Information

Boiler Efficiency 

Minimizing energy costs while maintaining high boiler reliability is one of the main objectives of every utility system operator. Optimization of boiler combustion controls, minimizing stack losses, and installation of heat recovery equipment are obvious big-ticket items. Water treatment, although typically a smaller payback item, is also important.

There are many opportunities to improve boiler system efficiency through the use of chemical treatment.

Reduce Boiler Scale
Boiler scale creates a problem in boiler operation because scale typically possesses a low thermal conductivity (relative to a clean metal surface). The presence of scale is equivalent to having a thin film of insulation across the path of heat transfer from the furnace gases to the boiler water. This heat-insulating material retards heat transfer and causes a loss in boiler efficiency. Stack gas temperatures may increase as the boiler absorbs less heat from the furnace gases.

Heat transfer may be reduced as much as 10%-12% by the presence of scale. A scale approximately 1/8-inch thick may cause an overall loss in boiler efficiency of about 2%-3% in fire tube boilers as well as in the convective sections of water-tube boilers. Even more important than the heat loss is that scale can cause overheating of the boiler tube metal and can result in subsequent tube failures. Costly repairs and boiler outages are the result of such a condition.

Scale formation in boilers can be controlled by chemical treatment in combination with the proper operation and maintenance of all make-up and feedwater pretreatment systems. Phosphates, chelates, and polymers are among the treatments in common use today to condition and/or maintain solubility of scale-forming solids within the boiler.

Optimize Boiler Blowdown Rates
Proper control of blowdown is a critical part of boiler operation. Insufficient blowdown may lead to deposits or carryover, while excessive blowdown will waste water, heat, and chemicals. The American Society of Mechanical Engineers (ASME) has developed a consensus on operating practices for boiler feedwater and blowdown that is related to operating pressure. These suggestions apply for both steam purity and deposition control.

The ASME limits are a good starting point in establishing blowdown needs. Operating experience with a particular boiler then determines whether or not it is practical to deviate from these limits. A steam purity study can help set new boiler limits which will minimize solids carryover, while also maintaining minimum blowdown rates.

Once specific limits for boiler water solids have been set, a practical way is needed to control solids level on a day-to-day basis. Conductivity, TDS, silica, chlorides, and/or alkalinity are often used to control the rate of blowdown. These tests, however, are often subject to interference and may be difficult to measure accurately in high-purity feedwater systems.

An inert fluorescent tracer can be used to accurately measure boiler cycles and chemical feed. Typically, the increased accuracy and confidence from using an inert fluorescent tracer will allow boiler cycles to be increased, resulting in significant savings.

Reduce Iron and Copper Pick-up in the Feedwater
Oxygen scavengers developed within the last 20 years may be more than their name implies. Some provide both chemical oxygen scavenging and passivation of the pre-boiler or feedwater system. (Passivation is the formation of a very thin, dense, protective iron oxide film, commonly magnetite, on the metal surface. Passivation typically increases the corrosion resistance of a metal surface.) These oxygen scavengers are used to prevent corrosion in the feedwater system. Reduced corrosion minimizes the transport of corrosion products to the boiler and also to the superheaters if feedwater is used for attemperation.

Reduce Feedwater to the Boiler
The maximum achievable cycles in a boiler is often limited by TDS or conductivity. In boiler systems using sulfite as an oxygen scavenger, switching to a non-sulfite scavenger can reduce feedwater TDS and improve boiler system efficiency. Sulfite is commonly fed to maintain a residual in the boiler water (Table 1). In contrast, the non-sulfite scavenger dosage is based on a small feedwater residual, typically about 1 ppm. In addition, some of the alternative scavengers do not themselves contribute to boiler water TDS or conductivity. For a typical boiler operating at 200 psig, 550,000 lbs/day steaming rate, and 5.5 cycles, the savings from switching to a non-sulfite scavenger would total $23,000 in fuel costs annually.

Steam/Condensate System
One economically attractive method of maximizing energy efficiency and boiler reliability is by increasing the amount of condensate return. Returned condensate, being condensed steam, is extremely pure and has a high heat content. Increased condensate return can improve boiler system economics through water and energy conservation.

As more condensate is returned, less make-up is required, saving on water and make-up water treatment costs. The high purity allows for greater boiler cycles of concentration, thus reducing water and energy losses to blowdown. The high heat content (typically in excess of 180ºF) can provide substantial energy savings. Additional savings will also be noted in reduced water treatment chemicals, water and sewer costs. For a typical boiler operating at 600 psig, 2,400,000 lbs/day steaming rate, and 13 cycles, the savings from increasing the condensate return 10% would total $132,000 in fuel costs annually.

Corrosion in condensate systems can limit the quality or quantity of returned condensate because of iron and copper corrosion products, which can deposit on boiler heat transfer surfaces. This reduces heat transfer efficiency and could cause tube failure. Condensate corrosion control is required to protect process equipment, lines, tanks, as well as to maintain the condensate as a quality feedwater source. Corrosion of the condensate system can result in increased maintenance and equipment costs, energy loss through steam leaks and loss of process heat transfer efficiency.

To prevent condensate corrosion, volatile neutralizing amines are typically used to neutralize carbonic acid and raise the condensate pH. These programs are most effective when fed to maintain a minimum pH of 8.5, ideally 8.8 to 9.2. A blend of several amines will assure that corrosion protection is distributed throughout the entire steam/condensate system. Filming amines and a new, patented chemistry are alternative condensate treatments.

Summary
There are many opportunities to improve boiler system efficiency through the use of chemical treatments. Water treatment is an important aspect of boiler operation that can improve efficiency and availability, or result in damage if neglected.