Bituminous coal, devolatilization

Gillen, J. E. Mills, R. E. 1998. Method for Obtaining Devolatilized Bituminous Coal from the Effluent Streams of Coal. Fired Boilers. US Patent No. 5,779,764. 

Figure 7 shows carbonized coal particles of the sieve fraction 5 to 3 mm. It can be seen that for all types of coals practically all particles display devolatilization pores. This result, obtained by counting, is particularly interesting for the low volatile bituminous coal (No. 6 in Figure 7) because in high temperature cokes produced under normal conditions from another coal of the same rank, the particles of carbonized coal show no pores and will be found as so-called unmelted particles in the coke. 

Anthony et al. [10] studied rapid devolatilization of monolayers of lignite and bituminous coals supported on wire mesh heating elements in helium. They calculated... 

The same trend has already been found in experimental studies [6, 9, I8j. In Figure 3 the conversion of fuel-N to NO and NHj during devolatilization of single fuel particles (ranging from bituminous coal to different biomass fuels) at 800 C and 10 kPa oxygen concentration is shown. It can be seen that these experiments provide the same qualitative trend, though for the data obtained in the Formation Rate Unit (FRU, refer to [6]) the sum of NO and NH] have to be viewed, since the residence time in this lab-scale unit is very short. 

Gale, T.K., Bartholomew, C.H., and Fletcher, T.H. Decreases in the swelhng and porosity of bituminous coals during devolatilization at high heating rates. Combustion and Flame, 1995,100,94. 

A large volume of work has been reported on rapid devolatilization of coal (heating rates approximating process conditions (21,22). Recently, the effects of coal minerals on the rapid pyrolysis of a bituminous coal were reported by Franklin, et al ( 23). They found that only the calcium minerals affected the pyrolysis products. Addition of CaCO3 reduced the tar, hydrocarbon gas and liquid yields by 20-30%. The calcium minerals also altered the oxygen release mechanism from the coal. Franklin, et al. attribute these effects to CaCOj reduction to CaO, which acts as a solid base catalyst for a keto-enol isomerization reaction that produces the observed CO and H2O. 

Further experimental data and further model comparisons relate to the rapid pyrolysis of different coals. In the absence of air, this experimental device heats and converts small coal particles (10-200 pm) in gas and distillates. Figure 20 shows a very satisfactory agreement between experimental data relating to a bituminous coal and model results at 1,260 K. It is noteworthy that despite the strong differences between carbon deposit and bituminous coal, the characteristic times for the dehydrogenation processes are practically the same. Further data on this subject, as well as a detailed model for the analysis of the pyrolysis and devolatilization process of coal particles, are available in a recent paper (Migliavacca et al., 2005). 

Coals were devolatilized at rates comparable with those encountered in combustion and gasification processes. Rapid pyrolysis was attained with pulse-heating equipment developed for this purpose. This technique permitted control of the heating time and the final temperature of the coal samples. Subbituminous A to low volatile bituminous coals were studied. All bituminous coals exhibited devolatilization curves which were characteristically similar, but devolatilization curves of subbituminous A coal differed markedly. The products of devolatilization were gases, condensable material or tar, and residual char. Mass spectrometric analysis showed the gas to consist principally of H2, CHh, and CO. Higher hydrocarbons, up to C6, were present in small quantities. 

Figure 1. Devolatilization of bituminous coals by rapid heating...

In contrast to the results obtained with bituminous coals, the weight-loss curve of subbituminous coal exhibited no peak instead, it reached a plateau in Figure 2. From 800° to 1000°C the volatile yield remained level at about 42 wt % of the coal. Beyond this region the production of volatiles increased sharply. The fact that the devolatilization curve of subbituminous A coal differs distinctly from those of bituminous coals indicates a need for further study of other subbituminous coals and lignites. Low rank materials such as these are of interest in coal gasification because their reserves are abundant and because they are situated in deposits with shallow ground cover. 

Reactivity of petroleum coke, like all solid fuels, is a function of chemical structure. Recognizing that the vast majority of all petroleum coke is produced in delayed cokers, analysis focuses upon delayed petroleum coke. Reactivity measures used here include maximum volatile yield and both devolatilization and char oxidation kinetics. Black Thunder Powder River Basin (PRB) subbituminous coal and Pittsburgh 8 bituminous coal are shown, for comparison, as reference fuels. 

The pyrolysis/devolatilization kinetics determined for the relatively volatile petroleum coke can be compared to those for Black Thunder subbituminous coal, and Pittsburgh 8 bituminous coal. This comparison is shown in Table 2.4. Note the higher pre-exponential constant. A, and activation energy, E, associated with the petroleum coke, relative to the reference coals. The kinetic parameters shown above are consistent with the lower maximum volatility of petroleum coke. 

Fig. 3 Comparative anode half-cell polarization of various carbon materials as reported by Weaver (1979), showing increase in rate at fixed polarization from graphite to devolatilized bituminous coal. The rate at 0.8 V spans four orders of magnitude.

Table 2.4. Short Table of Devolatilization Kinetic Parameters of Petroleum Coke, Compared to Representative Eastern Bituminous and Powder River Basin Coals...

The volatile nitrogen evolution for biomass can be compared to that for bituminous or PRB coal through the calculation of nitrogen/carbon (N/C) atomic ratios in char resulting from devolatilization at any given temperature. The results for any given fuel are then normalized to the initial N/C atomic ratios of the incoming fuel. 

---