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O2 Trim for Increased Boiler Efficiency & Emissions Compliance

Given today’s awareness to the advantages of minimizing energy usage and carbon footprint, boiler operators and plant managers are always on the lookout for ways to improve boiler efficiency and ensure emissions compliance. With improved efficiency, fuel usage is minimized which in turn reduces the carbon footprint (i.e. reduces CO2 emissions from the boiler) and reduces issues around emissions compliance. One way to increase boiler efficiency is to use oxygen trimming, or O2 Trim, at the stack.

A typical burner will operate from 3 to 4% O2 at 50% boiler load and higher. This stack O2 concentration corresponds to the amount of excess air at the burner, and excess air is required for burner operation to assure complete combustion of the fuel.  For example, for natural gas firing, 3% O2 corresponds to 15% excess air. During commissioning, the burner service engineer will set the fuel / air ratio so that there is always excess air over the firing range of the burner.  The service engineer must also keep in mind that ambient conditions (mainly air temperature changes) will affect air density which will affect the burner fan air flow output.  On cold days the fan will flow more air due to a higher air density, and on hotter days the flow will be less. Varying air flow conditions can adversely affect burner operation.

Boiler efficiency is affected by the excess air concentration in the flue gas. The rule of thumb is that for every 5% more excess air, boiler efficiency decreases by 0.5%. If not adjusted, the boiler stack can vary by at least 2% O2 (i.e., if normal operation is 3% O2, it can increase to 5% O2 on a cold day). That corresponds to about 1% boiler efficiency loss. Saving 1% efficiency over a year operation can save big on fuel costs. If the normal fuel bill is $10,000,000 per year, you would save $100,000. Adding an O2 Trim system would cost a fraction of that amount (assuming a 150,000 lb/hr steam boiler or smaller), providing a quick and worthwhile ROI. So, what exactly is an O2 Trim System?

Many burners use a control system where the fan air flow does not vary based on air temperature.  As explained above, the air flow can vary based on ambient conditions causing the stack O2 to vary; this can be solved by adding O2 Trim to the control system. O2 Trim is an air flow trimming system where stack O2 is measured (using an O2 probe) and the air flow is adjusted (trimmed) based on the reading. It’s a closed loop control system since changes in air flow will directly affect the stack O2 reading (assuming fuel flow is the same). By maintaining a consistent air flow rate, O2 trim reduces fuel usage in turn increasing boiler efficiency.

In addition to increased boiler efficiency, utilizing O2 Trim will ensure stable and safe O2 levels. On hot days with reduced fan air flow, the stack O2 level can drop to dangerously low levels and boiler emissions can go out of compliance. With O2 monitoring, alarms can be created to alert the boiler operator to either reduce fuel flow or increase air flow, to return to safe operating conditions.  

O2 Trim isn’t ideal for every boiler, though. Due to the residence time in the boiler and ducting, it takes time for the changes in burner fan flow to reach the stack. This causes a time delay with an O2 Trim system, which works well for boilers with slow load changes. However, for systems with rapid boiler load changes, the O2 Trim system typically can’t keep up easily and it is often “hunting” for the optimum air flow.

If you are interested in learning more about whether an O2 Trim System will benefit your operations, reach out to one of our qualified parts specialists or call 800-227-1966. Check out other articles on Nationwide’s Boiler Blog for more tips and tricks for improved boiler efficiency, routine maintenance, and more!

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Air Permitting for Rental Boilers in California

With California having the most stringent emissions requirements in the country, it is important to be well versed on any and all rules and regulations when buying a new boiler or renting a temporary boiler. This is our final installment of the 3-part series on air permit compliance for boilers in California (be sure to check out part 1 and part 2 if you haven’t already).

Nationwide Boiler maintains a fleet of rental units that are sub-9 ppm NOx and pre-permitted for use in the SCAQMD. We take care of the bulk of the permit work saving our customers an exponential amount of time (no waiting for the application to be approved), and we pay for any processing fees. This allows for quick installation and start-up of a temporary boiler, which is extremely valuable in an emergency outage. The only requirement of our customer is the source testing of the equipment, if the source test is due. Most of these pre-permitted boilers require source testing on an annual basis, and the source test must be done at a job site within the county’s jurisdiction.

The SJVAPCD does not allow the pre-permitting of rental boilers, but they do have a program called the Temporary Replacement Emissions Unit (TREU) which can be utilized when a rental is needed  in a pinch. This program contains an application shield provision which allows renters to install a temporary boiler in place of an existing permitted boiler that is down for repairs without having to get a new permit for the rental boiler. In order to qualify for the TREU Program, the rental boiler being installed must have a heat input equal to or less than the unit it is replacing. Plus, it must not have the potential to produce more emissions than the current permit allows. There is a time limitation to this program; the temporary boiler can be on-site for a maximum of 180 days within a 12-month period.

If you are outside of the two territories listed above, don’t fret! Nationwide Boiler can assist with the permitting process as needed. In addition, utilizing a pre-permitted boiler in a location other than the SCAQMD does have its advantages and can help expedite the permitting process.

With our headquarters being located in California, it is important to us that we are up-to-date with emissions regulations throughout the state. And with other areas of the country starting to experience a similar push for emissions reductions, we have the expertise and experience to help. Nationwide Boiler is proud to take the lead in helping customers everywhere understand and comply with current and future air regulation standards.
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Boiler Basics 101: Emissions Rules & Regulations for San Joaquin Valley APCD

Be sure to read Part I of this series to learn about emissions requirements in the SCAQMD.

In the last edition of Boiler Blog 101, we discussed air permitting rules and regulations in the South Coast Air Quality Management District (SCAQMD). In part II, we will cover the basics of air permitting for boilers in another California district that leads the country in air pollution control; the San Joaquin Valley Air Pollution Control District (SJVAPCD).

The SJVAPCD was founded in 1991 with a mission to improve the health and quality of life for all Valley residents through efficient, effective and entrepreneurial air quality management strategies. The SJVAPCD is made up of eight counties in the Central Valley of California; San Joaquin, Stanislaus, Merced, Madera, Fresno, Kings, Tulare, and Kern.

The rules that have been set forth specifically for boilers, steam generators, and process heaters are outlined below:

  QUALIFYING UNITS NOx LIMIT
Rule 4306 Units Greater Than 5 MMBtu/hr
Units Greater Than 20 MMBtu/hr
15 ppm
9 ppm
Rule 4307 Units 2.0 MMBtu/hr to 5.0 MMBtu/hr 30 ppm
Rule 4308 Units 0.075 MMBtu/hr to Less Than 2.0 MMBtu/hr 20 ppm
ADVANCED EMISSION REDUCTION OPTIONS

Rule 4320

Units Greater Than 5 MMBtu/hr
Units Greater Than 20 MMBtu/hr
9 ppm
7 ppm
*NOx limits listed are standard options for existing gaseous fuel-fired equipment. Enhanced options as well as limits
for new and replacement units will differ. Click each rule to review the full details; certain exemptions may apply.

Starting at the beginning of 2019, the SJVAPCD implemented a new rule titled, Best Available Control Technology (BACT). This rule requires that all new permits meet the lowest NOx limits that have been achieved, regardless of what is mandated. In addition, due to Assembly Bill 617, boiler owners in the SJVAPCD are mandated to meet Best Available Retrofit Control Technology (BARCT) for any existing equipment retrofits. This means that when your boiler needs to be upgraded or replaced, it must meet more stringent NOx limits, which can be as low as 7 ppm.

These rules can be overwhelming, which is why Nationwide Boiler’s AQMD experts are capable and ready to assist you with the permitting process needed for your facility. Furthermore, we can help you reduce NOx to comply with your air emissions regulations. Nationwide Boiler’s CataStak™ SCR system is proven to reduce NOx levels to as low as 2.5 ppm. Contact us today for more details.

Stay tuned for Part III, where we will discuss renting boilers in California.
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Understanding PPMVD and O2 Level Requirements

Have you ever wondered what ppmvd (ref. 3% O2) means? This term is often seen in air permits and is referenced as the emission unit for pollutants such as NOx from boilers.

PPMVD stands for “parts per million by volume, dry”.  For example, “9 ppmvd” refers to (9 / 1,000,000) x 100 = 0.0009% of the volume of flue gas. The reason they stipulate “dry” is because when the emission is measured, the instrument typically removes all moisture (water) from the sample. The “ref. 3% O2” is required as a benchmark for the oxygen content in the flue gas. This is because the boiler will operate at various O2 levels, and the air district needs to normalize the measurements.

Sometimes, emission limits are referenced with different O2 levels for equipment in the same plant.  For example, on a boiler application, NOx may be listed as 9 ppmvd (ref. 3% O2) whereas for a gas turbine it might be 9 ppmvd (ref. 15% O2).  What does it mean when the O2 levels are different?

The difference in the example above is that O2 is referenced at 3% and 15%. Looking at actual numbers will help explain the difference. Let's say a boiler or gas turbine is firing natural gas and the heat input is 100 million Btu/hr. The fuel flow is the same for each case, about 4500 lb/hr. The air flow, however, will be different. 

For boilers, the burner typically runs at about 15% excess air (this is 15% extra air flow above the stoichiometric requirement). In other words, 15% of the air flow passes through the system without being oxidized. This is why the stack O2 (O2 is the extra oxygen in the boiler flue gas flow) is about 3% on a dry basis. This also explains why the 3% O2 benchmark is typically used.Based on the 100 million Btu/hr input and 15% excess air, the boiler air flow is about 81,000 lb/hr. 

On the other hand, gas turbines typically run at higher excess air levels as compared to boilers. To get 15% O2, dry in the stack, the excess air works out to about 230%. The air flow would be about 233,000 lb/hr with the gas turbine application. Stack flow, or the sum of fuel and air, differs in boilers vs. gas turbines:

Boiler stack flow:                            4500 + 81,000 = 85,500 lb/hr
Turbine stack flow:                         4500 + 240,000 = 244,500 lb/hr

You can see that for the same heat input of 100 million Btu/hr, the stack flow will be quite different for these two applications. This also explains why the O2 benchmark is different for these two cases. When comparing the NOx values in lb/mmbtu, we have:

9 ppmvd (ref. 3% O2) NOx firing natural gas is equivalent to 0.011 lb/mmbtu
9 ppmvd (ref. 15% O2) NOx firing natural gas is equivalent to 0.032 lb/mmbtu

This means that even though both 9 ppmvd volumetric numbers are the same, the numbers are actually different on a weight basis (i.e. lb/mmbtu).

Nationwide Boiler provides temporary and permanent solutions with our CataStak SCR System for boilers, gas turbines, and other demanding applications, to meet the most stringent emissions requirements. Contact us today for more infromation. 1-800-227-1966

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