Reverse Osmosis Improves Boiler Efficiency

Introduction

Providing a continuous, reliable source of good quality feedwater for industrial and commercial boilers is a primary focus of water treatment technology. Ion exchange softeners have been the workhorse in accomplishing this task for over 70 years. More recently, however, reverse osmosis (RO) has gained a strong foothold in producing boiler makeup. This is because RO offers several advantages over traditional water softening. This includes operating the boiler at lower blowdown rates, improving steam and condensate quality, and minimizing the consumption of boiler maintenance chemicals.

Industrial and Commercial Boilers

The United States is home to 163,000 industrial and commercial boilers. Of these, 43,000 industrial boilers, having a combined output capacity of 1.6 million MMBtu per hour, provide process steam to primary industries such as food, paper, chemicals, refining and primary metals. An average industrial boiler is rated at 36 million Btu per hour (37,000 pounds of steam per hour).

Commercial boilers account for 120,000 units that provide heat and hot water for office buildings, hospitals, and universities. The average commercial boiler is smaller in size than the typical industrial boiler at 9.6 million Btu per hour (10,000 pounds of steam per hour), but because of the far greater number of units in service, commercial boilers have a total output capacity of 1.1 million MMBtu/hr; which is 68% of the industrial output.

Improving Overall Boiler Efficiency

Boiler feedwater quality restricts the maximum specific conductance (conductivity, as micromhos/cm) that can be reliably maintained in the boiler. Conductivity is a measure of the total dissolved solids content of the water. This includes hardness, alkalinity, chlorides, sulfates, iron, etc. The higher the dissolved solids content, the greater the specific conductance. Hence, the ratio between the conductivity in the boiler to the conductivity of the feedwater determines the concentration ratio; also known as cycles of concentration. Optimum boiler efficiency is obtained by operating at maximum cycles of concentration.

Traditional water softeners do an excellent job of removing calcium and magnesium. This process eliminates scale-forming impurities that otherwise form ‘lime’ deposits on heat transfer surfaces. But softening does not reduce the overall total dissolved solids. The boiler feedwater is soft, but still retains a high conductivity.

A typical industrial boiler producing steam at the rate of 37,000 pounds per hour with 50% condensate return, has a makeup demand of 19,500 pounds per hour (approximately 40 gallons per minute). In our example, this produces a boiler feedwater having a conductivity of 200 micromhos/cm. If our maximum conductivity limit in the boiler is set at 4000, our concentration ratio or cycles of concentration is 20.

To maintain the conductivity at 4000 micromhos/cm, the boiler must have a blowdown rate of 5% of the feedwater; in this case, just under 4 gallons per minute (5,750 gallons per day).

A major advantage of reverse osmosis over traditional softening is that it removes 99% of the dissolved solids from the makeup. In our design, a sodium softener is used to provide feedwater to the RO. This subsequently reduces the specific conductance of the makeup to 10 micromhos/cm. When blended with returned condensate, the boiler feedwater becomes 30 micromhos/cm instead of 200 micromhos/cm as with traditional ion exchange feedwater treatment. Thus,the boiler blowdown is reduced to 1% of the feedwater demand. In this example, the blowdown rate is now just 0.75 gallons per minute (1080 gallons per day).

Significant energy savings can be realized due to the reduction in heat loss in the boiler blowdown. This can be calculated from the Btu content of the blowdown, which is dependent on boiler pressure. In our example, an 81% reduction in blowdown heat loss is achievable by this strategy. However, if the boiler is already equipped with blowdown heat recovery equipment, the potential savings via reduced blowdown are limited.

Improving Steam Quality

Steam purity and quality are adversely affected by boiler carryover and certain volatile gases that exit the boiler with the steam, such as carbon dioxide. The carbon dioxide potential of the steam is determined by the bicarbonate and carbonate alkalinity of the feedwater. The following equation can be used to estimate the carbon dioxide potential using the P and M alkalinity of the feedwater.

Carbon dioxide potential = 0.79(M-2P) + 0.35(2P)

Frequently, the P alkalinity is zero (0), which reduces the equation to (0.79)M.

Sodium ion exchange softeners do not remove carbonate or bicarbonate alkalinity, whereas reverse osmosis is capable of reducing the total alkalinity by 95% or more. In our example, the feedwater M alkalinity decreased from 55 ppm (43 ppm carbon dioxide potential) with sodium softening to 3 ppm (2.4 ppm carbon dioxide potential) with reverse osmosis.

Carbon dioxide (CO2) condenses in the steam to form carbonic acid, which depresses the pH. The greater the CO2 content, the lower the pH of the condensate. Typically, boilers operated on softened, high alkalinity makeup produce enough carbon dioxide to depress the condensate pH below 6.0. Under low pH conditions such as these, corrosion of steel, copper and brass is a potential problem. With reverse osmosis, however, because of the lower CO2 content of the steam, the pH of the condensate is frequently near-neutral (pH 7). This is important in food plants and commercial buildings where steam comes into contact with food or is used for direct steam humidification. This same benefit can be applied in producing feedwater for clean steam generators where no chemical additives are permitted.

Reducing Chemical Consumption

The successful operation of a modern steam plant requires the application of various water treatment maintenance chemicals. These include oxygen scavengers, anti-scalants, polymers, and steam/condensate corrosion inhibitors. The dosage of these chemicals is essentially determined by the boiler cycles of concentration (blowdown rate) and the carbon dioxide potential of the steam (feedwater alkalinity).

Numerous specialty, proprietary water treatment formulations are marketed to treat industrial and commercial steam boilers. The trade names and numbers vary from one vendor to the next, but the basic water chemistry requirements remain the same.

Based on a recent survey, neutralizing amines account for the biggest expenditure for boiler water treatment chemicals. Amines typically equal 50% to 75% of boiler chemical expenditures. Polymers, oxygen scavengers (such as sodium sulfite) and caustic soda are included in the remaining cost.

The cost to chemically treat a million pounds of steam varies depending on the quality of the feedwater, chemicals used and boiler operating conditions. In our example, using sodium softened makeup, the cost is $60 per million pounds of steam. This is a reasonable budgetary number. (Although, in my experience, I’ve often seen chemical costs over $200 per million pounds of steam.) Generally, the cost per million pounds of steam is higher for commercial boilers than for industrial boilers.

Referring back to our average industrial boiler producing 37,000 pounds of steam per hour, 24 hours/365 days, the annual chemical cost is $19,500.

Now let’s consider the advantages of retrofitting the makeup water system to include reverse osmosis. This does two things. First, it reduces the feedwater conductivity so the boiler operates at higher cycles of concentration. This effectively reduces the blowdown rate. Less blowdown means a reduction in boiler chemicals going down the drain. In our illustration, the blowdown rate was reduced from 5% of boiler feedwater to 1%. This represents an 80% reduction in oxygen scavenger, polymer and caustic soda consumption; all other things being equal.

Second, because an RO system reduces the feedwater alkalinity, the carbon dioxide potential in the steam is decreased significantly. In our example, the CO2 potential drops from 43 ppm to 2.4 ppm, a 94% reduction. This is a significant enough decrease that in some cases neutralizing amines can be eliminated altogether. If this goal is unattainable, however, at a minimum, the expenditures for neutralizing amines will be significantly reduced.

Overall, and being conservative, it’s safe to assume that improving boiler feedwater quality by retrofitting with reverse osmosis will reduce water treatment chemical expenditures by 75% as compared to sodium softening alone. In our example, the annual chemical savings for an average industrial boiler is $14,625.

Summary and Conclusion

Reverse osmosis offers several advantages over traditional sodium softeners for producing good quality boiler feedwater.

Reducing the total dissolved solids in the boiler feedwater allows the unit to operate at higher cycles of concentration, which is manifested in reduced blowdown. For boilers without blowdown heat recovery equipment, this saves Btus that would otherwise be discharged to drain. If your boiler has heat recovery equipment, however, the energy savings, if any, will be much less.

Improving the feedwater quality by reducing total alkalinity offers the advantage of lowering the carbon dioxide potential of the steam. This reduces the amount of carbonic acid that forms in the condensate resulting in a less-corrosive pH condition. Reverse osmosis has the potential to completely eliminate neutralizing amine requirements, which is advantageous where steam contacts food or is used for direct steam humidification.

By virtue of operating the boiler at higher cycles of concentration (reduced blowdown) and with reduced feedwater alkalinity, expenditures for specialty chemicals is greatly reduced. This can be as much as a 75% reduction in chemical costs, which goes a long way toward offsetting the cost for the reverse osmosis equipment.

Each boiler plant is different, so results will vary by location. However, if you have not evaluated the potential benefits of retrofitting your feedwater system with reverse osmosis technology, you may wish to explore this opportunity for significant energy and chemical savings.

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