BOILER MACT BACKGROUND GEOENERGY WET ESPS FOR BOILER MACT APPLICATIONS The previously promulgated MACT rules for industrial boilers were vacated by the federal court of appeals in 2007. With this vacature, the court instructed the EPA to develop new rules that would comply with the intent of Congress as expressed in the Clean Air Act. After a variety of attempts, the EPA issued the final rule on December 21, 2012. The publication of this rule in the Federal Register is expected before the end of February 2013. After publication, affected operators will have three years to comply. The final rule is 282 pages long and is necessarily complex because it applies to over 14,000 boilers and process heaters. However, the bulk of the compliance burden falls on roughly 1000 large industrial boilers and a large share of this burden will be directed toward biomass-fired units. The rule will require maximum achievable control technology (MACT) to control four categories of hazardous air pollutants (HAPs). These are Total particulate matter Hydrogen chloride Mercury Carbon monoxide Of these, two are surrogate categories; particulate matter for all HAP metals and carbon monoxide for organic HAPs. (The rule also addresses dioxin and furan emissions but essentially waives applicability of this category for a variety of technical reasons.) The scope of this paper is to discuss the role of wet electrostatic precipitators in achieving compliance with the new MACT requirements as they apply to large boilers fired with biomass. WET ESP APPLICABILITY Many of these large industrial boilers are not biomass fired and use coal, natural gas and, in a few cases, liquid fuels. As a general rule, non-biomass boilers have no controls, as is the case when natural gas is the fuel, or utilize dry controls such as fabric filters or dry electrostatic precipitators. Dry ESPs and fabric filters are technologies that are very suitable for achieving the MACT requirement for particulate emissions. Also, there are injectable sorbents that can be used with these dry systems to achieve compliance with the requirements for hydrogen chloride and mercury. Examples of such sorbents are sodium bi-carbonate and powdered activated carbon. Compliance with MACT requirements for carbon monoxide is normally associated with boiler operating practice, not add-on controls. With respect to boilers that are not fired with biomass the experience-based conclusion is that dry technologies are the accepted approach for MACT compliance.
Biomass-fired boilers tend to be different. Many existing biomass fired industrial boilers with downstream controls are treated with wet scrubbers. The reason that existing large biomass boilers are frequently treated with wet scrubbers is historical. Most are older boilers installed when compliance requirements were much less rigorous than today. As a result, lower-cost and less effective medium/low energy wet scrubbers were installed. These wet systems also had the advantage of not being susceptible to downstream fires that are a possibility when firing a variable fuel such as biomass. Given the foregoing, our analysis of the control technology landscape leads to the conclusion that if the boiler is not biomass fired the best direction for MACT compliance is to work with existing dry controls or to install dry controls. However, if the boiler is biomass fired and equipped with a wet scrubber the answer to the question of how to comply with the MACT requirements is not so clear. In approaching this question, the obvious first step is to determine if an existing wet scrubber can reliably achieve less than the required emission. If the answer is no, the second part of the question is what can be done to an existing wet scrubber to ensure compliance. Presently, there are no wet scrubbers on biomass fired boilers that routinely meet the MACT standards for particulate matter. These wet units do generally comply with the rule for hydrogen chloride emissions and because there is normally little or no mercury in biomass fuels compliance is well established for mercury. There are two approaches to bringing an existing wet scrubber into MACT compliance for particulate matter. The first is to increase the efficiency of the scrubber by increasing the energy input. The correlation between scrubber energy input and efficiency is well established. The second approach is to add another technology downstream of the scrubber; e.g., a wet ESP. Increasing the efficiency of a wet scrubber by adding energy is a difficult proposition. Most wet scrubbers on biomass fired boilers operate at less than 20 inches w.c. pressure drop. Achieving MACT compliance for a stoker boiler firing biomass will require a particulate emission of less than 0.037 lb/mm BTU but a typical wet scrubber in this category performs at a level of closer to 0.1 lb/mm BTU. Thus, a major increase in pressure drop would be required to be reliably below the requirement. With existing scrubber systems this means, at a minimum, scrubber internal revisions, and increased fan horsepower. It may also mean changes to the system ductwork and foundations. It also would mean significant boiler down time during the scrubber modifications. In addition, such a scrubber upgrade would still result in higher operating costs. There is another downside to improving the performance of a wet scrubber to achieve boiler MACT compliance: This is the new ambient air quality standards for PM 2.5 recently issued by the EPA. These standards will put increasing pressure for emission reductions on the states and in turn industrial operators. Thus, even if the costs associated with increasing the energy input to a wet scrubber to achieve boiler MACT are acceptable it is unlikely that the final emission will be much less than the required MACT level and may be inadequate to achieve future PM 2.5 requirements. On the other hand experience with wet ESPs has demonstrated the capability of this technology to greatly reduce the particulate emissions from scrubber-equipped biomass-fired boilers. (A detailed discussion of this experience is provided in the following section.) 2
Our conclusion is that for a wet scrubber on a biomass-fired industrial boiler the best path to MACT compliance is to add a wet ESP. The advantages of this approach are: Enhanced emission reduction performance Less down time required Lower energy GEOENERGY WET ESP PERFORMANCE ON BOILERS Lundberg has installed Geoenergy E-Tube Wet ESPs on six individual boilers at various pulp mills. A summary of these installations is shown below. WET ESP INSTALLATIONS ON BOILERS Location Northwest Northwest Northwest Northwest Southeast Southeast Steam Rate (lb/hr) 200,000 200,000 600,000 210,000 600,000 500,000 Equipment Installed Wet ESP Wet ESP 2 Wet ESPs 1 Wet ESP 2 scrubbers w/wet ESPs 2 scrubbers w/wet ESPs Inlet Gas Flow (acfm) 200,000 200,000 285,000 182,000 372,300 228,000 Inlet Gas Temp (deg. F) 138 138 139 400 425 545 Inlet Particulate (gr/scfd) 0.05 0.05 0.05 0.10 1.40 0.30 Pressure Drop (in. w.c.) 1" 1" 1" 1.5" <2" 1.5" Outlet Particulate (gr/scfd) 0.0033 0.0010 Not available Not available 0.0071 0.0077 Emission control performance at each of these has exceeded the guarantee requirements and greatly exceeded the present requirements for MACT. Of particular interest are the data reported for the wet ESPs installed at one of the boilers in the northwest downstream of an existing scrubber system. In this case, we have the results of 46 stack tests for total particulate matter for two wet ESPs on a single boiler. The tests were performed each quarter for a period of two years as required by the state agency. These data are plotted on the following graph to show the performance of the wet ESPs at varying levels of gas flow. 3
WET ESP PERFORMANCE ON BIOMASS FIRED BOILER Note that in all cases, no test exceeded 0.01 lb/mm BTU. The present MACT standard for this type of boiler is 0.037 lb/mm BTU. Further evidence of the effectiveness of wet ESP technology as an add-on to an existing wet scrubber can be seen below. The photographs are self-explanatory. High Voltage Power Off High Voltage Power On 4
There are two other noteworthy aspects that applications of wet ESPs downstream of an existing wet scrubber provide. First, an add-on wet ESP requires no additional water. This is because the water required for a new wet ESP for electrode flushing is always less than the make-up water requirement for an existing scrubber. Second spent flush water from the wet ESP can feed forward to act as scrubber system make up water. The diagram below shows this. Existing Scrubber Scrubber with Add-On Wet ESP GEOENERGY TECHNOLOGY We believe that the Geoenergy E-Tube wet ESP technology is the best approach to any MACT-driven industrial boiler emission control project where a wet ESP is required. Geoenergy technology is the best for a variety of important reasons. A brief summary of the features supporting this conclusion follows. Round, metal tubes These offer the most durability and corrosion resistance. Also, round tubes beat any other configuration (plates, hex, square) for electric field optimization. Recent data obtained from a test program conducted by Powerspan Corporation at a coal fired utility boiler demonstrates the superior performance of round tubes over hexagonal tubes. This program involved a comparative evaluation of two multi-field wet ESPs. One was constructed with 10-inch round tubes and one with 10-inch hex tubes. Both units had the same tube count and overall length. Each used the same discharge electrodes and power supplies and were operated at the same voltage. The results of multiple inlet/outlet tests for particulate matter showed the round tube design to be clearly superior to the hex tube design. The results of multiple inlet outlet tests for particulate matter showed the round tube design to be clearly superior to the hex tube design. 5
Disk-in-Tube Electrode Arrangement High Voltage Corona Upflow configuration with tube cooling Upflow is less costly than downflow because no inlet or outlet mist eliminators are required. This also minimizes system pressure drop. Finally, upflow allows a higher discharge point saving stack and/or ducting costs. The upflow configuration includes an innovative tube cooling feature to further enhance performance and operability. The tube cooling feature works like this. The gas stream entering the wet ESP collection sections is saturated with water vapor. As shown in the diagram below, ambient air is pulled across the tube bundle in the manner of a shell and tube, air-to-air heat exchanger. This ensures that the collecting tube stays wet and facilitates cleaning. 6
UPFLOW WET ESP WITH TUBE COOLING Improved Materials of Construction Experience has shown that chlorides are usually present in biomass fired boiler gas streams. To resist stress corrosion cracking induced by chloride, the high voltage sections of Geoenergy Wet ESPs on this application are fabricated of duplex stainless steel which is highly resistant to this form of corrosion. Rigid mast electrodes These are very robust and clearly superior to any type of wire. Also, this design does not require any bottom frame or anti-sway hardware. High frequency power supplies These provide significantly higher operating voltages, more overall precipitating power (milliamps x kilovolts) without adding to the power consumed. 7
Unequaled experience 15,000,000 acfm of installed capacity is much greater than any other company in North America. SUMMARY Wet ESP technology is well suited for achieving compliance with the recent EPA MACT rule for industrial boilers. Existing wet ESP installations on large industrial boilers have clearly demonstrated the effectiveness of this technology. In addition, installing a wet ESP system downstream of a wet scrubber on an existing boiler is particularly attractive for a variety of process and economic reasons. 8