Chemisorption, char

The pyrolysis of wood, oxygen chemisorption and oxidation of wood chars were carried out in a computerized coupled TG-FTIR system containing Cahn-R-100 electric balance, DuPont Model 990 thermal analyzer and Nicolet MX-1 Fourier transform infrared spectrometer. All of these sequential processes are carried out within the thermal balance without interruption. 

Chemisorption of oxygen on char has often been discussed previously in terms of free radical concentration in the char (1.5,6). For cellulose chars Bradbury and Shafizadeh (1) found that free spin concentration reached a sharp maximum at HTT 550°C, coinciding with maximum CSA and drew the obvious conclusion that the extent of CSA was at least partly related to free radical content of the char. However, in subsequent work on cellulose char, DeGroot and Shafizadeh (3) have found that unpaired spin concentration continues to increase up to HTT 700"C. Ihe CSA of the char must therefore depend on factors other than free radical concentration. 

Figure 1. Initial chemisorption rates (140°C) and pyrolysis weight loss against maximum charring temperature. Pyrolysis at 5°/min in nitrogen. (Reproduced with permission from Ref. 19. Copyright 1989 Elsevier Scientific Publishing Company, Inc.)...

The conditions used for char preparation in the present chemisorption studies (i.e., progressive slow charring of wood) are intended to be relevant to "real life" smoldering combustion situations. Most previous studies of chemisorption have used chars from cellulose (i.e., avoiding hemicellulose and lignin... 

Influence of Metal Ions on Oxygen Chemisorption and Ignition of Chars. We have carried out extensive studies of the influence of metal ions in wood on pyrolysis mechanisms (5.6) and this approach has now been extended to oxygen chemisorption of the chars. The metal ions occur in wood predominantly as the counterions of the uronic acid components of the hemicelluloses (12). We have shown that they can be almost completely removed by very mild acid treatment without any other major change in the chemistry of the wood. Table II shows that the major metal ions in cottonwood are Ca, K and Mg. The acid-washing process removed 98X of the metal ions in... 

Figure 1 has shown that the maximum chemisorption of oxygen on chars from untreated wood occurs at HTT 450°-500°C. However, in order to understand better the effect of metal ions on the total process consisting of pyrolysis and subsequent chemisorption and oxidation of wood char, it was necessary to carry out pyrolysis, isothermal chemisorption and oxidation reactions in a single experiment. A typical overall pyrolysis, isothermal chemisorption (140°C) and oxidation curve is shown in Figure 2. The temperature program is (1) heat from 25° to 500°C at 5°C/min, (2) cool at...

Effect of Acid-washing and Ion-exchange on Oxygen Chemisorption and Ignition of Cottonwood Chars... 

Chars Oxveen ChemisorDtion qa (mmole g i) qmaxD (mmole g 1) Initial Rate of Oxygen Chemisorption x 10 (mmole g" min ) Ignition Temperature0 (°C)... 

Figures 21 14) and 22 show the weight increase and heat of reaction due to chemisorption of oxygen on fresh char determined by thermogravimetry (TG) and differential scanning calorimetry (DSC). In low-density fibrous cellulosic materials where the heat loss is restricted but oxygen can penetrate by diffusion, the heat flux from chemisorption could play a significant role in the ignition of the active...

Figure 21. Differential scanning calorimetry and thermogravimetry of oxygen chemisorption on cellulose char at 118 C. The analysis was carried out on 2.5-mg samples in aluminum pans using a Cohn R-lOO electrobalance and a DuPont calorimeter cell attached to a DuPont model 990 thermal analyzer, and nitrogen and oxygen gas flows (60 mL/min, dried by passing through H2SO4) were rapidly interchangeable for DSC and TG.

Figure 22, Differential heat of chemisorption as a function of the amount of oxygen adsorbed on cellulose char at 118 °C.

The surface areas of chars prepared from cellulose samples at different HTTs were determined by application of the Dubinin-Po-lany equation to CO2 adsorption at room temperature and compared with the area occupied by surface oxides calculated from oxygen chemisorption at 230 C. The results shown in Figure 25 indicate that cellulosic chars have large surface areas that vary according to the HTT, and peak at about 550 °C. The surface oxides formed by chemisorption occupy only a portion of the total surface area, and the chemisorption also shows a peak for chars formed at about 550 °C, corresponding to the temperature of smoldering combustion. 

Scheme 7. Oxygen adsorptioHy chemisorption, and oxidation of char.

Two Colorado oil shale samples one from the Parachute Creek Member and the other from the C-a tract, were retorted, de-charred and then subjected to temperatures between 800 K and 1100 K in order to study the mineral reactions which take place. Comparisions between these two samples include the reversible nature of ankeritic dolomite and free calcite as well as the temperatures at which significant silication takes place. Results for the C-a tract samples indicated silication appears to take place in stages and that ankeritic dolomite decomposition can be prevented by relatively low CO2 concentrations. Ankeritic dolomite and calcite decomposition rates were similar for the two samples and there was strong evidence that calcite recarbonation takes place via non-activated chemisorption of C(>2 ... 

This suggested mechanism is consistent with a number of observations made here and with previous work reported in the literature. For example it was found that recarbonation rates were relatively insensitive to temperature. This would indicate non-activated chemisorption and, as Fischbeck and Snaidt report (10), mineral recarbonation is often independent of temperature when the temperature dependencies of the decarbonation rate conr-stant and the equilibrium constant are similar. This is exactly what is observed in western oil shales (Ref (9), Equations (6) and (7)). Previous work has also pointed to the role of chemisorption phenomena in mineral decomposition reactions. Spencer and Topley (11) have suggested that finely grained oxides can chemisorb HoO as well as H2 and Soni and Thomson (12) observed higher recarbonation rates when CO2 was produced on the surface during oil shale char combustion. 

Char combustion kinetics have been previously reported for Antrim shale by Rostam-Abadi and Mickelson (9). In that study the authors reported that the rate was second order with respect to the char remaining and that there was noticeable chemisorption of (>2 Attempts to fit our data for the Antrim shale to a second order rate expression were unsuccessful and, in all cases, the data appeared to follow first order kinetics. Although we did not have the precision to measure O2 chemisorption, this phenomenon is consistent with our previous observations (6 ) of catalytic activity in those shales containing decomposed mineral carbonates. That is, the catalytic activity of CaO was attributed to its ability to chemisorb 02 As will be discussed in more detail below, the Antrim shale sample did not contain such carbonates and no catalytic behavior was observed. However, the magnitude of the rate constants reported by Rostam-Abadi and Mickelson (9) are very similar to those measured here. 

On the basis of the studies conducted here, it is readily apparent that the presence of minerals can drastically alter the reactivity of the residual char on spent oil shale More detailed quantitative studies of the mineral compositions are necessary in order to be able to assess their importance under typical oil shale processing conditions and will be the subject of future manuscripts from this laboratory. However, at this time, there are several conclusions which can be made. First, the combustion of the char in all six of the shales followed first order kinetics with respect to the oxygen partial pressure and the char available For the western shales this is in agreement with previous works which studied PCM and Anvil Points shales, but it does conflict with the results of Rostam-Abadi and Mickelson (9) who reported second order kinetics for Antrim shale. Secondly, we found that CaO has a catalytic effect on char combustion, most likely due to a chemisorption process And finally we found that Na20, as derived from the thermal decomposition of nahcolite, has a pronounced catalytic effect on the char combustion rate of saline zone shale ... 

Rgure 13.7 Chemisorption of oxygen and HCI on immersion in dilute hydrochloric acid of a heat-treated activated carbon firom carbonized sugar char. Heat treatment at 950°C followed by dry exposure to Oj at room temperature. (Reprinted from Ref. [14] with permission from Elsevier.)... 

Zhang, Z.L., Kyotani, T., and Tomita, A. (1989). Dynamic behavior of sufface oxygen complexes during oxygen chemisorption and subsequent temperature-programmed desorption of calcium-loaded coal chars. Energy Fuels, 3, 566—71. 

Teng, H., and Suuberg, E.M., Chemisorption of nitric oxide on char Irreversible carbon oxide formation, Ind. 

---