Dear all,
I would like to ask regarding the calorific value of biomass and synthesized activated carbon. From my readings, higher carbon content will lead to higher calorific value.
Nevertheless, from my experiment, the calorific value of the synthesized activated carbon from CO2 activation process (19,852 J/g) is slightly lower compared to its starting material (21,343 J/g). Referring to the CHNS analysis, carbon content of synthesized activated carbon is relatively higher compared to the starting material. (Note: calorific value analysis is done on dry basis material).
Appreciate if you can give your opinion regarding this. Thanks in advance.
CALORIFIC VALUE
The heating value or calorific value of a substance, usually a fuel or food, is the amount of heat released during the combustion of a specified amount of it. The calorific value is a characteristic of each substance. It is measured in units of energy per unit of the substance, usually mass, such as kcal/kg, kJ/kg, J/mol, Btu/m³. Heating value is commonly determined by use of a bomb calorimeter.
The energy value of coal
The energy value of coal, or the fuel content, is the amount of potential energy in coal that can be converted into actual heating ability. The value can be calculated and compared with different grades of coal or even other materials. Materials of different grades will produce differing amounts of heat for a given mass.
While chemistry provides methods of calculating the heating value of a certain amount of a substance, there is a difference between this theoretical value and its application to real coal. The grade of a sample of coal does not precisely define its chemical composition, so calculating the actual usefulness of coal as a fuel requires determining its proximate and ultimate analysis.
Chemical composition
The chemical composition of the coal is defined in terms of its proximate and ultimate (elemental) analyses. The parameters of proximate analysis are moisture, volatile matter, ash, and fixed carbon. The elemental or ultimate analysis encompasses the quantitative determination of carbon, hydrogen, nitrogen, sulfur, and oxygen within the coal. Additionally, specific physical and mechanical properties of coal and particular carbonization properties are also determined.
The calorific value Q of coal is the heat liberated by its complete combustion with oxygen. Q is a complex function of the elemental composition of the coal. Q can be determined experimentally using calorimeters. Dulong suggests the following approximate formula for Q when the oxygen content is less than 10%:
Q = 337C + 1442(H - O/8) + 93S,
Where C is the mass percent of carbon, H is the mass percent of hydrogen, O is the mass percent of oxygen, and S is the mass percent of sulfur in the coal. With these constants, Q is given in kilojoules per kilogram.
Charcoal describes any burnt organic matter and it can contain high levels of toxins which are harmful to health. Some of these toxins may be categorised as carcinogenic organic substances, generically known as polycyclic aromatic hydrocarbons, anthracenes and creosotes. Many of these substances are banned from human food.
Activated charcoal is processed under high temperatures - up to 1000 degrees C – using purified steam to remove any contaminants. The end product is pure activated carbon, also known as medicinal charcoal or activated charcoal. There are 3 main differences between charcoal and activated charcoal:
Charcoal burns in air. Activated charcoal does not;
Charcoal does not adsorb chemical substances. Activated charcoal does;
Charcoal may be toxic. Activated charcoal is non-toxic and safe to eat.
Producer gas with a calorific value more than 3.0 MJ/kg depending on the charcoal yield can be used for thermal application or power generation thus ensuring the overall efficiency is high and also reducing the net harmful emissions to the atmosphere. The producer gas calorific value depends on the charcoal extraction and ranges from 3.0 MJ/kg to 4.5 MJ kg when char extraction is varied from 35 % to 5 %.
For example, if 33 % is the yield of charcoal per kilogram of coconut shells, whose energy content is about 25 MJ/kg, with respect to the coconut shell's calorific value at 16 MJ/kg. If the energy in the gas generated can be effectively used, this is also accounted to estimate the energy efficiency. The energy efficiency is defined as (Yield of charcoal from the reactor or gasifier/kg of coconut shell fed into the reactor or gasifier' X calorific value of charcoal +yield of gas per kg of coconut shell x calorific value producer gas from the reactor or gasifier)/calorific value of coconut shell per kg...
hello
this link could help you
https://www.ncbi.nlm.nih.gov/pubmed/22209404
regards
Dear Prof Paul Mirham,
Referring to CHNSO analysis, oxygen content is reduced as compared to the starting materials. Based on your explanation, may I know how can it relate to the reduction of Calorific value of the activated carbon while comparing it to the starting material?
Tq.
I believe CO2 activation is similar to the gasification reaction (might be wrong, though), which entails a loss of carbon from the solid fraction (C_hot + CO2 -> 2CO), might it be that:
1.- the small C increase in your measurement is related to unreacted CO2 retained within the porosity of the material and detected in the elemental analysis?
2.- since you are actually concentrating ash as you activate, you are actually losing combustible C in the solid fraction?
I believe this might be the case since it's the very loss of C and formation of cavities in the sites of reaction that increases surface area in activated carbon.
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