Developments in Sorbent Injection Technology
for Sulfuric Acid Mist Emissions Control魔书
Paul S. Nolan
社保号查询Chemical Lime Company, 3700 Hulen Street, Fort Worth, TX 76109
E-mail: paul.; Telephone: (817) 806-1546; Fax: (817) 732-8144
Summary
As the U.S. electric power industry faces increasingly stringent emission control regulations, the economic benefits of using integrated solutions requires due consideration of the impacts of applying a given t of process technologies for the site-specific conditions of any given plant. The widespread application of lective and non-lective catalytic reduction (SCR and SNCR, respectively) for the control of nitrogen oxides (NO x) has resulted in attendant impacts on equipment and process downstream of the economizer, leading to incread formation of sulfur trioxide (SO3) and subquent potential emission of undesirably high concentrations of sulfuric acid mist. Moreover, the incread u of fuels or fuel blends with higher vanadium content (a catalyst for SO3 formation) has also contrib
uted to the phenomenon. In the abnce of direct federal regulatory measures and under some ts of circumstances, the effect can be minimal or even lf-controlled, but for many mitigation has effectively become esntial. With the pressure to address mercury and fine particulate emissions at the same time, utilities are eking solutions cost-effective solutions with synergetic effects.
The particular combinations of fuels burned, the combustion conditions, the lection and design of the NO x control technology, and the types of particulate collection and desulfurization techniques employed all contribute to the ultimate physical characteristics and chemical composition of the flue gas leaving the stack. Within the past three years, Chemical Lime has had veral opportunities to work with industrial and utility customers on trials and demonstration tests of hydrated lime injection as a means of reducing flue gas SO3 concentration and/or opacity resulting from the formation of sulfuric acid mist. The nature of the tests have ranged from one extreme of being very short (veral hours) “proof of concept” trials to extended demonstrations lasting a week or more that have been planned with the customer to explore potential longer term impacts and compatibility with other emission control systems, especially dry electrostatic precipitators (ESP).数据透视图
The specific results of the individual tests have usually been considered confidential as the tests have been exploratory in design or directed at potential performance of competing technological app
roaches at a given plant. It can generally be said, however, that the not-too-surprising primary conclusions are that, properly applied, calcium hydroxide as the strong chemical ba in commercial hydrated lime is an effective sorbent for the capture of strongly acidic, gas-pha SO3, and that the removal is primarily a function of the feed stoichiometry and the residence time available for diffusion of the gas to the reactive sites on and within the lime sorbent. Several published studies1 over the past decade have established this acid-ba relationship as fundamental. The apparent “ea” with which one can remove SO3 soon becomes more complex as choices must be made regarding which approach and technology might prove to be most cost-effective for a given t of site-specific conditions. For this reason, the overall significance of the various tests conducted points to the value of asssing the following considerations, some of which are necessarily interrelated, as part of the decision-making process.
Basic Objective Identification of the performance actually required – one must consider the mechanism(s) of SO3 formation and conversion to sulfuric acid via reaction and dilution with water as the flue gas pass through the time-temperature profile(s) associated with the boiler and its chemistry (including the u of alkaline additives for slag control), SCR catalysis or SNCR chemistry, and potential “inherent” capture as a result of localized condensation and/or physicochemical proces
s further downstream – each of which may affect where the sorbent injection may most effectively be utilized. It is also important to consider the mechanisms of sulfuric acid condensation, the significance of aerosol droplet size distribution particularly as it relates to opacity, and potential neutralization by alkaline components of the fly ash.2
八下英语课文翻译Injection Location Selection of the injection location(s) can have a substantial impact on overall sorbent utilization. Sorbent effectiveness is verely diminished when injection is directed at areas where the gas-pha SO3 has already condend into aerosol droplets. Once this occurs, reactions can esntially only take place as a result of inertial impaction between the droplet and the lime particle, a process that requires a much higher pressure drop than occurs in a typical flue gas path. It therefore becomes important to consider that SO3 concentrations can be easily altered by the degree of uniformity of
ammonia or urea injection for the SNCR or SCR process that results in some neutralization by excess ammonia, variation of catalyst activity with respect to SO2 oxidation to SO3 over a period of time, and condensation due to potentially significant temperature gradients, particularly near the “cold end” of air preheaters and as a result of air in-leakage near expansion joints. Understanding the concentration profile at the injection plane can permit one to bias the injection accordingly to impr
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ove utilization.
One other aspect of sorbent injection location that may prove to be worth considering is the possibility of lecting more than one injection location. While not widely practiced to date, it may be advantageous to direct some of the material into areas where an initial partial reduction is deemed advisable, for example, as a means of minimizing ammonium bisulfate formation while maintaining a relatively low particulate loading to lesn potential impacts on heat transfer surfaces. Additional sorbent can then be directed as needed to a downstream location, perhaps even between an ESP and a wet flue gas desulfurization system that, if operational conditions permit, will capture and u the residual alkalinity.
ps图片格式Sorbent Dispersion Effective dispersion of the sorbent at whatever locations are lected is critical to achieving high utilization. For smaller applications, it is often most cost effective to provide a simple blower arrangement to inject the sorbent into the flue. Becau hydrated lime is a fine powdery material, it generally follows the gas path and readily dispers into the gas within a few feet of the injection port or lance, forming a cloud veral feet wide. As the application becomes larger, however, it becomes increasingly important to understand both the means of injection and the velocity (and sometimes temperature) profile of the flue gas into which the sorbent is injected. For s
uch larger applications, multiple injection lances become necessary in order to deliver reasonable portions of the sorbent to lected areas of desired coverage. Consideration of flow imbalances that may depend on ductwork geometry and varying load conditions become important, as well as an understanding of the effective residence time of the disperd sorbent required to accommodate the diffusion and reaction kinetics. Generally speaking, minimum residence times of two or three conds are conducive to higher removals at reasonable stoichiometric feed rates. The u of cold flow models or computational fluid dynamic (CFD) models can be instructive in identifying not only how the sorbent will disper under various ts of load conditions, but also areas where eddy currents, recirculation patterns, and other geometric boundaries are liable to give ri to conditions that can affect performance either positively or negatively.
Downstream Effects on Dry ESPs Applications involving dry ESPs must be carefully considered due to the potential for developing a “back corona” condition that can lead to significantly diminished particulate collection. Dry ESP design parameters need to be reviewed with respect to the unit’s ability to capture higher resistivity ash. For some, the designs may be able to accommodate the relatively minor increa in ash alkaline components, while for others minimal levels of humidification to temperatures still above the acid dewpoint might be expected to restore, if not improve, performan
属虎的命运ce. Sorbent Properties In preceding studies, consideration of sorbent properties has been relegated to being one of lower importance due to the perception that, for the most part, they are commodity chemicals that have relatively low variability beyond basic purity levels. This is changing, however, as Chemical Lime and its parent company, Lhoist, are in the process of developing calcium hydroxide-bad SO3 sorbents with physicochemical properties that appear to provide improved reactivity and utilization. On-going bench-scale and small pilot studies that indicate perhaps 50 to 100 percent improvement in sorbent utilization is expected to translate into lower feed stoichiometric requirements that will also lesn the potential impact of the downstream effects on dry ESPs. A more extensive, full-scale evaluation of the sorbents is planned for this summer.
References
1. R.K. Srivastava, et al., “Emissions of Sulfur Trioxide from Coal-fired Power Plants,” Paper prented at POWER-GEN
International 2002, Orlando, FL, December 2002, and references therein
蜜汁烤肉2. W.P. Buckley and B. Altshuler, “Sulfuric Acid Mist Generation in Utility Boiler Flue Gas,” Power Engineering,
November 2002
