In colder weather, people have long been warming up around campfires and woodstoves. Lately, this idea of burning wood or other biomass for heat has surged in popularity as an alternative to using fossil fuels.

Now, in the journal ACS Sustainable Chemistry and Engineering, scientists report a step toward a "greener" way to generate heat with biomass. Rather than burning it, which releases pollutants, they let fungi break it down to release heat.

The benefit of biomass, which consists of plant material and animal waste, is that there is no shortage. It is produced continuously in enormous quantities as a waste product from paper and agricultural industries.

But burning it emits fine particles and volatile organic compounds, or VOCs, linked to health and environmental problems. So scientists have been trying to figure out how to use biomass with minimal emissions.

One approach involves adding microorganisms that can degrade the materials. In this process, heat is released without giving off fine particles or VOCs. So far, most investigations into this method have involved room-temperature conditions.

But for sustained use, these reactions would need to take place at temperatures above ambient conditions as heat is produced. Leire Caizan Juanarena and colleagues wanted to warm things up to see how much heat they could coax out of the process.

The researchers incubated two fungi species that do well in hot climates - lignin-degrading Phanerochaete chrysosporium and cellulose-degrading Chaetomium thermophilum - with blocks of birch wood, either sterile or with its natural biota. C. thermophilum produced the most heat (0.63 Watts per kilogram) when degrading wood with its usual microbial inhabitants.

To generate enough heat for an average home in the Netherlands, for example, the researchers say they need to determine optimal nutrient, moisture and temperature conditions, as well as other parameters, to increase power production to 6 W/kg.