5 Steps to Increase Steam Boiler Efficiency

Author: Joe Ham | Business Development Manager

Boiler Room
Close to 40% of all fossil fuel burned by industry is consumed in steam production.  Simple steps can have a large operational impact on the cost of fuel, water, treatment, and labor.  The chart shows a typical breakdown of these costs where fuel consumption is the primary expense.

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Boiler Operating Costs
Here are five strategies for improving your boiler operating efficiencies.

1. Maintaining a Clean Heat Exchanger

Clean boiler heat exchange surfaces allow for maximum heat transfer from the burning fuel to the boiler water.  Feedwater quality is critical to prevent deposit formation within the boiler and maintain peak efficiency.  Water-side deposits will insulate heat exchange surfaces and sharply increase fuel consumption.  Industry standards suggest that 1/16” of deposit can increase fuel costs by 12.5%.  Since fuel consumption is typically the major cost in boiler operation, this can have a major impact on overall efficiencies.

Regular monitoring of water chemistry and critical operating parameters (i.e. stack, tube, or coil temperatures) is recommended to provide early indication of fouling.  Visual inspections of the water-side of a boiler should also be performed to ensure the effectiveness of the current treatment program.

2. Automating Boiler Blowdown

The production of steam in a boiler leaves behind dissolved and suspended solids in the water.  These solids accumulate with continued steam production necessitating regular blowdown of boiler water.  The boiler water that is removed during blowdown has been chemically treated and heated to operating temperature.  As a result, excessive boiler blowdown increases the treatment, fuel and water costs.  Too little boiler blowdown can cause carry over of boiler water in steam also resulting in excess fuel and chemical consumption.  The installation of an automated blowdown control system allows for active monitoring of the boiler conductivity and optimizes the volume of blowdown based on fluctuating steam loads.  A well designed automated blowdown system will lead to increased boiler efficiencies and promote improved steam quality.

3. Reduction in Blowdown Requirements

Boiler blowdown rates are mainly dependent on the amount and type of minerals entering the system.  The installation of pretreatment equipment that will alter or remove the minerals in the make-up water can significantly reduce the required blowdown and the consumption of water treatment chemicals.  Examples of this equipment include softeners, dealkalizers, de-ionizers and reverse osmosis units (ROs).
Water Softener Illustration
Reverse Osmosis Equipment
To ensure you achieve the maximum amount of savings, it is important to thoroughly evaluate all of the pretreatment options for your specific application and water source.

4. Maximize Condensate Return Rate and Temperature

Returning high temperature condensate will reduce the cold water make up rates and save on water, energy, and treatment costs.  Since condensate typically contains low levels of dissolved solids, it will also reduce the required blowdown rates providing further savings.  Your system design and use of steam will dictate the amount of condensate that can be returned.  With potential savings typically over $30 per thousand gallons of condensate, the economic justification is often available to install the equipment necessary to return more condensate or repair any failures in the existing network that are leading to condensate losses.
condensate piping
Maximizing the energy in the condensate that is already being returned is another consideration.  Returning pressurized condensate will reduce the flash losses and increase the overall energy savings.  Insulation of the return piping will also minimize heat losses to the environment.

5. Metering and Benchmarking Utility

Though the installation of meters on a steam system may not directly impact the efficiency of a boiler, the data they collect will allow you to gain insight into the current operating conditions and provide a clearer understanding of the overall cost of steam production.
With the availability of the usage data from metering, opportunities for improved steam plant efficiency can be identified and quantified.  For example, control of boiler pressure, firing order and production scheduling based on steam plant load can have a major impact on the cost of steam generation.   By benchmarking and tracking steam costs, optimal operating profiles can be identified and any excursions quickly corrected.

Conclusions

Your plant’s steam system offers a multitude of opportunities to identify energy reduction projects.  This includes having a water treatment vendor that can design and maintain pretreatment equipment, a chemical program, and automation controls to optimize water conditions within the boiler.  Klenzoid engineers and chemists are recognized thought leaders in the development of industry best practices.  Our data-driven processes will get your steam systems operating at the lowest possible life-cycle cost and proactively maintain them there.

Joe Ham has a degree in Chemical Engineering from the University of Michigan. As a Business Development Manager, he works with clients to achieve the lowest operational costs for their heat transfer systems through well-maintained water treatment programs. Since Joe spends many hours driving from one client site to another, he is a true podcast addict.

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Dealkalizer Technologies

Some important design considerations for the chloride cycle dealkalizer are:

  • Feed water must be softened
    • Calcium chloride can precipitate and foul the beads
  • Minimal impact on total dissolved solids
  • Potential small decrease in blowdown requirements
  • Relatively low capital cost, reasonably effective, simple to operate

 

Some important design considerations for the WAC dealkalizer are: 

  • Additional softening required. WAC can remove as much hardness as there is available alkalinity – any residual hardness needs to be removed before the boiler.
  • Efficiency reduction with increasing flow rate, decreasing kinetics.
  • Handling of acid
    • Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
    • Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
  • Higher capital cost, very effective, easy to operate, larger footprint

Ion Exchange Explained



A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this.  Water softeners use strong acid cation (SAC) resin for ion exchange.  SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.

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How to Minimize Amine Requirements



Amines should be dosed at the minimum rate required to neutralize carbonic acid, and to maintain pH levels of 8.0 to 9.0 in condensate.

In situations where incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs or even PELs. A number of alternatives are available to decrease alkalinity levels from incoming water:
  • Reverse osmosis (RO) Weak-acid dealkalization (WAC)
  • Chloride-cycle dealkalization
  • Demineralization (Demin)
RO, WAC and Demin units remove alkalinity from incoming water sources, and are often implemented to reduce energy and/or water consumption in steam plants because they decrease the overall mineral concentration of dissolved solids from incoming water. However, the chloride-cycle dealkalizer is a standout choice if the goal is to simply reduce incoming alkalinity on a budget. It operates much like a softener unit, and can decrease alkalinity levels by up to 95%.


Chloride-Cycle Dealkalizer Operation

Chloride cycle dealkalizers use strong base anion (SBA) ion exchange resin to swap carbonate and bicarbonate ions for chloride ions.  The footprint is similar a sodium softener, and they also use salt as the primary regenerant.  A small amount of sodium hydroxide if also often used to increase the effective capacity per regeneration.



The reduction of alkalinity in the feedwater, reduces the formation of carbonic acid in condensate, thus reducing the required amount of amines to neutralize the carbonic acid to maintain pH levels of 8.0 to 9.0 in condensate.

Implementation of a chloride-cycle dealkalizer can reduce your amine requirement by up to 90%.

PELs & OTLs



There are 2 important concentration guidelines:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
The following table describes the limits set by Occupational Safety & Health Administration (OSHA) and American Conference of Governmental Industrial Hygienists (ACGIH):


Exceeding PELs poses a health risk to occupants. These PELs should never be exceeded for any period of time. See this link for a related article from the Centers for Disease Control and Prevention (CDC).

http://www.cdc.gov/mmwr/preview/mmwrhtml/00001848.htm

It is best practice to also follow OTLs to minimize the likelihood of complaints from occupants, especially from those with sensitivities.

A More Detailed Look at the Components of Steam



Oxygen

Liquid water always contains some concentration of oxygen (O2). The solubility of oxygen is primarily determined by the temperature of the water. Higher temperatures reduce the solubility of oxygen in water (see graph).
Because oxygen is extremely corrosive in high temperature water, steam boiler treatment programs use chemical and/or mechanical means of eliminating dissolved oxygen in water. An effectively treated steam boiler, and the steam it produces, will have near-zero dissolved oxygen concentrations.

Carbon Dioxide

Carbon dioxide (CO2) is released by the heating of carbonate (CO32-) and bicarbonate (HCO3-) in boiler water. These ions are naturally present in water from lakes, rivers and underground wells, and their concentration determines the alkalinity of the water source. The amount of carbonate alkalinity entering the boiler is proportional to the volume of carbon dioxide gas that will be in the generated steam. Carbon dioxide eventually forms carbonic acid in condensate. Higher alkalinity values result in greater carbonic acid concentrations.


The Release of Carbon Dioxide



The above reactions describe the release of carbon dioxide gas from sodium bicarbonate (1) and sodium carbonate (2).

The heat energy in boiler water is sufficient for the first reaction to proceed to 100% completion.  The completion of the second reaction is dependent on increasing pressure and temperature.

Higher carbonate and bicarbonate levels in boiler feedwater will lead to proportionally higher concentrations of CO2 in steam.

Amines

The amine compounds used in boiler water treatment are selected based on their boiling point, and their distribution ratio. The distribution ratio is a measure of how far the amine will travel before condensing. An optimal blend of amines will protect the entire condensate piping network (near and far). Amines are considered volatile organic compounds, and their concentration must be monitored to prevent exposure to levels beyond permissible limits.


Lesson about Amines to Impress Your Water Treatment Professional



Amines are a functional group in organic chemistry, and are derivatives of ammonia. They are separated into three main groups, primary, secondary and tertiary amines. These groups are defined by the number of hydrogen atoms replaced by organic substituents.

The most commonly used amines for neutralizing carbonic acid in condensate are:
  • cyclohexylamine (CHA)
  • diethylaminoethanol (DEAE)
  • morpholine
These amines are selected for their availability, basicity (ability to neutralize acids), boiling points, and most importantly, distribution ratios.

Distribution ratios (DR) are a measure of the how far amines will travel with steam before condensing. A proper blend of amines will include low DRs to protect condensate piping closest to the boiler, and high DRs to protect piping in longer and more complex condensate networks. Below is a table with the properties of the amines discussed above.

Other Types of Humidification Systems

Pan Humidifiers:

Pan humidifiers are essentially small shallow basins filled with water. The basins are heated with electric elements or steam, with the intent of evaporating water.

Pan humidifiers are found in smaller HVAC systems, and are susceptible to biological and corrosion fouling.

Water Spray Humidifiers:

This design uses an array of nozzles to atomize liquid water directly into the air stream. The phase change from liquid to vapour causes a noticeable drop in air temperature.

This type of system is most susceptible to biological and corrosion fouling. Facilities with year-long continuous cooling loads requiring high RH are best suited for this technology.

Steam to Steam or Clean Steam Generators:

These systems are small steam boilers, specifically designed to produce steam from high purity water sources, such as demineralization, or reverse osmosis. The energy input comes from steam raised elsewhere in the facility by a traditional steam boiler.

This design is typically more costly, and adds complexity, but produces steam with no boiler water treatment compounds.


Clean steam generators can only produce steam at low pressures.  The packaged heat exchangers rely on the higher energy content of higher pressure steam.

Water purity is critical for clean steam generators.
  • Low hardness levels (>3ppm of calcium, magnesium, or iron) will lead to fouling of heat exchange surfaces.
  • Water with even moderate alkalinity levels will release CO2 gas which will corrode any condensate piping components.
  • Moderate levels of total dissolved solids (TDS) will lead to priming or carry over, which may damage the steam control valves and/or contaminate the steam.
Therefore, Reverse Osmosis (RO) systems are ideal for humidifier makeup.  These units are designed to remove nearly all of the minerals from incoming water sources, and produce water with TDS concentrations of 0-5 ppm.

Steam to steam generators do cycle up.  Despite high purity makeup, there are always some dissolved solids.  If the generators do not purge some volume of water regularly, the bulk water will concentrate beyond acceptable levels, causing water discolouration and may lead to fouling and/or corrosion to system components depending on materials of construction.

Effects of Humidification on Occupant Comfort and Building Materials

RH levels have a direct impact on the health of patrons in a facility.

When humidity is too low occupants will get dry skin, irritated sinus, throats and eyes.

When humidity is too high mold/mildew problems can occur in the building, thus increasing the risk of illness to occupants. These health impacts are of increased concern with health care facilities who treat immunocompromised patients.

RH levels also have an impact on building materials.

The amount of moisture the material can hold will determine the extent to which it shrinks and swells with fluctuations in humidity. The effect is especially pronounced in wood and drywall, where gaps and cracks will form over time.

Windows are also prone to condensation in cold climates because they generally have little insulation value. The likelihood of condensation on windows increases as the indoor relative humidity rises, and the outdoor temperature decreases.

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