*3.6. Orthogonal Test of Evaporation Performance*

The orthogonal method is adopted to compare the degree of influence of various conditions on the evaporation performance, which includes five factors: flue gas flow rate, flue gas temperature, wastewater flow rate, initial wastewater temperature, and droplet size. Each factor is set at three levels with a total of 18 cases. The orthogonal results of each factor on the droplet evaporation performance are shown in Table 4. The orthogonal test results show that the evaporation ratio of cases 19, 21, 24, 27, 28, and 29 cannot reach 100%, and the evaporation

ratio of case-21 is the lowest, only 90.39%. Compared to other cases, this case has the lowest flue gas flow, the largest wastewater flow rate, the largest flue gas/wastewater ratio, and the lowest initial temperature of the wastewater. Therefore, the input heat is insufficient, the evaporation performance is poor, and the desulfurization wastewater cannot be completely evaporated. According to the comparison of the R-value of the complete evaporation distance of each case, the factors affecting the droplet evaporation performance are ranked as follows: flue gas flow rate > wastewater flow rate > flue gas temperature > wastewater initial temperature > droplet size. Considering the evaporation ratio and the complete evaporation distance, the optimal setting is 14.470 kg/s for flue gas flow rate, 385 ◦C for flue gas temperature, 0.582 kg/s for wastewater flow rate, 25 ◦C for wastewater initial temperature, and 60 μm for droplet size.

**Figure 18.** Effect of droplet sizes on evaporation performance.


**Table 4.** Orthogonal table of each factor.

**\*** The evaporation ratio is defined as: · (*mout* <sup>−</sup> · *min*−*flue gas*)/ · *mwastewater*, where · *min*−*flue gas* indicates the mass flow of the flue gas injected into the spray drying tower, · *mout* indicates the mass flow of the mixture gas flowing out of the spray drying tower, · *mwastewater* is the mass flow of the wastewater (liquid) injected into the spray drying tower. *Ki* (*i* = 1, 2, 3) indicates the test index sum of each factor at the same

level; *ki* (*i* = 1, 2, 3) indicates the test index average value of each factor at the same level; *R* indicates the range, which refers to the difference between the maximum value and the minimum value of the sum of each level index in the same factor, *R* = max{*k*1, *k*2, *k*3} − min{*k*1, *k*2, *k*3} in any column. For more details about the orthogonal test please refer to [29].
