How to Make Steam-Activated Charcoal ?

Steam-activation is primarily used for coconut charcoal and coal.

In the production of steam-activated charcoal, first the coconut shell or coal is heated to create a char. This char is then “activated” in a furnace at high temperatures of 1,700° to 1,800°F with steam in the absence of oxygen. In the steam-activation process, all volatile compounds are removed, and at the same time layer after layer of carbon atoms are pealed off, enlarging the existing internal pores, and leaving behind a carbon skeleton. The carbon + steam reaction results in producing hydrogen gas and carbon monoxide (C+H2O=H2 +CO). As the carbon monoxide gases off it takes carbon atoms with it. Typically 3 pounds of raw charcoal will produce 1 pound of activated charcoal. This is a perfect example of the saying “Less is More”. Less carbon atoms yields More internal space.

fig1

How to Make Steam-Activated Charcoal ?

Once the activated charcoal is cooled off, to remove the soluble ash content, it may be either “water-washed”* (which requires a lot of water) or it is “acid-washed” (to remove the acid-soluble ash content) and then repeatedly “water-washed” to remove any trace of the acid solution.
(*Not to let anything go to waste, the charcoal “vinegar” is sometimes collected and sold as commercial ascetic acid or processed into table vinegar.)

Because of the very high temperatures required for steam activation (600 – 1,200 °C), temperatures you cannot achieve in a conventional oven (260 °C), this method is all but limited to industrial technology.

Another huge limiting factor is the cost of production. The world uses a tremendous amount of Activated Charcoal annually and so production needs to be on an industrial scale that can produce millions of tons of AC at a very low price.

This is typically done in large rotating steel cylinder kilns (up to 180ft long producing up to 12.5 metric tonnes/hour) with a sophisticated delivery system of heat and steam. If money were not an issue, then individuals would need to first design an even more sophisticated miniature version. There would be the issue of washing/rinsing, the disposal of waste ash from the pyrolysis, managing the exhaust gasses, and other challenges. The net product would far exceed the cost of the mass-produced product, and quality would likely also be an issue, since cooking temperatures and times are quite critical. Aside form the fascination of building one’s own, it seems the cost would be prohibitive to make steam-activated charcoal “at home”.

So, how can you make steam activated charcoal? It should be obvious that, for small personal quantities, you are not set up for the technical challenges or the financial outlay. Well then, how can you make chemically activated charcoal? Is it less expensive and easier?

Some Raw materials of Activated Carbons

A carbonaceous substance can be used as the raw material for activated carbon.

Materials for activated carbon in use worldwide are as follows:

Powdered activated carbon
  • Sawdust
  • Hard wood chips
  • wood charcoal (carbon from sawdust)
  • Grass ash (peat)
Granulated activated carbon
  • Charcoal
  • Coconut shell charcoal
  • Coal (lignite, brown coal, bituminous coal, anthracite coal, etc.)
  • Oil carbon
  • Phenolic resin
Fibrous activated carbon
  • Rayon
  • Acrylonitril
  • Coal tar pitch
  • Petroleum pitch
  • Phenolic resin, etc.

Coconut shell
Coconut shell

Powdered activated carbon Raw Materials

Activated Carbon Manufacture: Steam Activation

The use of steam for activation can be applied to virtually all raw materials. Steam activation is the most widely used process to activate carbonaceous materials. Steam activated carbons are produced in a two-stage process.

First, the raw material in the form of lumps, pre-sized material, briquettes or extrudates is carbonized by heating in an low oxygen atmosphere so that dehydration and devolatilization of the raw material occurs. Carbonization reduces the volatile content of the source material to under 20%. A coke or charcoal (depending on the raw material) is produced which has pores that are either small or too restricted to be used as an adsorbent.

A variety of methods have been developed but all of these share the same basic principle of initial carbonization at 500-600 degrees C,followed by activation with steam at 800-1,100 degrees C.

Since the overall reaction (converting carbon to carbon dioxide) is exothermic it is possible to utilize this energy and have a self-sustaining process:

  • C + H2O (steam) —> CO + H2 (-31 Kcal)
  • CO + ½ O2 —> CO2 (+67 Kcal)
  • H2 + ½ O2 —> H2O (steam) (+58 Kcal)
  • C + O2 —> CO2 (+94 Kcal)

A number of different types of kilns and furnaces can be used for carbonization/activation and include rotary (fired directly or indirectly), vertical multi-hearth furnaces, fluidized bed reactors and vertical single throat retorts.  Each manufacturer has their own preference.

 

As an example, production of activated carbon using a vertical retort is described below.

Raw material is introduced through a hopper on top of the retort and falls under gravity through a central duct towards the activation zone. As the raw material moves slowly down the retort the temperature increases to 800-1000 degrees C and full carbonization takes place.

The second stage, which can take place later in the same kiln, is activation which enlarges the pore structure, increases the internal surface area and makes it more accessible. The carbonized product is activated with steam at very high temperatures. The chemical reaction between the carbon and steam takes place at the internal surface of the carbon, removing carbon from the pore walls and thereby enlarging the pores.

The activation zone, at the bottom of the retort, covers only a small part of the total area available and it is here that steam activation takes place. Air is bled into the furnace to convert the product gases, CO and H2 into CO2 and steam which, because of the exothermic nature of this reaction, reheats the firebricks on the downside of the retort, enabling the process to be self-supporting.

Every 15 minutes or so, the steam injection point is alternated to utilize the “in situ” heating provided by the product gas combustion. The degree of activation (or quality) of the product is determined by the residence time in the activation zone.

 

 

Activated Carbon Manufacturing: Chemical Activation

Chemical activation is generally used for the production of activated carbon from sawdust, wood or peat. The process involves mixing an organic chemical compound with the carbonaceous raw material, usually wood, and carbonizing the resultant mixture. The raw material and reagent are mixed into a paste, dried and carbonized in a rotary furnace at 600 degrees C. When phosphoric acid is the activating agent the carbonized product is further heated at 800- 1000 degrees C during which stage the carbon is oxidized by the acid. The acid is largely recovered after the activation stage and converted back to the correct strength for reuse.

Chemical activation

The raw material is mixed with an activating agent, usually phosphoric acid, to swell the wood and open up the cellulose structure. The paste of raw material and phosphoric acid is dried and then carbonized, usually in a rotary kiln, at a relatively low temperature of 400 to 500 degree Celsius. On carbonization, the chemical acts as a support and does not allow the charcoal produced to shrink. It dehydrates the raw material, resulting in the charring and amortization of the carbon, thereby creating a porous structure and an extended surface area.

Chemical Activation is generally used for the production of activate carbon from sawdust, wood or peat. Chemical activation involves mixing the raw material with an activating agent, usually phosphoric acid, to swell the wood and open up the cellulose structure. The paste of raw material and phosphoric acid is dried and then carbonized, usually in a rotary kiln, at a relatively low temperature of 400C to 500C. On carbonization, the chemical acts as a support and does not allow the char produced to shrink. It dehydrates the raw material resulting in the charring and amortization of the carbon, creating a porous structure and an extended surface area.

Activated carbons produced by this method have a suitable pore distribution to be used as an adsorbent without further treatment. The process used means that the activated carbons are acid washed carbons although they have a lower purity than specifically acid washed steam activated carbons. This chemical activation process normally yields a powdered activated carbon. If granular material is required, granular raw materials are impregnated with the activating agent and the same method is used. Granular activated carbons (GACs) produced have a low mechanical strength, and are not suitable for many gas phase uses. In some cases, chemically activated carbons are given a second activation with steam to impart additional physical properties.

Activity can be controlled by altering the proportion of raw material to activating agent, between the limits of 1:05 to 1:4. By increasing the concentration of the activating agent, the activity increases although control of furnace temperature and residence time can achieve the same objective.

Activity is controlled by altering the proportions of raw material to reagent used. For phosphoric acid the ratio is usually between 1:0.5 and 1:4; activity increases with higher reagent concentration and is also affected by the temperature and residence time in the kiln.

Activated carbons produced by this method have a suitable pore distribution to be used as an adsorbent without further treatment. This is because the process used involves an “acid wash” which is used a purifying step in steam activated carbons, post activation. Chemically activated carbons, however, have a lower purity than specifically acid-washed steam activated carbons as they contain small amount of residual phosphate.

This chemical activation process mostly yields a powdered activated carbon. If granular material is required, granular raw materials are impregnated with the activating agent and the same method is used.  The granular activated carbons produced have a low mechanical strength, however, and are not suitable for many gas phase uses. In some cases, chemically activated carbon is given a second activation with steam to impart additional physical properties.

Raw Materials of Activated Carbon

A carbonaceous substance can be used as the raw material for activated carbon.
Kuraray Chemical selects materials considering the difficulty in obtaining the material, amount of material required, price, reactivity with gas or chemicals, and appropriateness of quality for the products.

Materials for activated carbon in use worldwide are as follows:

Powdered activated carbon
  • Sawdust
  • Hard wood chips
  • wood charcoal (carbon from sawdust)
  • Grass ash (peat)
Granulated activated carbon
  • Charcoal
  • Coconut shell charcoal
  • Coal (lignite, brown coal, bituminous coal, anthracite coal, etc.)
  • Oil carbon
  • Phenolic resin
Fibrous activated carbon
  • Rayon
  • Acrylonitril
  • Coal tar pitch
  • Petroleum pitch
  • Phenolic resin, etc.

Coconut shell
Coconut shell

In addition to the more common raw materials discussed earlier, others can include waste tires, phenol formaldehyde resin, rice husks, pulp mill residues, corn cobs, coffee beans and bones.

Most of the developed nations have facilities to activate coconut shell, wood and coal. Third world countries have recently entered the industry and concentrate on readily available local raw materials such as wood and coconut shell. Coconut shell contains about 75% volatile matter that is removed, largely at source by partial carbonization, to minimize shipping costs. When producing coconut shell activated carbon from coconuts, only the shell (see fig.) is used and 50000 coconuts are needed to produce 1 ton of activated carbon.

The cellulosic structure of the shell determines the end product characteristics, which (at 30-40% yield on the carbonized basis) is a material of very high internal surface area consisting of pores and capillaries of fine molecular dimensions.

The ash content is normally low and composed mainly of alkalis and silica. Coal is also a readily available and reasonably cheap raw material. The activate obtained depends on the type of coal used and its initial processing prior to carbonization and activation.

It is normal procedure to grind the coal and reconstitute it into a form suitable for processing, by use of a binder such as pitch, before activation. (This is typical for extruded or pelletized carbon). An alternative method is to grind the coal and utilize its volatile content to fuse the powder together in the form of a briquette.

This method allows for blending of selected materials to control the swelling power of the coals and prevents coking. If the coal is allowed to “coke” it leads to the production of an activate with an unacceptably high proportion of large pores.

Blending of coals also allows a greater degree of control over the structure and properties of the final product. Wood may be activated by one of two methods, i.e. steam or chemical activation, depending on the desired product. A common chemical activator is phosphoric acid, which produces a char with a large surface area suitable for decolorization applications.

The carbon is usually supplied as a finely divided powder which since produced from waste materials such as sawdust, is relatively cheap and can be used on a “throw-away” basis. Since activated carbon is manufactured from naturally occurring raw materials, its properties will obviously be variable. In order to minimize variability it is necessary to be very selective in raw material source and quality and practice a high level of manufacturing quality control.