Assessing pellet fuels quality: A novel application for reflected light microscopy

Abstract

As the interest in the renewable resources has been increasing worldwide, the wood pellet sector has the potential to become a mainstream fuel of the future in the heat market. This development is especially noted in the European Union countries which consumed 50 % of global wood pellets in 2018. While only about 2 % of the United States energy consumption was derived from wood and wood waste in 2018, the U.S. pellet production continues to grow exponentially due to high demand from overseas markets. The U.S. pellet production was 8.2 million tons in 2018, making the United States the second largest producer of pellets in the world, surpassed only by China.In this growing market, a question has emerged: How can pellet quality be reliably assured? Current standards test the quality of pellets based on a variety of physical and chemical properties. However, some impurities in pellets (glass, plastic, metal, ceramics, coal, and coke) cannot be identified this way. Those impurities can have negative impacts on the environment, human health, and the durability of stoves.A quick and simple way to identify and quantify impurities in pellet fuels such as wood pellets and grilling briquettes is by using petrographic methods. In this study we used reflected light microscopy to identify a range of contaminants including bark, glass, plastic, coal, coke, slag, mineral matter, and metals in 514 commercially available wood pellets made in Poland, Ukraine, Germany, and the United States. Our results demonstrate that optical microscopy could become an effective tool for assessing the purity of wood pellets, and as such, it might be a valuable addition to physical and chemical tests used in the current standards.