Activated Carbon Manufacturing Process: Carbonisation, Steam Activation and Pore Development Explained

The activated carbon manufacturing process transforms a simple carbonaceous raw material into one of the most versatile adsorbent materials used across water treatment, air purification, food processing and pharmaceuticals. Understanding how activated carbon is made helps procurement engineers and plant managers select the right grade for their application, interpret quality certificates and anticipate how performance may vary between suppliers and batches.

At its core, the process involves two stages: carbonisation, which converts the raw precursor into a porous char by driving off volatile compounds, and activation, which dramatically enlarges the pore network to create the enormous internal surface area that makes activated carbon so effective. What activated carbon actually is and how it works is covered in our primer article. This guide focuses on the manufacturing science behind the product you specify and purchase.

1. Raw Materials Used in Activated Carbon Production

Any material with a high carbon content and low inorganic content can in principle be used as a precursor for activated carbon manufacture. In commercial practice, four materials dominate:

Coconut Shell

Coconut shell is the premium precursor for water treatment and gas-phase applications. Its naturally tight, ordered cellular structure carbonises to produce a dense char with predominantly microporous character. The resulting activated carbon has iodine numbers typically exceeding 1000 mg/g and outstanding hardness, making it resistant to attrition in packed bed systems. Acid-washed activated carbon grades are commonly made from coconut shell to meet drinking water and pharmaceutical standards.

Coal

Bituminous, sub-bituminous and lignite coals all serve as precursors, each producing carbon with a different pore structure. Bituminous coal yields a balanced micropore-mesopore distribution useful across a wide range of industrial applications. Coal-based granular grades are frequently used in granular activated carbon water treatment systems and in solvent recovery units.

Wood and Lignocellulosic Materials

Wood, sawdust, bamboo and agricultural residues produce activated carbons with broader pore distributions, including significant mesopore and macropore content. Wood-based carbons are well-suited to decolourisation applications in sugar refining, edible oil processing and wastewater colour removal. They are more commonly produced by chemical activation than steam activation.

Peat

Peat produces carbons with a broad pore size distribution. Its use has declined due to environmental concerns around peatland extraction, though it remains in production in some regions for specialised industrial applications.

2. Stage 1 – Drying and Preparation of Precursor Material

Before carbonisation, the raw precursor must be dried and sized. Coconut shell is crushed to the target granule size range. Coal is crushed, ground and in some cases mixed with a binder before being extruded into pellets of defined diameter. Wood is chipped, dried and in chemical activation processes mixed with the activating reagent at this stage.

Moisture content is reduced to below 15 percent, typically using rotary dryers or belt dryers operating at 150-200 degrees C. Controlling moisture at this stage matters because water entering the kiln along with the feed requires energy to evaporate and can disrupt temperature uniformity during carbonisation.

3. Stage 2 – Carbonisation: Converting Biomass to Char

Carbonisation takes place in a rotary kiln or multiple-hearth furnace operating in an oxygen-limited atmosphere, typically with nitrogen or combustion gases blanketing the product to prevent ignition. Temperature rises progressively from ambient to 400-700 degrees C over a residence time of 30 minutes to several hours depending on kiln design and precursor type.

During carbonisation, three categories of change occur in the precursor material:

• Moisture loss: Residual moisture evaporates between 100 and 200 degrees C.
• Volatile matter pyrolysis: Organic compounds (tars, methane, hydrogen, carbon monoxide) are driven off between 200 and 600 degrees C. This is where the initial pore network begins to form as volatile pathways open up in the material.
• Carbon skeleton consolidation: Above 600 degrees C, the residual material consolidates into a stable, highly carbonaceous char with a rudimentary pore structure.

The char leaving the carbonisation stage has a surface area of only 10-100 m2/g, far below the 800-1500 m2/g typical of finished activated carbon. The activation stage does the heavy lifting in developing porosity.

4. Stage 3 – Steam Activation and Pore Development

Steam activation is the most widely used physical activation method. The char from carbonisation enters a rotary kiln or multiple-hearth furnace at temperatures between 800 and 1000 degrees C in the presence of steam. The fundamental chemical reaction is:

C + H2O = CO + H2 (endothermic, requires energy input)

Steam selectively reacts with the more reactive carbon atoms in the char, burning them away and progressively opening, enlarging and interconnecting the nascent pore network. This burn-off process is the mechanism by which internal surface area multiplies from under 100 m2/g in the char to 800-1200 m2/g or more in the finished carbon.

Degree of Burn-Off

The mass lost during activation, expressed as a percentage of the char mass entering the kiln, is known as the degree of burn-off or activation yield. It typically ranges from 30 to 60 percent. Higher burn-off produces higher surface area and pore volume but also lower apparent density and reduced mechanical strength. The kiln operator controls burn-off by adjusting steam flow rate, temperature and residence time.

Effect on Pore Size

As activation progresses, initially narrow micropores widen to become mesopores, and existing mesopores merge to form macropores. The distribution between pore size classes shifts with burn-off level. Coconut shell carbons tend to maintain predominantly microporous character even at high burn-off, while coal-based carbons develop more mesopore content as activation deepens.

5. Chemical Activation: Phosphoric Acid and Zinc Chloride Routes

Chemical activation combines the carbonisation and activation stages into a single heat treatment by pre-impregnating the raw material with an activating chemical before heating. The two most common reagents are phosphoric acid (H3PO4) and zinc chloride (ZnCl2). Both are used primarily with wood-based precursors.

Phosphoric Acid Activation

The raw material (typically wood or lignocellulosic waste) is mixed with concentrated phosphoric acid at an impregnation ratio typically between 1:1 and 2:1 (acid:material by weight). The wetted material is then heated to 450-500 degrees C in an inert atmosphere. Phosphoric acid acts as a dehydrating agent, disrupting the natural polymer structure of the wood, promoting cross-linking and inhibiting tar formation. The result is a carbon with predominantly mesopore character and very high BET surface area, often exceeding 1500 m2/g.

After activation, the carbon is washed extensively with hot water and sometimes dilute alkali to recover and recycle the phosphoric acid and reduce residual phosphorus in the product. The washing step generates process effluent that must be managed in compliance with applicable environmental discharge standards.

Zinc Chloride Activation

Zinc chloride activation follows a similar impregnation-heat treatment-washing sequence. ZnCl2 is a Lewis acid that promotes dehydration and aromatisation at temperatures of 500-700 degrees C. The resulting carbons have high mesopore and macropore content, making them useful in applications involving large molecules such as colour bodies in sugar or textile wastewater. Post-activation washing is critical because zinc is a regulated heavy metal in most water quality standards.

6. Pore Size Distribution: Micropores, Mesopores and Macropores

The IUPAC classification divides activated carbon pores into three size classes based on pore diameter (measured as the width of the pore opening):

Micropores provide the majority of the adsorption capacity in most activated carbons. The adsorption mechanism in micropores is enhanced by overlapping potential fields from opposing pore walls, which dramatically increases the interaction energy between the pore surface and the adsorbate molecule. This is why micropore content, reflected in the iodine number, is the primary quality indicator for most liquid-phase applications.

Mesopores and macropores serve primarily as transport highways, allowing adsorbate molecules to reach the micropore interior of each particle. In applications involving high molecular weight compounds such as colour bodies, dyes or large organic molecules, mesopores also contribute directly to adsorption capacity. Powdered activated carbon grades used in effluent treatment are often selected for mesopore content when the target contaminants are higher molecular weight.

7. Post-Activation Processing: Washing, Drying and Sizing

Once activation is complete, the hot carbon is quenched and subjected to a series of post-processing steps that determine the final physical and chemical characteristics of the product.

Washing and pH Adjustment

Steam-activated carbons may be used unwashed or acid-washed depending on the application. Unwashed activated carbon retains surface oxides and may have an alkaline pH. Where applications demand low ash, low heavy metals and neutral pH, the carbon is washed with water or dilute hydrochloric or phosphoric acid. The acid-washed activated carbon product that results meets the more stringent specifications required in food contact, pharmaceutical and potable water treatment applications.

Drying

After washing, the carbon is dried in rotary or belt dryers to reduce moisture to 2-5 percent. Moisture content is a key specification because it affects apparent density measurements and can indicate product deterioration during storage and transport.

Sizing and Screening

The dried carbon is passed over vibrating screens to separate product into the target mesh size fraction. Oversize material is crushed and rescreened or returned to the process. Fines below the minimum mesh opening are removed by aspiration or elutriation. For applications requiring specific particle size distributions, air classification may supplement mechanical screening.

The final product form depends on the manufacturing route: rotary kiln activation of crushed shell or coal yields granular activated carbon. Ball milling of activated carbon produces powdered activated carbon. Die extrusion before kiln processing produces extruded activated carbon pellets.

8. Quality Control Testing: Iodine Number, Methylene Blue, BET Surface Area

Quality control in activated carbon manufacturing relies on a battery of standardised tests that probe different aspects of pore structure and surface chemistry. The most important tests used by both manufacturers and buyers are:

Iodine Number (ASTM D4607)

The iodine number measures the amount of iodine (MW 254) adsorbed per gram of activated carbon from a standard solution. Because iodine is a small molecule, it accesses primarily micropores. The iodine number is the most widely used single indicator of micropore content and liquid-phase adsorption capacity. Typical values: 500-700 mg/g (low grade), 800-1000 mg/g (standard grade), 1000-1200 mg/g (premium grade).

Methylene Blue Adsorption

Methylene blue (MW 374) is a larger molecule than iodine and accesses mesopores in addition to larger micropores. Its adsorption value complements the iodine number by indicating mesopore content. High methylene blue values relative to iodine number indicate a predominantly mesoporous structure, well-suited to adsorbing high molecular weight colour bodies and large organic molecules.

BET Surface Area

BET surface area, measured by nitrogen adsorption at 77K, is the most rigorous measure of total internal surface area. The technique, named after Brunauer, Emmett and Teller, applies a multi-layer adsorption model to the nitrogen adsorption isotherm to calculate total surface area in m2/g. For most commercial activated carbons, BET surface area ranges from 800 to 1500 m2/g. BET analysis can be extended (using BJH analysis) to provide pore size distribution across mesopores. (Source: ASTM D6556, Standard Test Method for Carbon Black – Total and External Surface Area by Nitrogen Adsorption)

Additional Quality Parameters

Standard quality certificates for activated carbon also report: moisture content (percent, typically tested per ASTM D2867), ash content (percent mineral residue after combustion), apparent density (g/cc, the bulk density in a loosely packed state), pH (typically 6-11 depending on grade), and hardness number (percent of granules retained on a standard sieve after a standard abrasion test).

For acid-washed grades, heavy metal content (lead, arsenic, cadmium, mercury) and chloride levels are additionally tested to confirm compliance with food-grade and potable water standards such as NSF/ANSI 61 and BIS IS 877. (Source: BIS IS 877:1992 – Specification for Activated Carbon, Bureau of Indian Standards)

9. How Manufacturing Method Affects Final Carbon Properties

The choice of precursor and activation route creates a cascade of property effects that determine the suitability of the finished carbon for a specific application. Buyers who understand these relationships can interpret product data sheets more effectively and make better-informed purchasing decisions.

Coconut Shell Steam-Activated Carbon

High micropore content, high hardness, low ash, iodine number typically 900-1100 mg/g. Ideal for: potable water treatment, aquarium filtration, pharmaceutical applications, gold recovery, air purification. Supplied as granular activated carbon in 8×16, 12×40 and 20×50 mesh grades. Western Carbon’s Westaqua and Westpharm brands represent premium coconut shell grades for water and pharmaceutical applications.

Coal-Based Steam-Activated Carbon

Broader pore distribution, moderate hardness, lower cost per tonne. Suitable for: industrial wastewater treatment, municipal water filtration, gas-phase applications. Available as granular grades and in extruded pellet form for gas-phase applications requiring low pressure drop. Western Carbon’s Westfiltra range covers coal-based granular grades for water treatment.

Wood-Based Chemically Activated Carbon

High mesopore content, lower hardness, highest BET surface area. Best suited to: decolourisation in sugar refining, edible oil bleaching, pharmaceutical decolourisation, and removal of large organic molecules from wastewater. Typically supplied as powdered activated carbon. Western Carbon’s Westoilpure and Westgold brands address edible oil and pharmaceutical decolourisation markets respectively.

10. Western Carbon Serves Customers Worldwide

Western Carbon & Chemicals supplies activated carbon manufactured to consistent specifications for clients across water treatment, process industries, pharmaceuticals and environmental applications. Our quality certifications and batch traceability give customers confidence in the product they receive. Contact our technical team to discuss your application requirements.

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12. Frequently Asked Questions