Unlocking the Potential of Coal-Based Activated Carbons
Activated carbon is an incredibly useful material made from carbonaceous source materials like coal, wood, and coconut shells. It has a very high surface area and porous structure, which gives it excellent adsorption properties. This makes activated carbon useful for diverse applications like water purification, air filtration, catalytic support, energy storage devices, and more.
In recent years, there has been growing interest in using coal as a precursor for producing activated carbons. Coal is an abundant and inexpensive carbon source. Bituminous coals in particular contain high carbon content and can produce activated carbons with high adsorptive capacities.
There are a couple approaches for converting coal into activated carbon:
Physical Activation - This involves carbonizing the coal at temperatures 400-900°C in the absence of oxygen to drive off non-carbon elements. This produces a carbon rich material called char. The char is then activated at temperatures 900-1000°C with oxidizing agents like steam or carbon dioxide. This etching process produces a porous structure and increases surface area dramatically.
Chemical Activation - This involves impregnating the coal with chemical activating agents like zinc chloride, phosphoric acid or potassium hydroxide before carbonization. These chemicals promote development of porous structure and surface area at lower temperatures 600-700°C. After activation, the chemicals are washed off.
Coal-based activated carbons can have high adsorption capacities comparable to or exceeding traditional materials derived from wood or coconut shells. They also tend to have larger pores and pore volumes which makes them suitable for applications like gas separation and storage, catalysis, water purification from organic contaminants, and more.
Some current research areas on coal-based activated carbons include:
Optimizing production methods for very high surface areas (>2000 m2/g)
Textural properties and pore size control using varying activation approaches
Reducing harmful elements through sufficient washing/treatment
Functionalization for targeted applications
Hybridization with other nanomaterials
One of the constraints for utilizing coal-based activated carbon had been higher levels of sulfur and ash contaminants compared to wood or coconut derived carbons. However, recent studies have demonstrated methods to reduce sulfur content to just 2-3% with sufficient hydrodesulfurization treatment. The ash content can also be brought down significantly to meet application needs through acid washing.
Overall, the abundance and low cost of coal makes it an attractive precursor for activated carbon production. With optimized production and treatment methods, coal promises to be a competitive alternative to conventional activated carbon precursors.
Unlocking the full potential of coal-based activated carbons will require addressing current limitations and an expanded focus on novel applications. There are certainly interesting opportunities in areas like energy storage, catalysis, and environmental remediation that merit further research on this class of materials.