ran laboratory testing on horse manure stored at four different stables in Italy. Some was stored outside; some inside. Some had been stored for days; some for years.

Regardless of the storage times and methods, all the manure—which was mixed with wood shaving bedding—had essentially the same moisture content and fuel potential, although older manure gradually lost its carbon content and, hence, its heating potential, said Da Lio.

The readings were high, for both the fuel potential and the moisture content, he said. Fresh horse manure has about as much carbon as wood shavings alone. Heating values of manure—once it’s dry—are equivalent to those of the promising new biofuel elephant grass (miscanthus) and only slightly lower than those of traditional wood logs.

Undried manure, on the other hand, has an extremely low heating value—less than 30% of the value when it’s dry, said Da Lio.

Manure’s high water content reaches upward of 80% with pure feces alone, dropping little by little with increasing amounts of bedding mixed in, he said. (For biofuel purposes, wood shavings trump straw as appropriate bedding, he added.)

But that’s only part of the problem. Manure is also complicated to dry, maintaining its moisture even after years of dry storage, Da Lio said.

Heating manure gets the outside layers of the pile dry but not the inside layers—and increasing the temperature doesn’t hasten the drying process. The core of the manure pile says considerably cooler than the outside, even when it’s heated in a furnace at temperatures exceeding 100°C (212°F), he said. And the bigger the pile, the cooler the core stays relative to the outside.

The trick, then, is for engineers to find a better way to manage manure piles, so they’re not as dense, for example, letting heat in and water vapor out. “Ingenious modifications of the manure bed are required to improve the rate of drying,” Da Lio and his colleagues stated in their recent publication in Waste Management.

Other Tweaks Needed to Perfect Horse Manure as a Biofuel

Burning horse manure comes with additional concerns, said Da Lio. Chlorine levels are twice as high as in wood, likely due to the salt in horses’ diets, he said. And sulfur is about three times as high as it should be, probably because manure often gets mixed with soil. High concentrations of chlorine and/or sulfur could lead to faster corrosion of furnaces and other metal equipment used in the burning process.

Manure also produces much more ash than wooden logs, he said. The good news, though, is horse manure ash melts at higher temperatures that are still within a normal range for a furnace, so more intense heating could prevent ash-related issues such as deposit buildup, Da Lio said.

More problematic, however, is the high ratio of nitrogen in horse manure—as much as 2%. Because nitrogen oxide, which comes from ammonia, is a greenhouse gas that contributes to global warming, scientists have set a concentration limit of 0.6% in combustible fuels. So researchers would have to find ways to lower nitrogen levels in manure—like stable management changes that reduce ammonia buildup—before burning it, he said.

From Barn to Basement: Developing a Direct-to-Furnace Solution With Raw Manure

Da Lio’s team isn’t the first to try to use horse manure as an energy source. Swedish researchers developed a combustion process for recycled horse manure mixed with wood chips more than a decade ago.

The Italian researchers’ goal, however, is to take manure directly from the stable to the furnace, Da Lio said. Scientists in a technology spinoff from the department are working to develop equipment that would include precombustion drying, he added.

“The optimization of the drying step by a relatively simple drying device (with design currently underway) allows for its profitable combustion,” Da Lio said.

Even so, legislation surrounding this biofuel is vague and varies from country to country, he said. As such, putting this system into practice might prove challenging unless laws are revised.

The study, “Effective energy exploitation from horse manure combustion,” was published in the June 1, 2021, edition of Waste Management.