| OCR Text |
Show an ideal model would treat the inter-relationship of these factors and devolatilisation with char-N, and hence N O x release during char combustion1'3,12"14. 1.3. NOx formation Nitrogen oxides arise from three routes, namely thermal (reactions of N2 with O at high temperatures), prompt (reaction of N 2 with fuel radicals), and fuel (release of fuel nitrogen during combustion). In many C F D models these latter two routes are represented by global reactions. The fuel-NO has a contribution from both the volatiles and the char nitrogen and it is assumed that all volatile nitrogen is released as H C N , and an empirical relationship is used to estimate the fraction of char-N converted to NOx. A more accurate prediction on NOx release can be made if the full NOx chemistry is used. This is discussed in more detail later. 2. Approaches to Improving Coal Combustion Models 2.1. Devolatilisation Devolatilisation and heating rates The rate of devolatilisation of different coals is a topic of great debate as illustrated in Figure 1, which emphasises the wide differences in rates measured using different experimental techniques. These discrepancies are thought to arise because of the differences in heat transfer mechanisms within a flame, drop-tube furnace or wire mesh apparatus. In general, the heat-up rate of a coal particle is given by: mpCp-^- = hAp(Tg-Tp) + EaAp(T$-Tp i) + {Qvap+Qvol+Qc) (10) The sum of {Qvap + Qvo, + Qc} is generally small and is neglected in the following calculations. There are uncertainties in the value of C p but it is known that this is a function of the particle temperature, and firstly increases, reaches a maximum at 773 K and then decreases; the value varies non-linearly between 1-2 kJ kg"1 K'1.15 The solution to equation (10) depends upon the circumstances, and two different cases are considered here: (i) in a flat flame (1673 K) the particle heat up is largely via conduction where the heat transfer condition at very low Reynolds number, h = 2k/D; radiation losses from the particle become important as the flame temperature is reached. With a coal particle initial diameter of 50 urn, the time temperature curve is exponential and the time to reach half of the final temperature (t,/2)is 10 ms16. 4 |
