There is ongoing debate about the management of natural forests and in specific fuel load reduction in natural forests in parts of the US and in Australia. There is also the question of residue utilisation from plantation forests. Briquettes is a relatively simple renewable energy product that can be produced from forest waste material.
Margules Groome periodically receives questions regarding the use of biofuels to replace coal in energy generation systems. While it might sound simple to replace one fuel with another, various factors that could complicate such a conversion comes to mind.
The “success” of coal as a fuel relates to its energy density (the amount of energy per mass) and bulk handling characteristics, making it an “easy” fuel to manage for large scale energy generation. The calorific value of hard coal such as anthracite range from 25 to 30+ Megajoules/kg while brown coal could have energy values of lower than 10 MJ/kg. Generally, the energy density of coal is rated at about 24 MJ/kg[1] while bulk density is between 600 and 900 kg/m3.
In comparison, biofuels have much lower energy densities as well as bulk densities, and thus require that more fuel is needed to produce the same amount of energy, resulting in higher transport and handling costs. The energy contents of different types of wood range between 17.5 to 21 MJ/kg and that of grasses and soft stem agricultural material between 15 to 19 MJ/kg[2].
Energy content is however not the only factor to consider with aspects such as moisture content (lead to energy loss due to water having to evaporate), ash content from inorganic substances, volatile content and chemical composition all playing a role in making some biofuels more suitable than others. Low density crop residue might simply not be suitable for large scale energy generation due to their bulkiness in unprocessed format, rapid combustion and fluctuating power output. These characteristics might require that biomass be processed before it can be used as biofuels. Processing could include chipping, dehydration, densification and the removal of incombustible material[3].
The manufacturing of biofuel briquettes has long been seen as a method of modifying biomass into particles of uniform size and mass. The heat produced during the pressing of comminuted particles into a mould degrades lignin in biomass and allows it to flow between particles. Lignin serves as a natural binder that will harden when the briquette cools and forms a stable particle. The briquette making process does not change the calorific value (which is a function of the type of biomass used) but the objective is to compress loose biomass so that the bulk density of biomass residues could be increased and thereby improving the handling and burning characteristics of the final product (influenced by compactness level and mould used).
The quality of briquettes is a function of:
- Calorific value and thus the biomass type from which it was made;
- Density, as higher density briquettes have a longer burning time and release more heat (Density is influenced by particle size and moisture content);
- Resistance to moisture, as lignin and other binders are water soluble (water vapor, released during combustion, could lead to briquettes crumble too soon during combustion).
In manufacturing briquettes, the pressing temperature, compacting pressure and moisture content will influence these quality attributes The pressing temperature needs to be high enough to allow lignin to spread without burning the volatile components in the biomass, sufficient pressure will reduce atmospheric humidity in the feedstock and an optimal moisture content of between 10 to 18% will prevent briquettes from falling apart[4].
While it might be difficult to replace coal for large scale energy production the use of briquettes has the added benefit of reducing carbon emissions and could serve as a long-term sustainable alternative to coal. For more information about briquettes contact Margules Groome Consulting.
[1] Energy density of coal – https://hypertextbook.com/facts/2003/JuliyaFisher.shtml
[2] Calorific value of different raw materials – https://cfnielsen.com/faq/calorific-values-for-different-raw-materials/
[3] Meincken and Tyhoda (2014) Biomass quality. In T. Seifert (ed.), Bioenergy from Wood: Sustainable Production in the Tropics, Managing Forest Ecosystems 26,Springer.
[4] Factors that influence briquette burning – https://www.renewableenergyworld.com/2014/08/11/factors-that-influence-your-briquettes-burning/#gref