7884079644

Fig. 3. Behavior of CO and C0₂ conceniration in the stack gases as a function of stoichiometric raiio ai ihe exit of the furnace (a_f) and steam power. The fitted lines correspond to Eq. (18) for values of the steam power of 20 and 50 t/h (lower and upper lines).

Steam power (t/h)

■ 20 o 30

0.0165

08)

the different stoichiometric ratios and steam powers anaiyzed. The statistical model relating carbon monoxide, expressed as a fraction of the total carbon oxides, to both a_f and £>_sh, is

= 0.0275 - 0.01485a_f

CO + C0₂

It is important to notę that the physical parameters measured in the tests are the 0₂, CO and C0₂ concentration in the exhaust gases, which are used to calculate the stoichiometric ratio, a_b, at the exit of the boiler applying the equation [2]

(19)

where 0₂, CO and C0₂ are the stack gases composition analysis. This equation is obtained using the combustion reactions as a function of the mole number, following the definition of the stoichiometric ratio previously stated in Section 3.

However, Eqs. (17) and (18) are correlated to the stoichiometric ratio at the furnace exit, <*f, because of the physical dependence of both C_uf and CO on a_f rather than on a_b. These two stoichiometric ratios are closely related throughout the air in-leakage. Aa, by a_b = a_f + Aa    (20)

Air in-leakage. Aa, represents the leakage of surrounding air, due to non-air tightness, into the boiler and can be calculated by [5]

(21)

where Aa^nom is the air in-leakage at the nominał steam power (D"_b^m = 45 t/h). This parameter, Aa^nom, was previously determined for all boilers tested and yielded a roughly constant value of 0.2.

Finally, to calculate the exhaust gases heat loss, q₂, the exhaust gas enthalpy, /₅, needs to be known. This enthalpy depends on the stack temperaturę, T_eg. The experimental data show a linear dependence of T_eg on the stoichiometric ratio, as seen in Fig. 4. As in Fig. 3, errors bars are only depicted for the lowest and highest values of the steam power (20 and 50 t/h). Fitting a curve to the measurements relating T_eg to <*f and D_sh, the following eąuation is obtained

T_eg (°C) = 172.32 + 24.76a_f + -j= - 0.213(Z>_sh)^{0 33}

(22)

as shown in Fig. 4 by the solid lines for 20 and 50 t/h, respectively. Even when the dependence of T_eg on D_sh is weak, it is noteworthy that the stack temperaturę is raised as the steam power decrease. When the steam power is decreased for a constant stoichiometric ratio at the furnace, combustion air flow ratę also decreases, reducing the average gas velocity in the furnace as well as the heat transfer in the waste heat recovering scheme. As a result, a higher exhaust gases temperaturę is measured at the exit of the boiler.

The three formulae, Eqs. (17), (18) and (22), based on the expected physical relations among the parameters, were obtained using the STATVIEW commercial codę [6], and are valid for steam power values ranging from 20 to 50 t/h and for stoichiometric ratios at the furnace exit, a_f, from 1.2 to 1.6. All the experimental measurements were included in the statistical fitting process and the regression coefficients R² were always greater than 0.9.

If the set of Eqs. (17), (18) and (22) is introduced into the methodology to calculate the heat losses r/₄, q₃ and q₂, their