Using “stable technology,” it is now possible to produce biogas from apple pomace, the pulpy residue remaining after the fruit has been crushed to extract its juice.
The key concept is a system of closed loops designed to reduce costs, recover resources from waste, promote reuse and recycling, and maximize use of bioenergy and biomaterials, according to an article describing the research.
Apples are among the most widely consumed fruits worldwide, both fresh and processed as juice, vinegar and cider, among other products. In making the products, the apple processing industry throws away much of the fruit as waste.
According to the Food and Agriculture Organization of the United Nations (FAO), global apple production totaled almost 86.5 metric tons in 2020. The main producers were China (46.85 percent), the United States (5.38 percent) and Turkey (4.97 percent).
“Biorefining with dry anaerobic digestion produces electricity and thermal energy, reduces greenhouse gas emissions, and valorizes waste via conversion to organic fertilizer,” said Tânia Forster Carneiro, last author of the article. Carneiro earned a PhD in industrial process engineering from the University of Cadiz in Spain in 2004 and is currently a lecturer in bioengineering and biotechnology at UNICAMP’s School of Food Engineering (FEA).
Anaerobic digestion is a microbiological process involving consumption of nutrients and production of methane, she said. Dry anaerobic digestion, with total solids in the reactor exceeding 15 percent, is considered an efficient method of recycling solid organic waste and far more environmentally appropriate than landfill disposal.
The results point to a yield of 36.61 liters of methane per kilogram (kg) of removed solids, potentially generating 1.92 kilowatt hours (kWh) of electricity and 8.63 megajoules (MJ) of heat per ton of apple pomace.
The bioenergy thus recovered could supply 19.18 percent of the electricity and 11.15 percent of the heat used to operate the anaerobic biorefining reactor designed by the researchers, who conclude that biofuels and bioelectricity can contribute to public policy, reduce fossil fuel consumption and cut the greenhouse gas emissions from organic residues.
The research group found the greenhouse gas emissions avoided by the biogas corresponded to 0.14 kg of carbon dioxide equivalent (CO2e) in the electricity and 0.48 kg of CO2e in the heat generated per ton of apple pomace.
“Anaerobic digestion is a stable technology and can be implemented in small to medium plants, assisting the circular economy transition and offering an added-value alternative to disposal of fruit residues as waste that benefits the entire supply chain,” Carneiro said.