In early 2019, Power Engineering published a chemical report that can significantly improve the reactivity of limestone and the removal rate of sulfur dioxide (SO2) in wet flue gas desulfurization (WFGD) systems. [1] This article outlines the full application of a coal-fired power plant in the eastern United States, where chemistry improves washing efficiency. This in turn enables operators to reduce the number of mud recirculation (recirculation) pumps, thereby saving operating and maintenance costs.
In addition, because there are fewer circulating pumps in operation, the back pressure of the device is reduced, thereby reducing the load of the supercharger and the induced draft fan. Under the Affordable Clean Energy (ACE) rules, the ability to reduce parasitic electrical loads increases the heat rate of factories, which can allow some factories to better connect to the grid. [2] This may allow some factories to remain open, thereby saving local employment opportunities. Subsequent use of the patented chemical in other factories has achieved similar results, and even more.
This article summarizes data on the application of this chemical in City Water, Light & Power (CWLP) in Springfield, Illinois. Although coal-fired power generation in the United States has decreased, there are still a number of core power plants. Now, more than ever, the remaining factories are facing the challenge of finding creative ways to reduce operating costs while improving factory performance.
Figure 1. Spray tower, wet limestone FGD process flow chart.
To briefly review, the main mechanical and chemical processes of wet limestone scrubbers are as follows:
Figure 2. The scrubber by-product dehydration on the vacuum drum filter. Photo courtesy of City Water, Light & Power.
The efficiency and completeness of the reaction depend on kinetics and several important chemical and mechanical factors, the most notable being:
In many factories, if SO2 absorption and limestone reactivity can be enhanced, it may bring significant benefits. Since one or more circulating pumps can be shut down during normal operation, one of its advantages is to reduce the load on rotating equipment (and the corresponding operation and maintenance costs). This possibility has been proven in all applications, including the factory in Reference 1.
The second potential benefit involves the choice of limestone. Some factories cannot obtain high-purity limestone at any time. Stones may contain significant concentrations of dolomite (MgCO3∼CaCO3) or inert materials that inhibit reactivity. Therefore, supplementary methods are needed to promote the reaction.
A common method for many years has been to add dibasic acid (DBA) to the scrubber process stream, but as described below, the new technology has significantly improved this chemical property. Dibasic acid is the common name for a mixture of relatively short-chain dicarboxylic acids (two COOH functional groups). It adds hydrogen ions (H) to help the limestone dissociate and circulate throughout the process to continue to assist the SO2 absorption chemistry. However, The availability, cost, and even efficiency of DBAs make it a less than ideal chemical method.
The patented ChemTreat product FGD1105 shows better cushioning capacity, as shown in the table below.
Figure 3a and 3b. Comparison of enhanced chemical buffer capacity of scrubbers. Data provided by ChemTreat.
When titrated with sulfuric acid and hydrochloric acid, the buffering capacity of FGD1105 is significantly higher than that of DBA, and is much higher than that of other major alternatives (formic acid and lactic acid). Cushioning capacity is a key feature of these products. CWLP personnel tested all three additives shown in Table 3b, and observed and recorded the special properties of FGD1105 and its outstanding performance improvements.
In plants where SO2 emission requirements are more stringent than the original scrubber design, enhanced additives may provide additional efficiency gains. For example, in CWLP, 95% SO2 removal rate is the maximum achievable under the original design conditions. To obtain 97-98% removal rate, additives need to be added. Based on this concept, another very important benefit of some plants is that, compared with low-sulfur coal, the enhanced chemical reactivity may allow the use of high-sulfur, cheaper coal, which may have the same material cost but come from farther away Places, increased transportation costs. Lower fuel costs greatly increase the opportunities for dispatching factories, thereby improving the viability of the factories and the continued local employment of the factories.
CWLP personnel began to evaluate FGD1105 as a potential replacement for Unit 4's DBA in the fall of 2019. Unit 4 is a 230 MW opposed wall-fired steam generator, fueled by locally mined coal. The device has a complete air quality control system (AQCS), including a selective catalytic reduction (SCR) system for reducing nitrogen oxides (NOx), a pulse jet fabric filter (bag filter) and belt There are 620,000 gallon absorbers. Scrubber additives such as DBA have been directly injected into the absorber to improve the SO2 removal efficiency and make the flue gas SO2 concentration below 0.20lb/MBtu, which is approximately equivalent to 65 ppmv (parts per million by volume). Add additives in batches based on the unit load and SO2 concentration measured by the Continuous Emission Monitoring (CEM) system.
One of the significant advantages of FGD1105 is that the product does not need to be heated like a DBA, and must be kept within the range of 135-150°F, which is of interest to CWLP personnel. In addition, FGD1105 has a freezing point of -11.2°F, and does not require heating to prevent freezing, reducing the need for material handling.
The evaluation used a 250-gallon FGD1105 suitcase. After the DBA system was isolated, it was connected to the suction port of the existing DBA feed pump through a pipeline. As with the previous DBA feed protocol, batch feed is the method of test selection. The evaluation was conducted for 16 days between September 4 and September 19, 2019. Factory personnel measured and recorded various FGD performance data, including unit load (MW), sulfur dioxide emission concentration and added product volume. They then compared these data with DBA's 16-day operation under similar operating conditions. The following is a summary of the comparison data.
FGD1105 and DBA: CWLP test
(Multiplier to match MW load = 1.08)
FGD1105 performed very well. An obvious advantage is that FGD1105 reduced the chemical raw materials by 21% (268 gallons) and 21% in the same period compared with DBA. As shown in the figure below, the frequency of FGD1105 feeding and DBA has also been reduced.
Another result that was observed almost immediately was the increase in the reaction kinetics of FGD1105. Before the evaluation, even though the data showed that the DBA was almost exhausted, the SO2 concentration was still close to the allowable limit. After the initial injection of FGD1105, the SO2 concentration quickly dropped by about 25 ppm and continued to decrease. If the SO2 concentration rises sharply, the improved reaction kinetics can meet compliance goals during load fluctuations, mechanical failures, and other failures.
Another observation of this evaluation is that by adding FGD1105, the stoichiometric ratio of limestone to SO2 decreases and the reactivity of limestone increases.
The calculation of the limestone utilization rate (LU) comes from the laboratory test of the by-product cake by the thermogravimetric analyzer (TGA) [3]. This data allows the calculation of the limestone stoichiometric ratio (LSR), which determines the percentage of gypsum (CaSO4∼2H2O), residual calcium carbonate (CaCO3) and residual hemihydrate (CaSO3-1/2H2O)-the air oxidation process.
These results confirm other evaluations that show that FGD1105 can increase the reactivity of 90% grade limestone to an equivalent 95% grade material. Due to the different costs of various limestone grades, this advantage can be very significant, and if the quality of the delivered limestone is variable, it can also provide very high performance and efficiency.
In general, CWLP personnel are very satisfied with the evaluation and are currently arranging for a further comprehensive evaluation of the facility's other FGD systems.
Some power facilities use this scrubber additive to successfully improve FGD performance and reduce plant heat consumption. Competition among power plants for grid access and the need to increase unit availability are affecting efficiency improvement plans. In some cases, factories will compete for operations within their own fleets. ACE regulations reward plants that increase thermal efficiency.
The ACE rules also allow states to implement separate greenhouse gas (GHG) reduction plans. Ultimately, each state plan will reward generating units (EGU) that operate more efficiently.
FGD1105 is an enhanced product that can significantly improve scrubber efficiency. It provides an exciting alternative for factories seeking to improve scrubber performance and reduce parasitic power loads. In some cases, it may provide the opportunity to switch to lower-cost limestone (although careful testing is required), or it may even use coal supplies with higher sulfur content but lower cost than current fuels.
About the author: Brad Buecker is a senior technical publicist at ChemTreat. He has 4 years of work experience in the power industry, most of which are in steam generation chemistry, water treatment, air quality control, and the results of City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Electric Corporation Engineering positions. Light Company's La Cygne, Kansas Station. Buecker holds a bachelor's degree in chemistry from Iowa State University, in addition to additional courses in fluid mechanics, energy and material balance, and advanced inorganic chemistry. He is a member of ACS, AIChE, AIST, ASME, NACE, Electric Chemistry Symposium Planning Committee, International Water Conference Advisory Committee and Power-Gen Planning Committee. You can contact him at bradley.buecker@chemtreat.com.
Branden Powell is the Environmental Process Supervisor for Urban Water, Light and Power (CWLP) in Springfield, Illinois. Brandon graduated from the University of Illinois at Springfield in 2001 with a degree in chemistry. After graduation, Brandon started his organic career as a chemist at TMI Analytical and then transferred to Prairie Analytical Systems, both of which are IEPA contract laboratories. At Prairie, Brandon has been a senior organic coordinator and laboratory supervisor. In 2015, Brandon became CWLP's factory laboratory supervisor. The following year, he was promoted to his current position, where he still supervised the factory laboratory and factory chemistry in the electrical department, and also supervised the factory's FGD operations, FGD wastewater operations, and factory discharge wastewater operations. Branden is a committee member, frequent writer and speaker of the University of Illinois Power Chemistry Symposium, which is held annually in Champaign, Illinois. You can contact him at Branden.Powell@cwlp.com.
Dave Karlovich is a strategic account manager for ChemTreat and a U.S. Navy veteran. He has more than 30 years of experience in supporting water treatment applications in the nuclear and fossil fuel power generation industries. His recent focus is on chemistry to improve the process side and wastewater treatment of wet flue gas desulfurization systems in factories across the country. He is the co-inventor of ChemTreat's patented FGD1105 technology, which improves SO2 removal and limestone reactivity in wet scrubbers.