Volatile matter and stoichiometry

Assuming that coal is represented by CH08O01 and DB by CH1 л O0 4, Figure 3.12 and Figure 3.13 plot the fraction of C atom remaining as volatile matter in coal and DB and the stoichiometric oxygen per kg volatile matter for coal and DB; if coal releases less VM for given H/C and O/C, then less C atoms leave with VM and more remain in char; typically coal has 40-50% as VM and as such, the stoichiometric O2 is about 2.5 kg per kg VM released; the DB has 80% VM and as such the stoichiometric O2 is only about 1.5 kg/kg of VM and lower compared to coal volatiles due to high O/C ratio in DB. At the same time, the % O2 consumed by coal during combustion of VM is only 50% while DB with VM consumes 78% of oxygen by the time all the volatile mater is burnt indicating that there is rapid depletion of O2 when DB is blended with coal resulting in lower O2 in the furnace which may lead to lesser NOx. In general AgB and AnB fuels with high VM may result in lesser NOx compared to coals. Stoichiometric A:F

The mass based stoichiometric A:F ratio is simply the ratio of the minimum amount air required by fuel for complete combustion on a mass basis. This is calculated based on the empirical chemical formula derived from the ultimate analysis neglecting the moisture and ash in the fuel. Flue gas volume

One can determine the volume of flue gas in m3/GJ from the knowledge of ultimate analysis, Boie equation and reaction equations of biomass with O2 supplied from air (Chapter 4, Annamalai and Puri, 2007). If the SATP is at 25°C, and 1 bar, then for any arbitrary 0 < O2% < 9% (volume), a fit is given as:

Fluegas STP volume, m3/GJ = {3.55 + 0.131O2% + 0.018 x (O2%)2}(H/C)2

— {27.664 + 1.019O2% + 0.140 x (O2%)2}(H/C)

+ {279.12 + 10.285O2% + 1.416 x (O2%)2} (3.16)


Figure 3.12. Fraction of C atoms remaining with VM fraction, coal.


Figure 3.13. Fraction of C atoms remaining with VM fraction, Dairy Biomass (DB).