In this review, we summarize the knowledge of the production and properties of charcoal that has been accumulated over the past 38 millenia. The manipulation of pressure, moisture content, and gas flow enables biomass carbonization with fixed-carbon yields that approachor attainthe theoretical limit after reaction times of a few tens of minutes. Much of the heat needed to carbonize the feed is released by vigorous, exothermic secondary reactions that reduce the formation of unwanted tars by augmenting the charcoal yield in a well-designed carbonizer. As a renewable fuel, charcoal has many attractive features:  it contains virtually no sulfur or mercury and is low in nitrogen and ash; it is highly reactive yet easy to store and handle. Carbonized charcoal can be a good adsorbent with a large surface area and a semimetal with an electrical resistivity comparable to that of graphite. Recent advances in knowledge about the production and properties of charcoal presage its expanded use as a renewable fuel, red...

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The Art, Science, and Technology of Charcoal Production†

Impact of pyrolysis temperature and manure source on physicochemical characteristics of biochar.

The removal of crop residues for bio-energy production reduces the formation of soil organic carbon (SOC) and therefore can have negative impacts on soil fertility. Pyrolysis (thermoconversion of biomass under anaerobic conditions) generates liquid or gaseous fuels and a char (biochar) recalcitrant against decomposition. Biochar can be used to increase SOC and cycle nutrients back into agricultural fields. In this case, crop residues can be used as a potential energy source as well as to sequester carbon (C) and improve soil quality. To evaluate the agronomic potential of biochar, we analyzed biochar produced from poultry litter, peanut hulls, and pine chips produced at 400°C and 500°C with or without steam activation. The C content of the biochar ranged from 40% in the poultry litter (PL) biochar to 78% in the pine chip (PC) biochar. The total and Mehlich I extractable nutrient concentrations in the biochar were strongly influenced by feedstock. Feedstock nutrients (P, K, Ca, Mg) were concentrated in the biochar and were significantly higher in the biochars produced at 500°C. A large proportion of N was conserved in the biochar, ranging from 27.4% in the PL biochar to 89.6% in the PC biochar. The amount of N conserved was inversely proportional to the feedstock N concentration. The cation exchange capacity was significantly higher in biochar produced at lower temperature. The results indicate that, depending on feedstock, some biochars have potential to serve as nutrient sources as well as sequester C.

Effect of Low-Temperature Pyrolysis Conditions on Biochar for Agricultural Use

We investigated the behavior of biochars in arable and forest soil in a greenhouse experiment in order to prove that these amendments can increase carbon storage in soils. Two qualities of biochar were produced by hydrothermal pyrolysis from 13C labeled glucose (0% N) and yeast (5% N), respectively. We quantified respiratory losses of soil and biochar carbon and calculated mean residence times of the biochars using the isotopic label. Extraction of phospholipid fatty acids from soil at the beginning and after 4 months of incubation was used to quantify changes in microbial biomass and to identify microbial groups utilizing the biochars. Mean residence times varied between 4 and 29 years, depending on soil type and quality of biochar. Yeast-derived biochar promoted fungi in the soil, while glucose-derived biochar was utilized by Gram-negative bacteria. Our results suggest that residence times of biochar in soils can be manipulated with the aim to “design” the best possible biochar for a given soil type.

Effect of biochar amendment on soil carbon balance and soil microbial activity

Combustion and gasification rates of lignocellulosic chars

A half century ago, Rosalind Franklin identified two distinct families of organic materials:  those that become graphitic during carbonization at high temperatures and those that do not. According ...

Do All Carbonized Charcoals Have the Same Chemical Structure? 1. Implications of Thermogravimetry−Mass Spectrometry Measurements

Charcoals produced by a modern, efficient method were studied in the kinetic regime, at oxygen partial pressures of 0.2 and 1 bar by thermogravimetric experiments and their reaction kinetic modeling. The charcoals were ground to an average particle size of 5−13 μm. A partial removal of minerals from the feedstock (corncobs) by an acid-washing procedure resulted in ca. 6 times higher specific surface area in the charcoal. Despite the increased surface area, this sample evidenced a much lower reactivity. A model based on three reactions gave an adequate description over a wide range of experimental conditions. Thirty-eight experiments on four charcoal samples were evaluated. The experiments differed in their temperature programs, in the ambient gas composition, and in the grinding of the samples. Characteristics of the combustion process were determined including activation energy values characteristic for the temperature dependence of the burnoff, formal reaction orders characterizing the dependence on the...

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Combustion Kinetics of Corncob Charcoal and Partially Demineralized Corncob Charcoal in the Kinetic Regime

The aim of this work was to study the thermal decomposition of different plant species obtained from energy plantations. Thermogravimetry/mass spectrometry (TG/MS) experiments have been performed with two herbaceous crops (Miscanthus sinensis, pelletized energy grass) and two wood samples (willow, water locust) in inert and oxidative atmospheres. Owing to the large number of data obtained in the experiments, a chemometric tool, principal component analysis (PCA) has been used to help the interpretation of the results. It has been found that the thermal decomposition of the studied wood species is similar, whereas that of the studied herbaceous samples exhibits significant differences. PCA has been found to be useful for finding correlations between the various experimental data.

Thermogravimetry/mass spectrometry analysis of energy crops

A high-yield activated carbon is produced from macadamia nut shell charcoal by (i) carbonization of the charcoal at 1173 K, (ii) air oxidation of the carbonized charcoal in boiling water (AOBW) at 503−553 K, and (iii) activation (a second carbonization) of the oxygenated carbon. In step ii, air is bubbled through a sparger to maintain a relatively high concentration of dissolved oxygen in the water, and the boiling water serves to control the temperature of the carbon during its gasification by the dissolved oxygen. Carbon dioxide is observed to be the only gaseous product of the oxidation chemistry. The oxidation results and the properties of the activated carbons from AOBW are similar to those obtained by controlled atmospheric air oxidation. However, the rate of CO2 formation is observed to increase with time to a plateau for AOBW, whereas the gasification rate decreases with time for atmospheric air oxidation. Multiple cycles, involving AOBW followed by activation, efficiently increase the specific su...

Activated Carbon from Macadamia Nut Shell by Air Oxidation in Boiling Water

The thermal decomposition of lignocellulosic biomass materials and their major components is discussed. Thermogravimetric and DSC curves at different T(t) heating programs were evaluated by the method of least squares. Pseudo-first order models, parallel, successive and competitive reaction schemes and complex reaction networks were employed in the modeling. The following topics are treated: thermal decomposition of cellulose at low (2°/min) and high (50 - 80°C/min) heating rates; low temperature phenomena; the validity of the Broido – Shafizadeh model; effects of mineral catalysts; cellulose pyrolysis in closed sample holders; thermal decomposition kinetics of xylan, lignin and lignocellulosic plant samples.

Keywords: Biomass, Cellulose, Hemicellulose, Lignin, Reaction kinetics, Thermogravimetry, DSC

Gábor Várhegyi

Papers

Do All Carbonized Charcoals Have the Same Chemical Structure? 1. Implications of Thermogravimetry−Mass Spectrometry Measurements

Combustion Kinetics of Corncob Charcoal and Partially Demineralized Corncob Charcoal in the Kinetic Regime

Thermogravimetry/mass spectrometry analysis of energy crops

Activated Carbon from Macadamia Nut Shell by Air Oxidation in Boiling Water

Kinetic modeling of biomass pyrolysis : 10 years of a US -Hungarian cooperation