Charcoal Properties

Dear friends: terra preta is fascinating in part because it involves so many disciplines. My viewpoint is that of a fuel scientist/chemical engineer.

My laboratory produces well-characterized charcoals for a wide variety of research endeavors, including carbon fuel cell studies, metallurgical charcoal applications, activated carbon production, and terra preta research (with my colleagues Dr. Goro Uehara, Dr. Jonathan Deenik, and Tai McClellan in the University of Hawaii’s College of Tropical Agriculture and Human Resources). With this message I wish to call your attention to the elementary properties of charcoal that I think about when I am producing a charcoal for one of our research endeavors.

Both the feedstock and the process (i.e. pyrolysis) conditions influence the properties of the charcoal product. For example, oak wood has little ash; consequently its charcoal also has little ash. On the other hand, rice hulls have much ash (nearly pure silica), and so does its charcoal.

Likewise corncobs produce a highly macroporous charcoal, whereas sucrose charcoal lacks a macroporous structure. But unfortunately, the properties of the feedstock do not completely determine the properties of the charcoal.

For example, if pyrolysis is carried out at a high temperature, some of the volatile ash components leave the charcoal. In our work it is not unusual to find that the charcoal contains as little as 20% of the amount of ash that we expected on the basis of the feedstock ash content. One carbon company produces an ash-free carbon for metallurgical applications by simply heating a fossil carbon (usually coal) to such a high temperature that virtually all the minerals in the fossil carbon vaporize.

Likewise the pyrolysis temperature (usually called the “heat treatment temperature” or HTT) exerts a big influence on the properties of the carbon.

Fuel scientists employ proximate analysis to measure this influence. Let’s be clear: there is nothing approximate about proximate analysis! Proximate analysis determines the moisture content (mc), volatile matter (VM) content, fixed carbon (fC) content, and ash content of a charcoal (or fossil carbon).

A good barbeque charcoal will have a VM content of 25 – 30%, whereas a charcoal destined for metallurgical use often has VM content below 10%. Increasing HTT lowers the VM content of the charcoal, but there is not a simple relationship between the HTT and the charcoal’s VM content. Why? The simplest explanation is that the thermocouple used to measure the HTT measures the temperature of the pyrolysis environment: it does not measure the temperature of substrate during pyrolysis! Pyrolytic reactors designed to maximize “oil” (or gas) yields - and minimize the charcoal yield - employ high heating rates. Under these conditions the pyrolysis reactions are endothermic; consequently there is a large temperature difference between the charcoal and its environment (i.e. the temperature of the charcoal can be hundreds of °C lower than its environment). On the other hand, a pyrolytic reactor that is designed to maximize the charcoal yield will evoke exothermic pyrolysis reactions in the substrate, since the reactions that form charcoal are exothermic. In this case the temperature of the charcoal can be much higher than its HTT. I can provide some interesting papers on this subject for anyone who is interested.

In summary, both the feedstock and the pyrolysis process conditions influence the properties of the charcoal product, but they do not determine the properties (i.e. knowing the feedstock and process conditions is not enough to predict the properties of the charoal). The only way to determine the charcoal’s properties is to actually measure them. We do proximate analyses of all our charcoals. Often we do gas sorption measurements to determine the carbon’s surface area and pore volume distribution. Sometimes we obtain an elemental analysis of the carbon, or an analysis of its ash content. For our carbon fuel cell work we measure the carbon’s electrical conductivity, and with colleagues in the Hungarian Academy of Sciences we do temperature-programmed desorption of biocarbons used in our fuel cell. We have done XRD, NMR, ESR, and MALDI-TOF MS analyses of some of our charcoals.
We have plans to expand our analysis capabilities into other areas soon.

Tests by my colleague Professor Goro Uehara and his co-workers in CTAHR have shown that the addition of some charcoals to the soil can be harmful to plant growth. Our analyses of the properties of this “harmful” charcoal indicate that it would have been perfect for barbeque. This illustrates the dangers of working with an uncharacterized charcoal purchased from your local grocery store. Professor Uehara and his co-workers will have more to say on this subject in the near future. In the meantime I emphasize that our understanding of charcoal’s beneficial and detrimental effects on plant growth must rest (in part) upon measurements of the charcoal’s properties. This is not an easy job and there are no short cuts that I can find.

Like most of you, at present my terra preta research is not funded, so the best I can do is provide well-characterized charcoals to my colleagues here at UH. In the future I may have the resources to provide well-characterized charcoals to other terra preta researchers as well. I will let you know when this becomes possible.

Best wishes, Michael.

Michael J. Antal, Jr.
Coral Industries Distinguished Professor of Renewable Energy Resources Hawaii Natural Energy Institute School of Ocean and Earth Science and Technology (SOEST) 1680 East-West Rd., POST 109 University of Hawaii at Manoa Honolulu, HI 96822

Phone: 808/956-7267
Fax: 808/956-2336
http://www.hnei.hawaii.edu

See discussion in March 2007