Development of Technology to Turn Food Waste Into Fuel
▲ Research Specialist Ahn Kwang-ho and Senior Research Fellow Kim I-tae, Department of Environmental Research
Food waste has typically been recycled in the form of compost and animal feed, using methods such as: the fermentation extinction method, in which woody biochips and food wastes are mixed together in the presence of air; anaerobic digestion, in which microorganisms decompose food waste in the absence of air; and drying and carbonization, in which the food waste is dried and carbonized. As the fermentation extinction and anaerobic digestion methods make use of microorganisms, they require longer residence time and effective management of microorganisms. The drying and carbonization method requires partial energy as compared to the fermentation extinction and anaerobic digestion methods and requires shorter amount time for treatment, which ultimately makes it a useful method for recycling large amounts of food waste in a shorter amount of time compared to the fermentation extinction and anaerobic digestion methods.
Renewable-energy Portfolio Standard (RPS)
Possibility of Replacing Wood Pellets Using Food Waste
Wood pellets are a type of material that is mainly used as a Bio Solid Refuse Fuel (Bio-SRF). Wood pellets are a solid refuse fuel used for coal-mixed firing, which is highly dependent on imports under the RPS system. As of 2019, compared to Korea’s domestic production of about 240,000 tons of wood pellets, a significantly higher volume of around 2.56 million tons were imported, which highlights the possibility of using biomass that utilizes food waste as a possible alternative to using wood pellets (Figure 1). Also, even though Bio-SRF is not as widely used as wood pellets for power generation, its use is consistently increasing (Figure 2). If biomass (produced using food waste) could be used to replace solid refuse fuels for coal mixed firing which are highly dependent on imports, it would save Korea approximately USD 280 million per year based on costs as of 2017.
Biochar, a combination of the words biomass and charcoal, retains intermediate properties that are somewhere between organic matter and charcoal. These properties are the result of thermal decomposition that happens at a certain temperature in anoxic conditions (without the presence of oxygen). Although woody materials have been conventionally used to make biochar, in more recent years, organic resources such as food waste have been increasingly used as raw materials to manufacture biochar. Biochar is porous, so it has the advantage of promoting air circulation when it is injected into the soil. Additionally, since it does not promote decomposition or the transformation of microorganisms, it does not emit the carbon found in the soil into the atmosphere. As such, it is currently being considered for various uses.
Current Status of Technology Development (Establishment of an Eco-friendly Pilot Plant)
This study sought to explore an efficient treatment method for food waste as well as the potential use of food waste as fuel by lowering the salt concentration of the food waste in order to resolve the issue of high concentration of salt, which is a major obstacle to converting food waste into fuel. Researchers used a desalting process and a carbonization process (300–500 ℃ as temperature of the carbonization furnace) to produce a biochar that satisfies the quality standards (Table 2) for Solid Refuse Fuel products. Figure 3 shows the biochar production facility (working capacity of 100 kg) built within the Gimpo City Resource Center which has been used by the researchers. In order to make the biochar, researchers fed dry food samples through the fuel input hopper and carbonized the material at a temperature of 300 to 500 °C. The resulting gas was burned off through an emission gas combustion unit, and the resulting biochar was stored in the outlet. Salt was then removed from the biochar using a desalting system with Dissolved Air Flotation (DAF) technologies. The final biochar product was then dried and converted into fuel. Under temperature conditions of 300–500 ℃ and a residence time of 10–30 minutes, the high calorific value of the materials increased from an average of 5,475 kcal/kg before desalting to an average of 5,568 kcal/kg after desalting, and the chlorine ion concentration decreased from an average of 2.42% before desalting to an average of 0.97% after desalting. Based on these results, researchers concluded that, under certain temperature conditions, it was possible to use food waste to obtain biochar that could meet the quality standards for Bio Solid Refuse Fuel (Bio-SRF).
Despite the feasibility of converting food waste into fuel using the above technology, it is difficult to turn food waste into fuel due to current legislation on renewable fuel materials (Table 3). In order for this technology for converting food waste to reach its full potential, it must be supported by appropriate legislation. Legislation should be promoted to allow food waste to be included as a Solid Refuse Fuel product if it meets certain criteria.
In March of last year, the Korea Institute of Civil Engineering and Building Technology (KICT) began working toward effectively treating and converting food waste into fuel by entering into business agreements with the Gimpo City Resource Center and the Korea Midland Power (KOMIPO). In the near future, it is considered that large amounts of food waste may be treated and utilized as compost and livestock feed, as well as be used as fuel through biomass gasification. Many researchers hope that technology and legislation can continue to advance so that food waste can be put to better use as a source of renewable fuel.