The effect of inorganic additives on the formation,composition, and combustion of cellulosic char

At temperature above 300°C the glycosyl units of cellulose are simultaneously depolymerized to a tar and decomposed to a char by evolution of H₂O, CO, and CO₂. When the glycosyl units are depleted, a stable char is formed containing about 30% aliphatic and 70% aromatic components. The aliphatic component is formed first, but on further heating is converted to polycyclic aromatic structures. The chars formed at lower temperatures are more combustible because the aliphatic component of the char is highly pyrophoric and is oxidized almost at the same temperature at which it is formed (～360°C). The aromatic component, however, is less reactive and is oxidized at ～520°C. Consequently, the chars formed at higher temperatures are less combustible. It has been shown that (NH₄)₂HPO₄, which is a well-known flame retardant and smoldering inhibitor, lowers the pyrolysis temperature and increases the char yield by accelerating the decomposition reactions. This affects the composition of the intermediate chars but the final products have about the same composition irrespective of additives. (NH₄)₂HPO₄ also lowers the rate of oxidation of the aromatic component and the corresponding heat release. NaCl, which is an enhancer of smoldering combustion, has a slight stabilizing effect on pyrolysis of cellulose. It lowers the oxidation temperature of the aromatic component and dramatically increases its rate. The corresponding heat release is also increased due to complete oxidation to CO₂. The rate of oxidation calculated from the dynamic thermal analysis data is more than tripled by NaCl and significantly reduced by (NH₄)₂HPO₄.     

Development of aromaticity in cellulosic chars

Cellulosic chars prepared at HTTs ranging up to 500°C contain aromatic structures as evidenced by the production of benzene polycarboxylic acid derivatives on permanganate oxidation. Analysis of these products indicates the concentration of the aromatic units and the degree of substitution of benzene polycarboxylic acids represents the extent of condensation or crosslinking of the structures. Further information on this subject is obtained by elemental composition of the char and the $\text{H}\text{C}$ ratio. These studies indicate a rapid weight loss and development of aromatic structures between 350 and 400°C, as the $\text{H}\text{C}$ ratio is gradually reduced from 1.5 to 0.7 and the aromatic carbon of the benzene polycarboxylic acids formed is increased to 2.5% of the carbon content of the original cellulose. Above 400°C the rate of weight loss is reduced with the formation of the “stable” char and the yield of the aromatic carbon remains constant. However, the aromatization process continues with rapid reduction in $\text{H}\text{C}$ ratio, due to the condensation and growth of the aromatic clusters as evidenced by the increased formation of the highly substituted benzene polycarboxylic acids. The presence of inorganic additives, representing flame retardants results in increased charring and enhancement of aromaticity and condensation.     

IR studies of carbons—II: The vacuum pyrolysis of cellulose

The pyrolysis of cellulose in vacuum from 22 to 765°C was followed by IR photothermal beam deflection spectroscopy. Series of spectra recorded at various stages of pyrolysis showed that although the main decomposition occurred near 300°C, some decomposition occurred as low as 190°C with the formation or highly absorbing aromatic systems. Mixtures of predominantly aliphatic material predominate in the 300–400°C range but decline in extent to be replaced by mixtures of aromatic nature above 500°C. Further degassing causes discrete spectral features to decline and disappear near 700°C; a continuum absorption remains. Band assignments are discussed. In particular, the 1600 cm^?1 band of spectra of carbons is shown by isotopic study not to be ascribed to carbonyls. but is thought to be a C = C mode made IR active by asymmetry caused by bound oxygen.     

Chemisorption of oxygen on cellulose char

Investigation of a series of chars prepared by rapid pyrolysis of cellulose in the temperature range of 400–800°C has shown that they have a high chemisorptive affinity for oxygen. Maximum chemisorption occured on chars prepared at a HTT of 550°C. The Elovich equation was used to describe the kinetics of the process. The extent of chemisorption decreased with increasing HTT of the chars, although the surface area of the chars stayed approximately constant; indicating the presence of less reactive areas on the surface of chars formed at higher temperatures. As chemisorption progressed there was a corresponding increase in the intensity of several IR absorption bands, which were attributed to the formation of stable oxygen-containing functional groups. The chemisorption process, preceded by physical adsorption, does not influence the gasification reaction. The presence of impurities from pre-pyrolysis doping of cellulose could promote or inhibit the rate of gasification but had negligible effect on the initial rate of chemisorption. The role of these two processes in char combustion was discussed in the light of known concepts for the carbon-oxygen reaction.     

Influence of inorganic additives on oxygen chemisorption on cellulosic chars

Chemisorption of oxygen on cellulosic chars is the initial step leading to gasification and is a significant factor in controlling chemical reactivity and heat release in smoldering and glowing combustion of cellulose. Oxygen chemisorption kinetics have been determined for chars (HTT 550°C) prepared from cellulose and cellulose treated with inorganic additives. Elovich kinetic analysis indicates that combustion behavior can be correlated with chemisorption kinetics. Addition of the same inorganic additives by grinding with pure cellulose chars had little or no effect on chemisorption kinetics. These data indicate that the mode of action on inorganic additives in enhancing or inhibiting the solid phase combustion of cellulose chars involves their influence on char functionality developed during pyrolysis. Chemisorption of oxygen on chars results in a decrease in free radical concentration, and heat treatment at 400°C in flowing nitrogen restores the original concentration. However, free radical concentrations do not differ significantly between additive treatments over most of the temperature range studied. Therefore, combustion behavior cannot be explained strictly in terms of changes in free radical concentration and other functional groups must also play a significant role.     

Structural characterization of Saudi Arabian heavy crude oil by n.m.r. spectroscopy

Saudi Arabian heavy crude oil was separated into six fractions, including five distillate fractions (<93, 93–204, 204–260, 260–343 and 343–454 °C) and a >454 °C distillation residue. Each fraction was analysed by ¹H and ¹³C n.m.r. spectroscopy, and combined gained information from these analyses provided reliable average structural parameters. These included estimation of aliphatic and aromatic content, average paraffinic chain length, and estimation of hydrogen, methyl and alkyl bearing aromatic carbons for each of the six fractions. The extent of branching in paraffinic chains and amount of aromatic bridgehead carbons were also calculated.     

The significance of heating rate on char yield and char properties in the pyrolysis of cellulose

The carbonization of powdered cellulose was investigated in the temperature range 200–950°C by measuring weight loss, carbon and hydrogen content, BET-adsorption of nitrogen and carbon dioxide, mercury penetration and particle-size distribution. Evidence is presented in support of a kinetic model according to which cellulose decomposition is controlled by dehydration at low temperature and by cleavage/scission at high temperature. Increased char yield and lower $\text{O}\text{C}$ ratio at low heating rate, as well as kinetic investigations into the effect of potential catalysts, support this model. The difference in reaction mechanism according to the heating rate appeared to influence the char properties considerably. Yield in micropore volume and surface area of slowly carbonized cellulose is up to four times larger than that of rapidly heated cellulose. Mercury pore volume, density and particle diameter depend on the heating rate, also. By adsorption of various gases, differences in relative size of the pore openings of different chars can be discerned. Micropore volumes measured with carbon dioxide were as much as seventy times larger than the corresponding volume measured with nitrogen. Thus, it is possible to obtain chars with molecular sieve properties by simple pyrolysis heating schemes.     

Evaporation of pyrolysis oil: Product distribution and residue char analysis

The evaporation of pyrolysis oil was studied at varying heating rates (～1–10⁶°C/min) with surrounding temperatures up to 850°C. A total product distribution (gas, vapor, and char) was measured using two atomizers with different droplet sizes. It was shown that with very high heating rates (～10⁶°C/min) the amount of char was significantly lowered (～8%, carbon basis) compared to the maximum amount, which was produced at low heating rates using a TGA (～30%, carbon basis; heating rate 1°C/min). The char formation takes place in the 100–350°C liquid temperature range due to polymerization reactions of compounds in the pyrolysis oil. All pyrolysis oil fractions (whole oil, pyrolytic lignin, glucose and aqueous rich/lean phase) showed charring behavior. The pyrolysis oil chars age when subjected to elevated temperatures (≥700°C), show similar reactivity toward combustion and steam gasification compared with chars produced during fast pyrolysis of solid biomass. However, the structure is totally different where the pyrolysis oil char is very light and fluffy. To use the produced char in conversion processes (energy or syngas production), it will have to be anchored to a carrier. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Pyrolysis and oxidation of heavy fuel oils and their fractions in a thermogravimetric apparatus

Using the derivative thermogravimetric technique, an investigation was made of the pyrolysis and oxidation of some heavy fuel oils and their separate paraffinic, aromatic, polar and asphaltene fractions. The thermal behaviour of fuel oil can be interpreted in terms of a low-temperature (< 400 °C) phase involving the volatilization of paraffinic and aromatic fractions, and a high-temperature phase in which the polar and asphaltene fractions pyrolyse and leave a particulate carbon residue. In an oxidizing atmosphere, the first phase (< 400 °C) consists of the simultaneous evaporation and oxidation of paraffinic and aromatic fractions. The second phase (400–550 °C) consists mainly of pyrolysis of oxidized polar materials, the asphaltenes, with a final phase involving the burning of the carbonaceous residue formed in the second stage.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VII. Raman spectroscopic study on the changes in char structure during the catalytic gasification in air

FT-IR/Raman spectroscopies have been used to identify the structural features of Victorian brown coal chars during the gasification in air at 400 °C. The deconvolution of the Raman spectra has allowed us to identify the main structural sites in char where preferential reaction with O₂ takes place. The presence of Na and Ca catalysts is shown to alter the reaction pathways between char and O₂. In the absence of a catalyst, the O-containing functional groups formed in char during gasification were closely associated with the aromatic structure and thus tended to loosen the aromatic structure. The non-catalysed gasification was slow and took place on some specific (especially sp³-rich or sp²–sp³ mixture) sites distributed throughout the char. In the presence of a catalyst (Na or Ca), the O-containing functional groups were not closely associated with the main aromatic structure throughout the char. The catalytic gasification reactions were localised on the sites associated with the catalysts. The preferential removal of smaller aromatic ring systems and the persistence of cross-linking structures in the presence of a catalyst mean that the large aromatic ring systems were increasingly concentrated with little flexibility, affecting the dispersion of catalyst.     

The influence of pyrolysis conditions on the subsequent gasification of lignocellulosic chars

Prune pit chars prepared by pyrolysis at heating rates of 1 and 15°C/min to 500, 700 and 900°C were subsequently gasified by exposure to CO₂ at 900°C for various lengths of time. Gasification rate was found to be dependent on the conditions during pyrolysis: slow heating below 500°C and prolonged exposure to high temperatures (~900°C) during pyrolysis in an inert atmosphere lead to lower rate gasification. Despite differences in gasification rate, the pore structure developed for a given mass loss due to the gasification reactions was apparently independent of the char preparation conditions. Pore volume in the gasified char (expressed on an absolute basis) passed through a maximum at 40–50% burnoff, apparently due to mass loss from the exterior of the particles.     

Evolution of biomass char structure during oxidation in O2 as revealed with FT-Raman spectroscopy

FT-Raman spectroscopy has been used to identify structural features and evaluate the structural evolution of biomass chars during gasification with air. Chars prepared from the pyrolysis of a cane trash sample with a fast particle heating rate in a novel fluidised-bed/fixed-bed reactor at 500, 700 and 900 °C were oxidised at 400 °C in air in a TGA. The data derived from the spectral deconvolution of Fourier Transform — Raman spectra suggest that the 500 °C char showed very different structural features after pyrolysis and during oxidation from the 700 and 900 °C chars, while the differences between the latter two chars were small. Preferential consumption by O₂ of smaller aromatic rings and structures of somewhat aliphatic characteristics left the char more enriched with larger aromatic ring systems. The changes in char structure are in agreement with the observed reactivity measured in O₂ in a thermogravimetric analyser.     

Fixed-bed pyrolysis of cotton stalk for liquid and solid products

The pyrolysis of cotton stalk was studied for determining the main characteristics and quantities of liquid and solid products. Particular variables investigated were temperature (from 400 °C to 700 °C), particle sizes (from 0.25 mm to 1.8 mm) and nitrogen gas flow rate (from 50 and 400 cm³/min). All experiments were performed at a heating rate of 7 °C/min. The results showed that particle size and nitrogen flow rate did not exert a significant influence, whereas temperature was very significant. The liquid products and the subfractions of pentane-soluble fraction were characterized by elemental analysis, FT-IR spectroscopy, ¹H-NMR spectroscopy, and the pentane subfraction was analysed by gas chromatography. The characterization of char was performed in terms of its elemental composition, surface area and FT-IR spectroscopy. The H/C and O/C ratios of the chars decreased with the rise in the temperature. FT-IR showed that results the hydroxyl and carbonyl functionalities were lost at high temperatures. According to the experimental results the liquid products can be used as liquid fuels, whereas the solid products can be transformed to activated carbon for adsorption processes.     

Thermal decomposition of chlorinated poly(vinyl chloride) (CPVC)

Chlorinated poly(vinyl chloride) (CPVC) shows reductions in flammability and smoke production over PVC. The thermal decomposition of pure CPVC (without stabilizer or lubricant) was studied by dynamic thermogravimetric analysis (TGA) at heating rates from 5 to 100°C/min in atmospheres of nitrogen, air, and oxygen. In each case, a two‐step decomposition was observed similar to that for PVC where dehydrochlorination is followed by pyrolysis/oxidation of the carbonaceous residue. The rate of dehydrochlorination was dependent on atmosphere, occurring slightly slower in nitrogen than in air, and slightly more quickly in oxygen than in air. The decomposition of the residual char was clearly dependent on the conditions in which it was formed. Under dynamic conditions, chars formed at high heating rates appeared more resilient to oxidative degradation than those formed more slowly. However, when chars were formed by heating at different rates and then held at 500°C, the char formed at the slowest heating rate was the slowest to be oxidized. The uptake of oxygen by the char appears to be rate‐limiting. At low heating rates char oxidation is similar in both air and oxygen. As the heating rate is raised, the rate of mass loss of char in air becomes progressively closer to that in nitrogen until at 100°C/min they are almost identical. This work is important to the understanding of the decomposition and flammability of CPVC and flame‐retarded CPVC, where the char formation is one of the flame‐retardant mechanisms.     

Characterization of whole coals by 13C CP/MAS spectroscopy at high field

Solid-state 50-MHz ¹³C spectra essentially free of spinning sidebands have been constructed for three bituminous coals by the addition of echo spectra having phase-altered spinning sidebands (PASS). The echo spectra are produced by a modified version of the Dixon pulse sequence. Quantitative analysis of the aromatic carbon content (f_a) from PASS spectra for the three coals compares favourably with results obtained by other methods. Values of f_a are found in the range 0.69–0.73. Removal of the unwanted spinning bands allows absorptions for specific structural units present in the coals to be distinguished and assigned. Spectra show an upfield shoulder at 13–15 ppm, and moderately intense absorptions at 20–24 ppm and ≈30 ppm which are characteristic of several aliphatic structures in different steric environments. In addition to the main aromatic band at ≈120 ppm, absorptions for substituted aromatic carbons appear at ≈140 and ≈155 ppm. Less intense signals from several carbonyl functional groups (160–190 ppm) and oxygen- and nitrogen-substituted aliphatic groups (50–90 ppm) are also present.     

I.R. studies of carbons—VII. The pyrolysis of a phenol-formaldehyde resin

Infrared spectra were recorded of the chars produced by pyrolyzing a Novolac phenol-formaldehyde resin in vacuum and in nitrogen, using photothermal beam deflection spectroscopy. The two pyrolysis techniques led to the same results. Thermal branching and cross-linking occurred near 350°C, with the formation of diphenyl ether structures. These reactions continued at higher temperatures when aryl-aryl ethers were formed. Autooxidation is not an important degradation pathway. Although some changes occur, the polymer network remains essentially intact until 500°C, the aromatic systems being held apart and stabilized by aliphatic bridges. In the 500–560°C range, however, drastic changes occur in that the network collapses, aliphatic bridges are destroyed, hydrocarbonaceous residues are eliminated and those remaining are altered, polyaromatic domains form, and the resulting char is much like other intermediate temperature chars.     

Porous properties of activated carbon produced from Eucalyptus and Wattle wood by carbon dioxide activation

This work focused on the preparation of activated carbon from eucalyptus and wattle wood by physical activation with CO₂. The preparation process consisted of carbonization of the wood samples under the flow of N₂ at 400°C and 60 min followed by activating the derived chars with CO₂. The activation temperature was varied from 600 to 900°C and activation time from 60 to 300 min, giving char burn-off in the range of 20/2-83%. The effect of CO₂ concentration during activation was also studied. The porous properties of the resultant activated carbons were characterized based on the analysis of N2 adsorption isotherms at −196°C. Experimental results showed that surface area, micropore volume and total pore volume of the activated carbon increased with the increase in activation time and temperature with temperature exerting the larger effect. The activated carbons produced from eucalyptus and wattle wood had the BET surface area ranging from 460 to 1,490 m²/g and 430 to 1,030 m²/g, respectively. The optimum activation conditions that gave the maximum in surface area and total pore volume occurred at 900°C and 60 min for eucalyptus and 800°C and 300 min for wattle wood. Under the conditions tested, the obtained activated carbons were dominated with micropore structure (∼80% of total pore volume).     

IR studies of carbons—VI. The effects of KHCO3 on cellulose pyrolysis and char oxidation

The pyrolysis in vacuum of cellulose containing 5 wt. % of KHCO₃ (KHC) was followed by IR photothermal beam deflection spectroscopy, and compared with observations made with pure cellulose (PC) [Carbon 21, 283(1983)]. The KHCO₃, and KOH and K₂CO₃ formed from it, drastically altered the cellulose carbonization and the subsequent oxidation of the KHC chars. KHC became charred at 140 °C and the cellulose disrupted at 220 °C. The surface layer was a mixture of weak acid K salts. K₂CO₃ was not formed until 310 °C and persisted until about 500 °C, when metallic K was evolved. Unlike PC, the surface layer contained no aromatic CH structures. The carbonization of KHC proceeded more rapidly and to a greater extent than with PC. The oxidation of KHC chars was also markedly affected; burn-off occurred at much lower temperatures, and the gasification temperature range was narrower, than with PC chars.     

Reactivity of Australian coal-derived chars to carbon dioxide

The reactivities to CO₂ of four chars derived from Australian coals at 610 °C, were measured thermogravimetrically. Reaction rates in 100% CO₂ (total pressure, 101 kPa) varied from 0.026 mg h^?1 mg^?1 at 803 °C for char derived from a Lithgow coal to 6.3 mg h^?1 mg^?1 at 968 °C for a Millmerran coal char. Activation energies for the four chars were in the range 219–233 kJ mol^?1. The results show that for Lithgow (Hartley Vale) coal char, reactivity increases with CO₂ concentration and decreasing particle size. The apparent reaction order for this char with respect to CO₂ concentration was found to be 0.7. For different chars, reactivity is inversely proportional to the rank of the parent coal. No general correlation has been established between total mineral content (ash) and char reactivity.     

The thermal history of char samples taken from a gasifier

The thermal history of coal chars obtained from a fixed bed gasifier is of importance in understanding gasifier performance and behaviour. Several methods of assessing thermal history have been considered. The results show that Raman spectroscopy is a good method for estimating the heat treatment temperature in a thermally homogeneous coal char sample, whereas reflectance measurements provide a rapid means of characterizing thermal heterogeneity of the char samples in the temperature range 400–1000 °C. The advantages of the techniques over other char characterization methods are discussed.