Boilers are pressure vessels used to heat water, produce steam for industrial and domestic needs, or generate power from steam turbines. Boilers are also used to heat the indoor environment of buildings [1].

A significant portion of the world’s power is produced using fossil fuels, including coal, oil, gas, etc., and boilers are often the best option for converting these energy sources into electricity. Increasing a steam boiler’s efficiency even little can significantly decrease the quantity of energy or fuel used to produce electricity.

Additionally, boilers are used by the majority of industrial heating systems to generate steam or hot water. As a result, an efficient boiler considerably influences energy savings associated with heating [2]. Using energy optimization strategies and enhancing the boiler’s overall efficiency can save significant energy, which ultimately results in reducing fossil fuel consumption and CO2 emissions.

This article will discuss the following important points:

  • Energy consumption in boilers
  • Techniques used to assess boiler energy efficiency
  • Energy losses and the reasons why they happened
  • Techniques for recovering waste heat
  • Utilizing technology to reduce heat loss
  • Boiler maintenance activities importance
  • Emission control systems

Studies have shown that the boiler’s high-temperature flue gas or exhaust dissipates significant energy. Other inevitable losses also happen because of different factors. However, waste heat can be converted into a useful form of energy, such as electricity, the cooling effect, etc., adopting various approaches. Regular maintenance allows a boiler to operate at peak efficiency and is thus a reasonable choice.

1 Types of Boilers

Fire-tube and water-tube boilers are the two main categories for boilers. Both boilers differ in terms of structure, steam output capacity, energy needs, and efficiency, all of which are discussed in this section.

Figure 1: Schematic of fire-tube and water-tube boilers

1.1 Fire-tube Boiler

In a fire-tube boiler, the tubes contain hot gas or fire, and water surrounds them. It is called a “fire tube boiler” because fire is inside the tubes. The heat of hot gases is transferred to the water through the tube’s walls.

The basic vertical, Lancashire, Cochran, and Velcon boilers are examples of fire tube boilers.

1.2 Water-tube Boiler

In a water-tube boiler, hot gases or fire surround the tubes contained by water.

The Benson, La-Mont, Stirling, and Loeffler boilers are some examples of water tube boilers.

Table 1: Comparison of fire-tube and water-tube boiler

2 How is Heat Lost in Boilers?

In a boiler’s combustion chamber, fossil fuel is burned, and the resulting heat is transmitted to water via hot flue gas. As the hot flue gas transmits heat to the water via convective heat transfer, a substantial amount of heat losses take place through the flue gas. Considering that the flue gas leaving a boiler normally has a temperature between 150 to 250°C, about 10 to 30% of the heat energy loss occurs during the process [3, 4]. Radiation, blowdown, fly ash, incomplete fuel consumption, and bottom ash losses are additional boiler heat losses [5]. The main sources of energy waste should be sorted out to recover them and operate a boiler plant as efficiently as possible.

3 Efficiency of Boiler

The efficiency of a boiler is determined by comparing the heat amount taken by the produced steam to the heat amount provided to the boiler. Another way of calculating the efficiency is by subtracting the total heat loss of the boiler from the total heat input to the boiler [6]. In order to optimize boiler efficiency, it is necessary to reduce the amount of heat lost by the boiler by optimizing factors such as fuel, surplus air, steam demand, flow rate, etc. A boiler must be supplied with more air for the combustion process than theoretically required to ensure full combustion. Otherwise, carbon monoxide can rapidly accumulate in flue gas, generating smoke in severe instances. On the other side, excessive extra air increases the volume of unneeded heated air and is expelled at the stacking temperature [7]. Figure 1 illustrates a typical heat balance for a boiler.

Figure 1: Typical boiler heat balance [1].

The flue gas contributes the largest heat loss in the boiler system as shown in Figure 1, ranging from 10-30% of the input heat. Heat recovery from the high-temperature exhaust can result in substantial energy savings because the most portion of heat is dissipated through the high-temperature flue gas [8]. A boiler system can be able to save a significant amount of energy by using the waste heat from the hot flue gas. However, the boiler efficiency can be increased by limiting this dissipation and delivering an optimal surplus air ratio utilizing a variable speed drive (VSD) [9], which is installed on the fan motor to modify the extra air ratio. Figure 2 shows how the efficiency of a boiler is affected by the temperature of flue gas.

Figure 2: Relation of flue gas temperature and boiler efficiency [10].

4 Fuel Consumption in Boiler and CO2 Emissions

Fossil fuel usage directly relates to CO2 release. Environmental legislation emphasizes lowering the emission of CO2 as this is substantially causing global warming. Hence, to minimize the use of fossil fuels and CO2 emissions, the efficiency of the present energy systems should be enhanced in some way. There are various approaches to minimize heat and energy consumption and CO2 emissions [11].

Most thermal power plants have an efficiency of around 30% for the net generated output, and the boiler loses the most energy. Consequently, since the performance of the boiler will impact the efficiency of a plant, it is possible to significantly increase the power plant’s performance by optimizing its boiler performance [4].

5 Possible Energy Losses in Boilers

There are different modes of heat loss based on fuel type, boiler type, operation circumstances, etc. Every component of the boiler system experiences heat losses. There are four general categories into which all losses can be grouped:

  • Heat dissipated by dry flue gases (excluding water vapor);
  • The latent and sensible heat that hot water vapors carry away;
  • Heat loss is caused by conduction, convection, radiation, incomplete combustion, unburned carbon, and losses from the surface outside;
  • Heat loss due to blowdown [12].

5.1 Stack Gas Heat Loss

The stack gas is where the majority of energy losses in a boiler occur. The primary determinants of heat loss are the volume and temperature of stack gas exiting the boiler. As a result, lowering any of these factors will result in less heat loss. The stack gas temperature will need to be lowered to that of the boiler environment to eliminate stack loss completely. However, these losses cannot be avoided due to the economic impossibility and constraints of the heat transfer concept [12]. Excessive flue gas generation occurs because of leakages in the boiler, extra air supply from forced draft fans operating, and leakage in the flue gas pipeline, which reduces heat transfer to the steam and raises pumping needs [13]. It is possible to lessen heat loss through the stack gas by taking the following measures:

  • Optimization of excess air
  • Ensuring a clean area for heat transfer
  • Implementing heat recovery system from flue gas

If less extra air is supplied for burning in the boiler, the volume of stack gas will be lowered. As a result, the velocity of the flue gas slows due to the stack gas’s smaller volume, giving the flue gas more time to absorb heat and lowering its temperature. Boilers typically range in efficiency from 75 to 90%. Thus, it is important to find solutions to reduce 10 to 25% of energy dissipations in boilers [14].

5.2 Conduction and Radiation Energy Losses

The areas around the steam distribution pipelines, auxiliary machinery, and the outside surface of the boilers have different temperatures. Therefore, heat is lost from the heated surfaces of the boiler systems by conduction, convection, and radiation. The temperature of the heated surface determines how much heat is lost, which in turn relies on the insulation. To reduce that energy loss, heated surfaces should be coated with insulating material with a suitable barrier to heat transmission [15].

Additionally, the insulation must be in good condition and of an appropriate thickness. The area of the heated surface affects heat loss. Because of their large surface areas, boilers lose a lot of heat through radiation when operating at low loads. Heat loss from convection and radiation for a typical boiler when running at full capacity is around 2%.

5.3 Boiler Short Cycle Losses

Boiler short cycle loss results in a situation when an overcapacity boiler is chosen for a chemical process with less heating or a plant or process extension plan is not implemented. Large plants frequently employ a single boiler to deliver steam to several manufacturing lines, but only a handful of them are really in use.

This large boiler quickly meets the building’s heating needs, which may cause the boiler to short cycle or allow the boiler to keep producing steam at a low firing rate. However, as the boiler operates at low fire or when short boiler cycling takes place, the thermal efficiency of the boiler drops. This efficiency drop results from the boiler’s fixed losses being magnified by its modest load. Radiation loss doubles at a 50% loading situation compared to a full load state. Pre-purge and post-purge procedures cause additional heat loss. With the help of the fan, the air is pushed into the boiler, dispersing any highly combustible gas mixture that may have built up inside.

5.4 Steam Leaks

Steam leakage takes place through pipelines, valves, traps, flanges, couplings, and process equipment. In some steam distribution systems, it can be of a substantial volume. The size of the hole and the operating pressure affect how much steam leaks from particular openings [1]. Research [16] that analyzed and measured plant losses concluded that a limited number of significant steam or condensate leaks were responsible for most of the overall loss. While the primary boiler had an efficiency of 80%, malfunctioning steam traps allowed 25% of live steam to escape.

5.5 Scaling and Fouling of Heat Transfer Surfaces

On the heat transfer surfaces of boilers, scaling, fouling, and soot formation function as insulators and limit heat transfer. This reduces heat transmission to the boiler’s water and increases flue gas temperature. The boiler’s thermal barrier to heat transmission increases as the flue gas temperature goes up over time under the same load conditions and excess air arrangement. Usually, a buildup of 1-1.5mm soot on fire side can raise fuel usage by 3-8%. Similarly, scale accumulation on the water side can increase fuel usage by 4-9% with a soot deposition of 1-1.5 mm [17].

Scale deposits arise when magnesium, calcium, and silica in most water sources merge to maintain a consistent material layer on the waterside of the boiler’s heat exchange tubes.

However, boilers that use heavy oil may require cleaning more than once a year. Additionally, precautions should be taken. It is important to take action to enhance water softening and maintain a lower level of total dissolved solids (TDS) to prevent scaling due to insufficient water treatment.

5.6 Blowdown Energy Losses

The solid particles in the intake water stream are left behind when the water in the steam drum evaporates and becomes steam. These solids form sediments or sludge in the steam drum, causing heat transfer resistance between the surface of the steam drum and water. Additionally, dissolved materials contribute to foaming and water bleeding into the steam. In order to keep the amounts of suspended and TDS within acceptable ranges, water has to be periodically drained from the steam drum bottom surface. However, the blowdown rate might range from 4 to 10% of the water flow rate, depending on the type of boiler, water treatment, working pressure, and makeup water quality [13, 18]. An auto-control system can be employed with a specific payback period of 1-3 years to ensure optimal blowdown rates [13].

5.7 Moisture Content

The water or moisture in the fuel absorbs sensible and latent heat to form superheated steam during combustion. The heat that is added in the combustion chamber to produce the superheated steam is lost in the chimney and the flue gas. The amount of heat absorbed by water is directly related to the fuel moisture content.

5.8 Incomplete Combustion

Carbon monoxide (CO) is the byproduct of incomplete combustion, which releases only 52% of the total heating capability of the fuel. Thus, products of incomplete combustion should be re-combusted for further energy release.

5.9 Moistest Air

Water vapors are present in the atmosphere as moisture. Water vapors from the combustion air are converted into superheated steam after combustion, sacrificing heat generated from the combustion process. As a result, the moisture in the air causes a certain degree of heat loss.

5.10 Unburnt Carbon and Fly Ash

Studies on boiler ash have revealed that the carbon concentrations are variable, which may be due to changes in the reactor’s retention time, incomplete combustion, temperature changes, and moisture content fluctuations. The carbon content of ash represents the direct loss of valuable fuel carbon that is anticipated to be consumed in the combustion chamber [15].

6 Boilers Energy Audit

An energy audit is a comprehensive way to look into industrial energy use and pinpoint the prominent causes of energy waste. A company may use this tool to examine and comprehend how much energy it uses. It also facilitates deciding on allocated energy distribution in various sectors of an organization, planning energy requirements, improving energy efficiency, reducing energy waste, and substantially lowering energy expenses [18]. A thorough energy management program can be defined and pursued with the help of an energy audit.

6.1 Energy Audit Tools

A walk-through tour, conversation with plant managers and operator interviews is the first part of the energy audit. The building operation is analyzed and any obvious areas of energy waste or inefficiency can be identified through walk-through visits, utility bills like power, gas, and water, as well as other operational data. Energy consumption data is essential for evaluating the energy demand patterns and energy consumption profiles of a facility.

6.1.1 Flow Meter

The flow meter is used for precise flow measurements of wet, saturated, and superheated steam and water.

6.1.2 Exhaust Gas Analyzer

To measure as precisely as possible how well the fuel is burned and how much of each pollutant gas is coming out of the boiler’s chimney.

6.1.3 Pressure Probe

The pressure needed in the boiler system varies depending on location to achieve optimum combustion efficiency. Pressure probes are used to detect pressures at various points within a boiler without disrupting its regular functioning.

6.1.4 Clamp-on Power Meter

This meter is used to assess the electrical output of various systems, as well as the current drawn, the load, and the power factor. Taking readings is easy since the meter can clamp onto whatever you need to measure without interfering with your workflow.

6.1.5 Tachometer

The tachometer can be used to determine the rotational velocity of any component of the boiler system that uses moving parts. It is more convenient to use optical tachometers because they are simple to read.

6.1.6 Thermocouple

Temperature measurements are made using thermometer/thermocouple sensors in different places. The most widely used temperature measurement sensors are thermistors and RTDs. Temperature sensors should be linked to a data logger to store the data. Another option is to utilize a thermal image camera or an infrared thermometer to monitor the temperature of a location within the system that is inaccessible to thermocouple sensors.

6.1.7 Data Logger

It is necessary to gather many system characteristics over extended periods, such as flue gas composition, power consumption by motors, and temperatures at various boiler sites. Several laboratory tools are also required to make it easier to identify the distinctive qualities of the fuel. Energy audits sometimes need laboratory-related work, such as thermogravimetric analysis, solid content determination, heating value determination, etc. A thorough financial study is also required concerning the investment amount, operational cost reductions, and implementation cost.

7 How can We Save Boiler Energy?

We can regulate and minimize boiler energy waste by implementing several techniques. Here are some important techniques that can be adopted for this:

7.1 Excess Air Control

The combustion chamber of a boiler receives air so that the fuel can burn. Only the oxygen in the supplied air is used in the combustion process, while the remaining portions absorb sensible heat from combustion, which is lost down the chimney together as a stack loss. The air volume for combustion purposes should be kept as low as possible to limit stack loss. Due to specific constraints, theoretical air does not ensure the complete combustion of fuels. The amount of combustion air that is given should exceed what is theoretically needed to ensure full combustion. Otherwise, incomplete fuel combustion might result in a larger percentage of CO in the flue gas or unburned carbon in ash. In order to reduce the loss of dry flue gas, a boiler should be run with a specific amount of extra air. An inlet vane control, damper control, variable speed control, and/or monitoring system for the flue gas oxygen content can be utilized to maintain the optimal airflow for complete combustion [18].

7.2 Improving Combustion Efficiency

The amount of energy released from the fuel during the combustion process and converted into useful heat correlates to combustion efficiency. It can be determined by checking the flue gas concentrations of oxygen or CO and the exhaust temperature. As a result, higher emissions of unburned pollutants such as CO and soot are also produced by reduced combustion efficiency [19]. In an ideal situation, a specific volume of air is needed to react with a particular volume of fuel. In real scenarios, combustion requires “extra” air to burn fuel entirely. So, it should be practiced to find out the optimal level of air according to your conditions that can give complete consumption with minimal heat losses.

7.3 Flue Gas Heat Recovery

Boiler exhaust temperatures range from 150 to 250°C [20], and this high temperature is associated with the constraint in the heat transfer area between combustion product and water/steam and condensate of flue gas. As a result, a significant quantity of heat energy is lost through flue or boiler gas emissions. High-temperature flue gas may cause the loss of 10–20% of the input energy [3]. Therefore, recovering some of the net heat content of flue gas can increase boiler efficiency. Utilizing a heat exchanger to warm up the feed water and combustion air is a highly accepted approach to recovering heat from the flue gas (an economizer).

7.4 Preheating Combustion Air

Combustion air preheating is the most effective approach to increase boiler efficiency and steam production [21]. The exchanger is a suitable way to transmit the heat energy from a high-temperature exhaust gas stream to the entering combustion air. Therefore, combustion air is provided with a substantial amount of the sensible heat it requires to participate in the process of combustion.

7.5 Condensate Recovery

In the majority of steam systems, latent heat from the steam is extracted and used for process heating. The resultant condensate is nevertheless quite energetic despite being at steam temperature. There will be considerable energy savings from returning condensate to the boiler’s feed water tank. Therefore, condensate recovery aids in lowering blowdown, water treatment costs, and water usage (water cost) [1].

7.6 Steam Traps

Condensate and non-condensable gases are removed from steam systems using steam traps. As seen in Figure 3, they are primarily used in buildings for condensate removal from steam headers and steam heating coils. The efficient operation of steam traps is critical because allowing live steam to pass from the steam end to the condensate end results in energy loss [22]. Also, the lower capacity and longer heating-up times could cause energy loss if the traps can not pull enough air out of the system when they first turn on or can not get rid of enough condensation.

Figure 3: Stream trap application

7.7 Heat Recovery Technology

The equipment that requires the least amount of capital investment is preferred from an economic standpoint for collecting waste heat and transferring it to a suitable heat sink. It is also better to utilize the waste sources already present in the system. The first option for using the heat recovered from boiler exhaust is to preheat the combustion air and boiler feed water.

Other solutions demand a bigger investment, such as heat pumps to offer a temperature rise [23], the organic Rankin Cycle for producing electricity [24], the absorption refrigeration cycle for air conditioning or cooling impact, etc. [25].

Choosing a technology is also influenced by some boiler exhaust characteristics (dust, sticky). The choice of technology is greatly influenced by the effectiveness of heat recovery, simplicity of installation, and project investment return period [26].

8 Maintenance and Boiler Energy Savings

Regular boiler maintenance can prevent unexpected failure, reduce unplanned boiler downtime, and save energy [27]. Daily, weekly, monthly, and yearly maintenance tasks should be completed. All maintenance work should be documented in writing. Checklists could be used to keep track of the maintenance work done and to let the operator/manager know the working condition of the boiler [28].

8.1 Cleaning Boiler

One of the most frequent issues with the steam production system is fouling on the fire and water side of the boiler tubes. Burning fuel causes deposits to accumulate on the fire side of the tubes, and the fuel types influence the generation of the deposits. Solid fuels have a significantly higher propensity to clog boilers than liquid and gaseous fuels [29]. Heavy oil-fired boilers face more fouling issues than natural gas-fired boilers, which experience much less fouling [30]. The most prevalent fouling causes are low air-to-fuel ratios, improper fuel preparation, or malfunctioning burners. On the waterside of the boiler tubes, deposits or scales are generated by dissolved minerals. The waterside deposit or scale raises the temperature of the tube wall and flue gas because it prevents heat from the tube wall from transferring to the water. Scale growth in an operating boiler may be detected automatically and chemically removed. In small low-pressure boilers, boiler feed water should be examined regularly, and in large high-pressure boilers, it should be hourly. Soot blowers are frequently used in large boilers to make cleaning the fireside tube surfaces easier. Small boilers should undergo routine examination and cleaning.

8.2 Improve Insulation

Directly applying insulation to the boiler’s outside walls and its steam pipes is a quick and affordable technique to reduce heat loss through radiation and convection. Insulation should be made of newly developed materials with a decreased heat capacity, including fibers and ceramics. Steam and condensate pipes, valves, boiler surfaces, and fittings should all be insulated since they reach temperatures higher than 50°C. Hot areas on the boiler casing should be examined and repaired, and worn-out or damaged insulations should be replaced. The temperature of the skin should not exceed 50°C. According to research, improved insulation can reduce costs by 6-26% [31].

8.3 Minimize Wasted Blowdown Water

The routine hot water discharge from the steam drum’s bottom is blown down to avoid scale buildup on the boiler tubes. This blowdown process wastes the energy generated by the hot water, and more repeated blowdowns amplify this energy loss. The energy savings from blowdown water can be maximized by using automatic blowdown controls to maintain a controlled TDS, conductivity, alkalinity, silica, and acidity in the boiler water. The system’s efficiency can increase by around 1% if waste heat from blowdown water is used to preheat boiler feed water, saving even more energy [32].

8.4 Other Activities

The proper operation of the water supply system, fuel supply system, safety devices, electrical component systems, etc., and the maximum heat transmission, require scheduled maintenance [33]. Other operations, such as frequent combustion tests utilizing flue-gas analysis [34], are also required to ensure the boiler operates at peak efficiency.

9 Boilers Emission Control

The combustion of fossil fuels in industrial and commercial boilers produces the following main emissions: CO2, CO, nitrogen oxides (NOx), sulfur oxides (SOx), volatile organic compounds (VOC), and particulate matter (PM). The portion of each constituent in the emissions gases may vary depending on the fuel type, its purity or pollutant ingredients, combustion process, etc. This section will discuss the emission control techniques of these constituents.

9.1 CO2  Emission Control Techniques

Carbon capture and storage (CCS) is the method of absorbing and storing CO2 before it is emitted into the air. This technique can absorb up to 90% of the CO2 produced from fossil fuels burning to generate power and perform industrial activities [35].

CO2 may be collected in several ways. The three primary ones are oxyfuel, post-combustion, and pre-combustion. CO2 is removed from flue gases generated by the combustion of fossil fuels using post-combustion technology. Pre-combustion processes convert fossil fuel into a mixture of hydrogen and CO2 before it is burnt. Fossil fuels can be burned with almost pure oxygen using oxyfuel technology, resulting in the emission of CO2 and steam emission as byproducts. The CO2 is compressed into a liquid condition once captured and then transferred via pipeline, ship, or road tanker. Then, if the geology is adequate, CO2 may be piped underground, often at depths of 1 km or more, to be stored in depleted reservoirs of oil and gas coalbeds or deep salt reservoirs.

9.2 NOx Control Technologies

There are two categories of NOx controls: post-combustion procedures and combustion control approaches. While combustion control approaches attempt to prevent NOx from forming during combustion, post-combustion strategies target NOx emissions after they have already been produced. Post-combustion procedures are often not employed on boilers with inputs of less than 100 MMBtu/hr since they are more expensive than combustion control approaches.

Post-combustion control techniques include:

  • Selective Non-Catalytic Reduction
  • Selective Catalytic Reduction

The level of NOx control differs depending on the approach. Most of the techniques discussed in section 8 are included in the combustion control techniques.

9.2.1 Selective Non-catalytic Reduction

A NOx reducing agent (e.g., ammonia or urea) is injected into the boiler exhaust gases during selective non-catalytic reduction at a temperature of around 750–870°C. The ammonia or urea converts the NOx in the exhaust gases into water and ambient nitrogen. NOx is reduced by up to 70% using selective non-catalytic reduction [36].

However, it is quite challenging to implement the technique in industrial boilers that modulate or cycle often. This is necessary because a particular flue gas temperature must be reached before injecting ammonia (or urea) into the flue gases. Additionally, the level of exhaust gases at the designated temperature continually changes in industrial boilers that modulate or cycle regularly. Therefore, it is impossible to use selective non-catalytic reduction on industrial boilers with large turndown capacities and often modulate or cycle.

9.2.2 Selective Catalytic Reduction

Ammonia is injected into the boiler exhaust gases while a catalyst is present during selective catalytic reduction. Contrary to selective non-catalytic reduction, the catalyst enables ammonia to reduce NOx values at lower exhaust temperatures. Further, the selective catalytic reduction can be carried out with exhaust gases as low as 260°C, depending on the catalyst type. This process can reduce NOx production by about 90% [37]. This process will be expensive to implement when boilers operate with less than 100 MMBtu/hr inputs.

9.3 Sulfur Compounds (SOx)

Shifting to low sulfur fuel, desulfurizing the fuel, and using a flue gas desulfurization (FGD) system are all ways to minimize SOx. Fuel desulfurization involves eliminating sulfur from the fuel before burning it, and it is mostly used for coal. Scrubbers are used during flue gas desulfurization to take out SOx emissions from the flue gases.

Systems for removing sulfur from flue gases can either be regenerable or not. The most popular non-regenerable FGD systems produce waste that needs to be properly disposed of. The waste byproduct is transformed into a usable product, such as sulfur or sulfuric acid, using regenerable FGD. Through FGD, SOx emissions can be reduced by 90–95% [36]. The main purpose of fuel desulfurization and FGD is to lower SOx emissions from large utility boilers. On industrial boilers, the technique typically cannot be economically justified.

The most economical way to reduce SOx for those who use industrial boilers is to burn low-sulfur fuel. Burning fuels with little or no sulfur content, such as distillate oil, can reduce SOx emissions without the need to build and operate costly machinery since the sulfur level of the fuel largely influences SOx emissions.

9.4 Carbon Monoxide (CO)

The high volume of CO emissions in boilers equipment is typically caused by incomplete combustion brought on by firing settings (such as an incorrect air-to-fuel ratio) or, in certain cases, a leaking furnace. The generation of CO may be controlled at a safe level by proper burner maintenance, inspections, and operation, as well as through equipment upgrades or the use of an oxygen control package.

9.5 Particulate Matter (PM)

Sulfates, nitrates, carbons, oxides, and unburned fuel components are a few of the numerous chemicals that make up PM emissions from combustion. Particulate pollutants have the potential to be corrosive, poisonous to both plants and animals, and dangerous to humans.

The fuel quality used in the boiler is the main factor affecting PM emissions. In general, natural gas has substantially lower PM levels than oils. Compared to residual oils, distillate oils produce significantly fewer particle emissions.

Sulfur, carbon residue, asphaltenes, and ash are the four fuel components that impact particle levels most when burning heavy oils. These elements are found in fuel oils, especially residual oils, significantly affecting particle emissions. The particle emissions for the oil can be approximated by knowing the fuel constituent levels.

Several particulate control techniques are used for various types and sizes of boilers. Scrubbers, baghouses, and electrostatic precipitators are frequently used with utility boilers. Utilizing clean fuels is the most efficient strategy for industrial and commercial boilers. Particulate matter emissions can be reduced by converting from a residual to a distillate oil or from a distillate oil to natural gas. Additionally, particle emissions can be reduced by appropriate burner setup, adjusting, and maintenance, but switching fuels is the most effective strategy.

9.6 Volatile Organic Compounds (VOCs)

VOCs consist of carbon, hydrogen, and occasionally oxygen. VOCs are problematic because they contribute to developing ground-level ozone and quickly evaporate once released into the atmosphere. They are commonly referred to as hydrocarbons and typically fall into one of two categories: methane or non-methane, depending on how well a boiler performs.

The main cause of VOC formation in commercial and industrial boilers is inadequate or insufficient combustion caused by faulty burner setup and adjustment. No auxiliary equipment is required to regulate VOC emissions from industrial and commercial boilers; regular burner/boiler maintenance can keep VOC emissions to a minimum. Maintaining proper air and fuel pressures in the combustion chamber or per the manufacturer’s recommended setting also plays a role in VOC emissions. VOC levels can exceed 100 times the normal limits in a boiler that has not been properly maintained.

10 Endnotes

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[17]      “Best Practice Guide for Improving Energy Efficiency,” ed: LJ Energy, 2021.

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[19]      T. Nussbaumer, “Combustion and co-combustion of biomass: fundamentals, technologies, and primary measures for emission reduction,” Energy & fuels, vol. 17, no. 6, pp. 1510-1521, 2003.

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[21]      T. Hasegawa, S. Mochida, and A. Gupta, “Development of advanced industrial furnace using highly preheated combustion air,” Journal of propulsion and power, vol. 18, no. 2, pp. 233-239, 2002.

[22]      J. Radle, “The importance of intensive steam trap management,” Chemical Engineering, vol. 114, no. 12, p. 40, 2007.

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