Page 135 - New Trends In Coal Conversion

98 New Trends in Coal Conversion

S c Y N;char MW NO
S char;NO ¼ h (4.9)
MW N $V
where S c , Y N,char , V, h, and MW N represent the char burnout rate (kg/s), mass fraction
3
of nitrogen in char, cell volume (m ), conversion factor, and molecular weight of N
(kg/kmol), respectively. The conversion factor, h, is to account for that some of the
char-N is converted to N 2 , as shown in Fig. 4.2.
The NO formed may be reduced to N 2 over the residual char due to its catalytic ef-
fect on NO formation and reduction. The NO reduction rate can be calculated as
follows:

S NO;reduction ¼ c s A BET MW NO $0:23e 17166:3=T X NO P atm (4.10)

3
where c s , A BET , X NO , and P atm are the fuel particle concentration (kg/m ), particle BET
2
surface area (m /kg), mole fraction of NO, and pressure (atm), respectively.
If neglecting the direct conversion fuel-N to NO x , the NO source term due to fuel
NO x mechanism can be summed up as follows:

S Fuel NOx ¼ S volatile;NO þ S char;NO S NO;reduction (4.11)

Compared with thermal NO x , which depends on the main combustion simulation
results such as temperature and oxygen, fuel NO x prediction also relies on:
• the fuel-N split ratio in volatiles and char.
• the volatile-N partitioning for HCN and NH 3 . For low-ranking (lignite) coal and biomass,
NH 3 is the major NO x precursor, whereas for higher-ranking (bituminous) coals, HCN is
the major precursor.
• the conversion factor of char-N to NO, h. The contribution of char-N to the NO x precursors is
often neglected. If also neglecting char-N conversion to N 2 , a conversion factor of char-N to
NO, h ¼ 1, can be used.

4.3.7 Ash behavior

In coal/biomass cofiring, noncombustible material in the fuel such as fly ash particles
and salt vapors may be deposited on wall surfaces in a boiler. When wall surface tem-
peratures are lower than local gas temperatures, the alkali salt vapors may be
condensed to form submicron droplets/particles in the boundary layer close to the
walls and get deposited on the wall surfaces. After the initial condensed deposit is
formed, the large and intermediate-sized particles start to stick on the wall surfaces.
Modeling of ash deposition in solid fuel combustion often considers the following
mechanisms (Baxter, 1993; Wang and Harb, 1997; Lee and Lockwood, 1999; Kær
et al., 2006; Weber et al., 2013):
• For the submicron salt vapor droplets/particles, the deposit is formed mainly via diffusion,
turbulent eddy impaction, and thermophoretic mechanisms.