Ways to improve the quality and efficiency of boiler manufacturing in Vietnam

Technologies to improve industrial boiler efficiency are well known and developed, but research is ongoing. As efficiency improves with steam pressure, within the boiler sector most breakthroughs have been made on new materials for large power boilers allowing for higher temperatures and pressure. For industrial boilers, as pressures are normally quite low (up to 20-30 bars), the use of new expensive materials is not necessary; efficiency improvement is, therefore, achieved by other techniques.

The available techniques for improving boiler efficiency are listed below, together with reflections on their use in Vietnam for the first 10 options:

1. Enhancing burner performance: for oil and gas boilers
2. Boiler Instrumentation and control (I&C) enhancement
3. Installing economizers and air heaters
4. Improving heat exchange from combustion gases to water
5. Boiler insulation
6. Boiler blowdown heat recovery
7. Switching from intermittent to continuous blowdown
8. Minimization of heat transfer surface deposits
9. Combustion tuning/excess air control
10. Use of cleaner fuels
11. Blowdown control (automatic blowdown)
12. Combustion optimization
13. Use of blowers to remove soft deposits on heat exchange surfaces

1. Enhancing Burner Performance

Older, incorrectly sized, or mechanically deteriorated burners are typically inefficient which results in incomplete combustion (high carbon monoxide emissions and unburned carbon) and the need for high excess air.

Burner upgrades will often need to consider controlling conventional emissions and more sophisticated combustion monitoring and controls may be an integral part of a burner upgrade. Burner replacement and retrofits can also be considered as a potential market to be developed in Vietnam. New burners for all types of boilers and fuels are commercially available and are focused on reducing emissions and improving efficiency of burners with single and multiple fuel capability, low- and ultra low-NOx emissions.

Burner technology requires a high level of skill and long manufacturing experience, which Vietnamese manufacturers do not presently have (they purchase burners from western Companies). The need for developing burner technology in Vietnam is marginal if we consider that manufacturing of biomass and coal boilers is the most relevant production in Vietnam. However, a market for small boilers in densely populated areas where use of coal and biomass is not allowed still exists and could be covered by companies specialized in burners manufacturing.

2. Boiler Instrumentation and Control (I&C) enhancement

Vietnamese boilers are normally equipped with basic I&C; a simple positioning control, where the steam pressure drives the fuel valve (for oil and gas burners) is common for boilers manufactured in Vietnam. For solid fuel boilers (coal and biomass) the control system is even more basic, and based more on combustion air flow rate control than on fuel flow rate control (which is often manually operated).

There are no fixed requirements for instrumentation. In fact, very little instrumentation is essential to operate the boiler safely. However, if maximizing boiler efficiency or minimizing emissions (e.g., CO, NO_x, and CO) is required, certain instrumentation is justifiable, such as temperature sensors and oxygen monitors. Additional instrumentation makes it possible to achieve better performance. If a CO monitor is installed, the boiler operator can fine-tune the process for uniform operation at minimum excess air without generating excess CO.

Combustion optimization and I&C may not be economical in all cases. Usually, the larger the boiler, the more likely optimization and I&C can be justified. Also, coal or biomass fired boilers may be better candidates for optimization and I&C systems than natural gas-fired systems because their operating parameters (e.g. fuel quality) may be variable and difficult to control

As CO emissions are a major concern to Vietnamese government and manufacturers, some rules from government to install CO sensors to control emissions at stack could be accepted by manufacturers as a measure to limit CO emissions. The same sensor could be used to optimize combustion and improve efficiency.

3. Installing economizers and air heaters

Air heaters are not as widely used as they were in the past due to more recent NO_x regulations (the higher temperature of the combustion promotes NO formation). Hence, many gas and liquid fuel boilers do not have air heaters. Economizers are favored in these cases, as they do not adversely impact the combustion air temperature and the resulting NO_x formation.

Economizers are basically tubular heat transfer surfaces used to preheat boiler feedwater before it enters the steam drum or furnace surfaces. Economizers also reduce the potential of thermal shock and strong water temperature fluctuations as the feedwater enters the drum or waterwalls. Similar to all gas/air handling equipment, economizers and air heaters will impose some pressure loss on the system. Also, reducing flue gas temperatures close to or below acid dew point incurs condensation/corrosion concerns that must be addressed by manufacturers

One additional point is that potential for a market for installing economizer and air heaters on existing boilers could be created by government by setting up measures that require boiler owners to improve boiler efficiency, as mentioned above.

4. Improving heat exchange from combustion gases to water

In firetube boilers, within the tubes, the combustion gas typically changes from a turbulent flow regime when it enters the tubes, to a laminar regime, with its boundary layer of cooler gas along the tube walls. This boundary layer has a well-known negative impact on heat transfer.

To obtain a turbulent regime within the tubes, the use of turbulators helps to regain the heat transfer characteristics of a turbulent flow regime by creating “turbulence” within the tubes. Physically, turbulators are simple devices (baffles, blades, coiled wire) that are inserted in the gas tubes to “break-up” the laminar boundary layer, resulting in the increased convective heat transfer.

The implementation of this technology requires a change in boiler design and possibly the use of heat exchange software tools to re-calculate the heat balance in furnace and firetubes. The result is that the flue gas exits at a lower temperature, and boiler efficiency is improved. For firetube boilers, turbulators are a cheaper alternative to economizers and air heaters. Turbulators are usually installed on the last boiler pass.

Efficiency improvements from the application of turbulators derive from the increased heat transfer from the flue gas, resulting in lower flue gas exit temperature. Vietnamese manufacturers could easily use this technology to improve oil and gas boiler performances. The effect of ash transportation should be checked for biomass and coal boilers.

5. Boiler Insulation

The outer surface area of the larger boilers can be relevant as significant heat loss can occur through the boiler shell. Proper insulation is important to keep these losses to a minimum with the refractory material lining the boiler the primary insulating material. The quantity of heat lost in this way is fairly constant at different boiler firing rates and, as a result, becomes an increasingly higher percentage of the total heat loss at the lower firing rates. Properly applied insulation can result in large savings in energy losses depending on type, thickness, and condition of the existing insulation. Radiation losses tend to increase with decreasing load and can be as high as 7% for small units or larger units operating at reduced loads.

Implementing this measure is technically straightforward and can easily be implemented by Vietnamese manufacturers. The potential for a market for replacing insulation on existing boilers is also attractive.

Implementing this measure is technically straightforward and can easily be implemented by Vietnamese manufacturers. The potential for a market for replacing insulation on existing boilers is also attractive.

6. Boiler blowdown heat recovery

Blowdown is required to maintain water quality. Depending on site-specific conditions and make-up of water quality, blowdown rates may vary greatly. The blowdown still contains energy, which could otherwise be used instead of being wasted. This waste heat can be recovered with a heat exchanger, a flash tank, or flash tank in combination with a heat exchanger. The resulting low-pressure steam is most typically used in deaerators. Cooling the blowdown has the additional advantage of reducing the temperature of the liquids released into the sewer system. The higher the blowdown rate and boiler pressures, the more attractive the option of recovering the blowdown becomes. Any boiler with continuous blowdown exceeding 5 percent of the steam rate is a good candidate for considering blowdown waste heat recovery.

This measure is simple and affordable for Vietnamese manufacturers

7. Switching from intermittent to continuous blowdown

Blowdown can be either intermittent bottom blowdown or continuous blowdown. Intermittent bottom blowdown may be sufficient if the feedwater is exceptionally pure or the boiler does not need deeply treated water. Intermittent blowdown is performed manually and, therefore, may result in wide fluctuations in blowdown patterns. Use of continuous rather than intermittent blowdown saves treated boiler water and can result in significant energy savings.

This measure can be easily implemented by Vietnamese boiler manufacturers

8. Minimization of heat transfer surface deposits

Boiler heat transfer surfaces are exposed to high temperature gases and products of combustion, which vary in composition amongst different fuels and operating conditions. Formation of soot, ash products from solid and liquid fuels, and incomplete combustion of carbon all contribute to the potential for surface deposits. Oxides may also be formed on the surface of the tubes. These deposits are further related to operational issues ranging from malfunctioning burners, to the condition of the heat transfer surfaces, to gas flow patterns within the boiler combustion zone. Minimizing heat surfaces deposit is normally an operational measure, but some  systems firing ash-laden fuels also include “cleaning” systems (soot blowers that typically use compressed air or steam) to periodically remove the unavoidable deposition on the boiler walls and tubes.

Solid fuels are the most common application for soot blowing systems. Boilers that burn more fuels (multi-fuel) generally have more problems with creation of deposits than in the heat exchange surfaces.

More advanced soot blowing systems, or Intelligent Sootblowing Systems (ISS), which use feedback signals, such as exit gas temperature or heat transfer sensors to trigger their operation could be used to improve boiler performance.

9. Combustion tuning/excess air control

One objective of the initial setting of the combustion system is to maximize the combustion efficiency (minimize unburned carbon and CO emissions), and demand to minimize NO emissions – as a result, this may require further tuning. Also, the combustion system may drift over time from its optimum setting or certain controls (e.g. dampers) and therefore, may not be operational due to wear.

Normally combustion tuning is an operational measure that is implemented either by boiler operators or by specialized engineering companies. For larger boilers, especially ones that change operating conditions (e.g. load) frequently, it may be economical to install an optimization system. These are software-based systems that monitor and optimize boiler performance based on a user-specified objective within pre-set operating constraints. An optimization system normally uses signals from sensors installed at several points of the boiler. One of the most important parameters to be taken under control is the excess air or air-to-fuel ratio.

Excess air can easily be tuned in oil and gas boilers, while it is more difficult to achieve in solid fuel boilers, where fuel feeding is often done manually by the opening of fuel doors. In biomass and coal boilers, excess air is regulated by dampers at induced draft fans, which can be either manually operated or controlled by steam pressure. Sometimes, fan motors have inverters which can be used to regulate fan speed and tune up combustion parameters.

Cost of combustion optimization systems are relatively low and are particularly important for large boilers subject to frequent load variations. Its implementation on small boilers could be analyzed and implemented with Vietnamese manufacturers.

10. Use of cleaner fuels

The use of clean fuels in a boiler presents interesting advantages in terms of performance and efficiency improvement. Unfortunately, it is not normally possible (or is too expensive) to select fuels cleaner than those available on the market.

Cleaning the fuel at the boiler site is also usually relatively expensive and generates waste which is difficult to manage and poses additional problems. As a result, the increasing cost of fuel is driving customers towards cheaper, but dirtier fuels which will result in lower efficiencies and higher emissions.

In Vietnam, the use of biomass to replace coal and oil is increasing. Rice husks, shell nuts, bagasse and other biomass fuels are residues from harvesting or production processes (i.e. palm oil). The low heating value and the associated high transportation costs per unit energy content do not normally allow cleaning techniques to improve boiler performances and limit the extensive use of biomass.

Fabrication of biomass briquettes and pellets from wood has been implemented over the last few decades in western countries in order to easily handle biomass fuels. Fully automatic fuel feeding systems for wood pellets and other biomass are in operation in Europe and the US with high efficiencies.

Production of pellets/briquettes is separated from boiler operation as boilers owners have wood pellets available on the market as an alternative to raw biomass (i.e. wood logs).This technology, which implies a new biomass production logistic, could be studied to verify its implementation in Vietnam. Plants for the production of pellets to deliver to customers could be set up in areas of biomass production. Pellets, sold in bags or containers, will be easier to manage than raw biomass and could result in higher boiler efficiencies.

For oil boilers, new technologies, such as micro emulsions, are already available and guarantee 3-5% efficiency improvement together with cleaner emissions and less deposits on heat transfer surfaces.

Micro-emulsion devices produce a quite stable (lasts for months) emulsion of oil and water which, when injected into the boiler furnace from the burner, result in optimized and more homogeneous combustion with lower unburned carbon, CO and smoke at stack emissions. The efficiency gain is higher than 3-5%, but some energy is lost to evaporate the water in the emulsion.