Page 186 - New Trends In Coal Conversion

Technologies for control of sulfur and nitrogen compounds and particulates 149

pilot-scale combustion facility. Under the staged case, an usual NO x trend was obtained by
the following measures: coal nitrogen can be rapidly converted to NO x and then reaches a
peak concentration in the primary combustion zone, followed by a significant reduction in
NO x in the reduction zone. The greater the staged level is, the earlier the reduction stage
starts. With the increase in the staged degree, raising the O 2 concentration promotion causes
a decrease in the rise of NO x emission. NO x emissions go up again when the oxidation of
unknown intermediate compounds that contain nitrogen occurs (Fan et al., 2017). Fan con-
clusions can be drawn from the tests: (1) In the staged cases, the NO x curves show significant
reductions in NO x in the reduction zone; (2) The burnout air addition caused the NO x to rise
again to some extent. The NO final emission level is determined by the burnout air injection
point; (3) There exists an NO reducing saturation phenomenon in an overlong reduction
zone; (4) There is no essential difference in the mechanism of NO formation and destruction
between staged combustion under oxygen enrichment and air-staged combustion; (5) The
low-level NO x emissions in the reduction zone would rise again to a larger value after
burnout air addition.
• Other researchers (Zhao et al., 2017) made a study about particulate filter systems or cata-
lysts. The de-NO x performance of the Cu-beta zeolite (BEA) selective catalytic reduction
(SCR) catalyst with Ce and Nb as additives was investigated for catalytic reduction systems.
The Cu-BEA catalyst with the Ce and Nb additives improved NO x conversion, compared
with the Cu-BEA catalyst without additives. This result is related to the strength of the in-
tensity ratio as (Cu2p3 B peak)/(Cu2p3 A peak) for the Ce and Nb added catalysts.
• Fern andez-Miranda et al. (2016), also in this field of studies, evaluated SCR catalysts under
an oxy-combustion atmosphere, focusing on the main differences in conventional air coal
combustion. Under the conditions of this study, a higher oxidation reaction in the CO 2 -
enriched atmosphere was observed. In these conditions, NO x conversion was less, producing
an elevated concentration of NO and free NO 2. In oxy-combustion conditions, the high
amount of CO 2 and H 2 O present make the active sites blocked for mercury adsorption.
• Regarding the supercritical coal-fired system, Chen (Chen et al., 2017) made a study about
a 660 MW supercritical coal-fired power plant with a feedwater heater coupled with a
steam ejector and applied to address the NO x removal problem under low load conditions.
The model results obtained were successful to the point that a good NO x removal effi-
ciency could be reached (70%). This method could be effective under extremely low loads,
and NO x removal efficiency remained efficient when the unit approximated the 20% THA
condition.

6.2.2 SO x emission control technology
6.2.2.1 What is SO x ?

SO x represents both SO 2 and SO 3 emissions that are considered one of the criterion
pollutants derived from anthropogenic activities established in the Clean Air Act of
1970 (Roy and Sardar, 2015). Sixty-five million tons of SO 2 are emitted every year
worldwide, which corresponds to 14% of the total criterion pollutant emissions derived
from industrial activity and energy production. Most of them are SO 2 , which accounts
for almost 95% of the share (Wang et al., 2005). SO 2 causes environmental damages
and human health problems such as urban smog and acid rain. It also produces the
acidification of soil and water (Roy and Sardar, 2015).