Best Practice Guide

Food and Beverage Growers and Processors

Introduction

Planning an Energy Program

Funding and Approval

Commissioning

On-the-Farm Energy Use

Lighting

Building Envelope

Central HVAC System

Boilers

Energy Use Basics

Proper Maintenance

Energy-Smart Investment

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Boilers

Proper Maintenance

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All fuel inputted to a boiler is converted into either usable steam or waste heat. Simply put,

Input fuel (latent energy) - steam generated (thermal energy) = heat losses through flue gas, hole, cracks, condensate, etc.

Boiler efficiency is expressed by the amount of fuel needed as an input - not by the amount of steam produced or system pressure. However, the easiest way to measure efficiency is to look at the amount of waste heat, that is,

Boiler efficiency = 100 percent - heat losses.

Thus, a boiler that is 75 percent efficient converts three-quarters of the input fuel into steam, but losses 25 percent of the fuel as waste heat.

Factors affecting boiler efficiency are the air-to-fuel ratio and combustion efficiency. Boiler efficiency can also be improved significantly by reusing waste heat exiting the stack.


Reduce Excess Air

Illustration of a water-tubed boiler. Photo courtesy of Oakridge National Laboratory.View Details.

Excess air is the result of the air-to-fuel ratio going into the boiler - this an important factor governing boiler efficiency and affects both combustion and heat loss. Excess air consumes energy as it is heated. Energy is then wasted as this heated air is released up the stack.

A "rule of thumb" is that boiler efficiency increases one percent for every 15 percent reduction in excess air or 40ºF reduction in stack gas temperatures . While one percent might seem small, in our hypothetical boiler example, one percent represents about $23,000 per year in energy cost savings.

However, without sufficient air, not all fuel will burn. This wastes fuel and may even be dangerous. Unburned fuel will continue to move through the system, leaving behind soot. Deficient air levels may also result in the formation of carbon monoxide and smoke. Soot and carbon monoxide can accidentally combust in areas not designed for this, which will shorten equipment life. Soot buildup also acts as an insulator, blocking some of heat transfer between combustion gas and steam generation.

Analyze flue gas to measure the amount of oxygen and the stack gas temperature, and then calculate boiler efficiency. Adjustments can then be made to optimize the level of excess air and incoming air temperatures.

Computer-based distributed control systems (DCS) are now common features, and are relatively cheap to install compared to annual operating costs. DCS automatically controls the fuel burner to "trim" oxygen levels as needed, keeping the air-to-fuel ratio optimized. Trim systems are particularly useful when stack temperatures are high or when fuel composition is highly variable. Payback on investment is usually less than one year.


Combustion
Combustion occurs as fuel and air are mixed within the combustion chamber. As mentioned above, the air-to-fuel ratio is important for proper combustion. Combustion efficiency is also influenced by air leaks and the temperature of incoming air.

Check and repair air leaks; this should be part of routine maintenance, as recommended by the boiler manufacturer. Air infiltration can occur in the furnace, burner or stoker, and steam generation section.

Consider the temperature of incoming air. Higher temperatures result in better combustion efficiency. Seasonal variation in air temperatures affects excess air and boiler efficiency. This is important as seasons change and ambient air temperatures fluctuate. An oxygen trim control is beneficial to accurately control excess air when combustion air temperature changes.


To improve efficiency, air can be preheated using an air economizer. One low-cost method is to modify the combustion air inlet so that it draws hot air released by the stack near the top of the boiler room or use flue gas recirculation.


Preventing Heat Loss
Heat loss can reach as much as 35 percent of fuel input. Most of this waste heat goes straight up the stack, like heat lost up the chimney of a fireplace. The lower the temperature of these gases, the more efficient the system. In other words, a 100 percent efficient system would, theoretically, expel flue gases at the same temperature as ambient air. In reality, the lowest temperature possible is roughly 300º F, unless a condensing heat exchanger or heat economizer is utilized.

An economizer recovers waste heat from the stack. The heat is then usually directed to boiler feedwater for preheating. Be sure to have the boiler system cleaned and tuned before installing an economizer so that an accurate measurement of stack gas temperature can be taken. Additional heat can be extracted from the flue gas (below 300º F) by use of a condensing economizer. In order to use this approach, it is necessary to have a requirement for heating a significant quantity of low temperature water. Also, condensing economizers and adjacent flues/stack must be constructed from corrosion-resistant materials.

Besides waste heat up the stack, heat can be lost in the burner and combustion chamber. Soot on the combustion side and deposits on the water side act like insulation, blocking some heat transfer and the efficient generation of steam. Just 1/8 inch of soot, for example, can decrease boiler efficiency by as much as 8 percent - in our boiler example, that would cost about $195,000 per year. Reduce this heat loss by keeping heat transfer surfaces clean.

To keep the fire-side surfaces clean, ensure there is adequate excess air to avoid soot build-up. A soot blower used periodically helps remove soot from the fire-tubes. Water-side maintenance depends on water treatment protocols (discussed directly below).


Boiler Water Treatment
Feedwater is treated before it is pumped into the boiler in order to remove impurities that might cause deposits or corrosion. Without proper treatment, impurities build up and reduce boiler efficiency.

Scale is the most common problem. Scale acts like insulation between the combusted fuel and the boiler water, in effect blocking some of the heat transfer. Scale can reduce a boiler's efficiency by as much as 12 percent (in our hypothetical system, this means as much as $309,000 in energy costs). Eventually, scale buildup causes boiler tubes to overheat and rupture.

Besides scale, a number of other impurities, such as calcium, magnesium, silica, iron oxide, oxygen, and suspended solids, need to be removed from boiler feedwater.

Most methods for removing impurities involve the addition of chemical treatments. Treatments should be balanced to minimize the use of chemical additives as much as possible (and thus, reduce costs) without interfering with the removal of impurities. Clarifiers and filters also help eliminate impurities, and are worthwhile investments. Other water treatment methods include reverse osmosis, electrodialysis, and ion exchange demineralization.

Besides blocking efficient heat transfer, impurities lead to energy waste because of the need for periodic boiler blowdown. Although blowdown effectively removes impurities, it is also one of the largest energy wastes since it involves discharging boiler water. This wastes both the thermal energy embodied in the water as well as the chemical additives used to treat the feedwater.

Electronic controllers can automate bottom blowdown so that it is done as infrequently as possible without allowing excessive build up of impurities. With continuous blowdown, discharged water can be directed through a heat exchanger to recover some of the thermal energy that would otherwise be lost. This heat can then be used to preheat boiler feedwater. However, the best option is to properly treat feedwater correctly and minimize blowdown.


Resources

U.S. Department of Energy (DOE), Industrial Technologies Program. Improving Steam System Performance: A Sourcebook for Industry. This Sourcebook is designed to provide steam system users with a reference that describes the basic steam system components, outlines opportunities for energy and performance improvements, and discusses the benefits of a systems approach in identifying and implementing these improvement opportunities.

Council of Industrial Boiler Owners (CIBO). CIBO Energy Efficiency Handbook. Burke, VA, 1998. The Energy Efficiency Handbook was pre-pared to help owner/operators get the best and most energy-efficient performance out of their boiler systems. This manual gives information and helpful operational tips on virtually every aspect of boiler operation, from Water Treatment, through Combustion and Heat Recovery, to Flue Gas Treatment and Cogeneration.

Harrell, Greg, Ph.D., P.E. Steam System Survey Guide. Oak Ridge National Laboratory, May 2002. ORNL/TM-2001/262. This Steam System Survey Guide provides technical information for steam system operational personnel and plant energy managers on some of the major opportunities available to improve the energy efficiency and productivity of industrial steam systems.

Boiler System Guide. Pacific Gas and Electric webpage. Online, comprehensive information on purchasing, operating, and maintaining an efficient boiler.

Steam System Assessment Tool. U.S. DOE, Industrial Technologies Program. Free, downloadable software that allows steam analysts to develop approximate models of real steam systems. Using these models, you can apply SSAT to quantify the magnitude-energy, cost, and emissions-savings-of key potential steam improvement opportunities.

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