An Overview of Coal Combustion in Steam Boilers
Steam boilers are vital in the process and manufacturing industries, in which fuel plays an integral role for combustion purposes. Various types of steam boilers are ideal for combustion with certain types of fuels such as oil-fired boilers, gas-fired boilers, coal-fired boilers, biomass boilers, etc. Coal is a widely used fuel in India, particularly bituminous and sub-bituminous coal. Coal is classified into three primary types, i.e., anthracite, bituminous, and lignite. Among these, anthracite is the most ancient type of coal whereas lignite was found comparatively later, consisting of volatile matter and moisture content with low fixed carbon. Coal combustion greatly depends on its chemical composition. The properties of coal can be classified as physical properties and chemical properties.
Reactions of Coal Combustion:
The major factor that contributes to releasing heat in steam boilers is the oxidation reaction that converts the elements of coal into their respective oxides. The coal combustion takes place in two stages,
- Evolution of volatile matter in the initial stages of heating along with physical and chemical changes
- Resultant combustion of the residual char
The ignition and combustion of volatile matter result in the oxygen diffusion to the particle surface and ignition of char. Char oxidation occurs in three steps as stated below:
- Oxygen diffusion from bulk gas to char surface
- The reaction between oxygen and char particle surface
- Reaction products diffusion from the char particle surface to the bulk gas
The overall reaction rate is determined by the rate of chemical reaction at low combustion temperature. With an increase in chemical reaction rate with raising the temperature, the carbon-oxygen reaction occurs quickly, leading to difficulty in the diffusion of oxygen to the surface. In such situations, the overall reaction rate is monitored with the diffusion rate of oxygen to the reacting char surface.
Coal Combustion System in Steam Boilers:
Fixed Bed:
In fixed bed combustion, coals sizing between 3 millimeters and 50 millimeters are placed on a grate and preheated with primary air. The air is blown under the bed to burn the fixed carbon along with secondary air that burns the volatiles released from the bed. The coal feeding system can be classified as underfeed, overfeed, spreader, and traveling-grate stoker.
The large-sized coal combustion reduces the heating rate of the particle to about 1°C per second, leading to a combustion time of about 45 to 60 minutes.
Fluidized Bed:
A fluidized bed in steam boilers consists of fluid by an airstream from the bottom of the bed that suspends the material. The bed material usually includes a mixture of coal and sand that acts as a fluid. The crushed coal preheated to about 850 to 950°C and then introduced to the bubbling bed. The coal particles are heated at 1000°C per second and devolatile, which burns the residual in around 20 minutes. Fluidized bed results in effective coal combustion due to a uniform temperature throughout the bed. A fluidized bed requires accessories such as cyclone collectors, separating fines that contain a high amount of combustibles, and recycling them back in the steam boiler system.
Pulverized Coal:
Pulverized coal combustion is highly used in large power stations as it performs fine grounding of coal, in which 70% to 80% by weight passes through a mesh screen. The powder is burned in the combustion chamber by suspending the particles in the combustion air. Due to finely ground coal having more surface area per unit than denser particles, coal combustion takes place quickly. However, this method leads to the excess cost of drying and grinding the coal, fouling and slagging of heat-transfer surfaces, and equipment for collecting fine particles.
Advantages and Disadvantages of Coal Combustion:
Advantages of Coal Combustion:
- Abundant Supply
- Easy Storage
- Low Capital Investment
- Safe to Use
- Simple Burning Process
Disadvantages of Coal Combustion:
- Non-renewable source of energy
- High Carbon Emission
- Harms Natural Habitats
- Coal is potentially radioactive
- Hazardous to the environment by increasing pollution
Alternative for Coal Combustion in Steam Boilers:
Coal is relatively expensive as well as harmful to climate with its carbon emission. Additionally, it is non-renewable and requires careful utilization by ensuring minimal wastage. Non-renewable fossil fuels are expected to deplete in the next three to four decades. Biomass is an excellent alternative as fuel for combustion in the steam boiler system. Biomass fuel includes animal waste, industrial waste, agricultural waste, etc., which is cost-effective and easy to procure, ranking as the fourth energy source globally with approximately 14% of consumption.
Depending on their elemental composition, biomass fuels show different combustion characteristics. It involves chemical reactions wherein carbon is oxidized to carbon dioxide while hydrogen is oxidized into water. Biomass can be ignited with various types of combustion processes like fixed bed combustion, fluidized bed combustion, and pulverized bed combustion. Biomass combustion reduces SO2 by up to 75%, and compared to coal it reduces the CO2 emissions by 93%
Rakhoh Boilers is a leading boiler manufacturer in Pune since 1983. With the expertise of our 500+ members in thermal solutions, we have emerged as a leading name in boiler supplier globally with more than 3000 successful boiler installations in over 26 countries worldwide. We manufacture a range of efficient industrial steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories. To ensure optimal productivity and efficiency of steam boilers, we provide boiler services like solid fuel conversion, energy audit, boiler automation, steam trap assessment, annual maintenance contract, etc.
To learn more about our products and services, visit www.rakhoh.com
- Published in Boiler
A Guide to Biomass Boilers | Working, Fuel Types, Advantages, and Disadvantages
Steam Boilers are undoubtedly an essential asset in most process and manufacturing industries. Fuels play a vital role in the combustion process of steam boilers. The ignition of fuel heats the water that causes vaporization to generate steam. Steam boiler varies with the fuels used by the process plant, such as solid fuel boilers, oil-fired boilers, gas-fired boilers, and biomass boilers. The increasing costs of fuels that consequently increase the production costs as well as environmental laws have escalated the demand for biomass boilers in recent years. Biomass boilers are emerging as an ideal solution for heating sustainably and efficiently. Let us have a detailed understanding of the working of biomass boilers along with their advantages and disadvantages.
What is a Biomass Boiler?
Biomass boilers are an ideal choice for renewable heating. For understanding biomass boilers, it is necessary to understand the concept of biomass. Biomass is a fuel acquired from organic matter such as wood, agricultural wastes, industrial wastes, etc. Such renewable steam boiler systems are available for various types of biomass fuel with a range of product capacity, depending on the requirements of the process plant. With the increasing costs of non-renewable fuels like coal and the depletion of fossil fuels in the next three to four decades, biomass boilers are the best solution for effective heating and steam generation.
In other words, biomass boilers offer heating from renewable fuels, efficient steam generation, and reduce greenhouse emissions.
Biomass Conversion into Energy:
Biomass can be converted into energy through various processes such as,
- Direct Combustion: Burning the fuel to produce heat. Direct combustion is the most commonly used method of converting biomass into energy. Biomass of any type is burned for heating in industries, buildings, and generating electricity through steam turbines.
- Thermochemical: The process involves conversion to produce solid, liquid, or gaseous fuels. Thermochemical conversion involves the process of pyrolysis and gasification that are decomposition processes. It includes heating the biomass feedstock materials at high temperatures in closed, pressurized vessels termed gasifiers.
- Chemical: Chemical conversion produces liquid fuels. Also known as transesterification, it is used for converting animal fats, vegetable oils, and greases into fatty acid methyl esters (FAME), utilized for producing biodiesel.
- Biological: It includes biomass conversion into producing liquid and gaseous fuels. Biological conversion involves fermentation in order to convert biomass into ethanol and anaerobic digestion that resultantly produces renewable natural gas. Renewable natural gas, also termed biogas or biomethane, is generated in anaerobic digesters at sewage treatment plants or dairy or livestock operations. It can also be captured and produced from solid waste landfills. If treated properly, renewable natural gas has the same utilization as fossil fuel natural gas.
Working of Biomass Boilers:
One of the primary benefits of adapting to biomass boilers is their ease of use. Like general steam boilers, biomass boilers use renewable fuels like wood, wood chips, wood pellets, agricultural wastes, industrial wastes, etc., to generate heat for steam.
Biomass boilers operate by burning the fuels suited for the steam boiler system, utilized for heating to generate steam. However, biomass boilers require a larger amount of fuel compared to fossil fuels. Biomass boilers are easy to maintain, but the ash built up by burning the fuel requires regular cleaning.
Types of Biomass Fuels:
Some of the most commonly used biomass fuels are as follows,
- Wood Chips:
Wood chips are the most economical type of biomass fuel that produces around 2.5p per kilowatt-hour (kWh) of electricity. It consists of a 30% moisture content that should not be stored for longer periods to avoid degradation. The drawback of using wood chips is the germination of spores that can cause incurable respiratory disease. Therefore, it is essential to keep the airflow and encourage the drying process.
- Wood Pellets:
Wood pellets are relatively expensive, as compared to wood chips. It is the densest form of biomass, as well as efficient and compact. Wood pellets release a high amount of dust that requires effective hoppers and dust collector systems.
- Wood Logs:
Wood logs are ideal for those having access to wood sources and large storage spaces. Due to its high moisture content, it requires to be stored in a dry place for a year to reduce its moisture content and allow its maximum efficiency. It can also be used as wood bark and sawdust.
- Agricultural Wastes:
Agricultural wastes such as briquettes, bailed bagasse, rice husk, coffee husk, palm shell, groundnut shell, coconut shell, cotton waste, etc. It is best-suited to agricultural industries and industries that can transport the fuels conveniently and store them easily.
- Industrial Wastes:
Industrial wastes like paper pulp, wood waste, textile waste, etc. can be used as fuels depending on the requirement and capacity of the steam boiler system.
Advantages of Biomass Boilers:
- Environment-friendly
- Renewable source of energy
- Cost-effective
- Easy to procure and transport
Disadvantages of Biomass Boilers:
- Requires large space for storage
- Requires Large Furnace
Efficient Biomass Boilers by Rakhoh:
With 38+ years of expertise and experience in thermal solutions, Rakhoh Boilers have aimed to balance advancement and sustainability with consistent innovations. We deliver efficient and reliable Biomass Boilers that operate effectively with various types of biomass fuels. As a leading steam boiler provider globally, we provide the best boiler services like fuel conversion, boiler automation, energy audit, annual boiler maintenance, steam trap assessment, turnkey solutions, etc.
Learn more about our products and services on www.rakhoh.com
- Published in Boiler
An Overview of Combustion Air in Steam Boilers
Steam boilers are integral in manufacturing industries for heating and drying purposes in process operations. Boilers generate steam by igniting fuels through combustion that requires an optimal amount of fuel, heat, and air. Although all three elements are vital for proper combustion, this article will discuss the role of air in detail.
Improper combustion leads to carbon monoxide poisoning that may cause more damage than the collective impact of pressure vessel failure and combustion explosion. Therefore, it is necessary to ensure proper maintenance, operation, and inspection of the combustion system. Various factors affect the combustion air supply, leading to the issue of carbon monoxide.
Carbon monoxide is caused by incomplete or improper combustion. It is colorless and odorless that cannot be detected with an instrument. In some cases, there are indicators for the production of carbon monoxide. During improper burning of natural gas, the formation of aldehydes along with carbon monoxide causes an unpleasant odor. Similarly, smoke or soot is released during the oil combustion, indicating unburned oil with its smell.
Carbon monoxide is occurred due to two primary reasons:
- Improper mixing of the air with the fuel used during the combustion
- Inadequate supply of combustion air to the fuel-burning equipment
The improper mixing of air and fuel in combustion usually occurs due to the adjustments of components consisting of the burner. It is also important to ensure that the burner is provided with an adequate air supply for proper and complete combustion. Since most of the steam boilers are inspected by boiler manufacturers and independent organizations, steam boiler design is less likely to be the cause of carbon monoxide.
Various factors should be considered for ensuring adequate air supply in combustion:
- The amount of air needed for the combustion process
- The amount of air needed for cooling and ventilation in the boiler room
- The impact of the venting system such as vent hoods, economizers, barometric dampers, and draft control systems
- Presence of exhaust fans that may be the source of providing the air or consuming the air from the boiler room
- Any other equipment that may draw the air needed for combustion
- Burner maintenance
- Boiler room maintenance
The amount of air required for the combustion process depends on the chemical composition of the fuel used. On the other hand, the amount of air needed for cooling and ventilation in the boiler room is determined by the heat loss from the boiler shell, boiler piping, or any other equipment that generates heat. Heat loss from the boiler shell usually ranges up to 4% of the boiler output, depending on the steam boiler size, pressure, temperature, boiler design, and insulation. In most cases, a large boiler faces less heat loss. To ensure the optimum amount of air in the burner; the boiler room temperature, at the burner fan inlet, should be maintained between 50°F and 100°F.
The venting system can impact the amount of air needed in the boiler room and the performance of the air fan in providing the required amount of airflow. For instance, draft hoods and barometric dampers consume the air from the boiler room that could have been used for combustion. The result of introducing an economizer must be considered before its installation. The flue gas economizer accelerates the pressure loss through the venting system. In case of substantial pressure loss, the overall combustion air fan performance is reduced.
Exhaust fans in the boiler room can be considered a severe challenge in the combustion air supply unless provided with adequate makeup air. Facilities that draw combustion air from the plant itself face more unbalance.
Regular maintenance of the burner and boiler room is essential to ensure adequate combustion air supply. A clean boiler room is vital for the proper combustion process. Dirt or impediments in the fan inlet screens or flan blades lead to soot formation and carbon monoxide. Timely boiler flue-gas analysis helps in detecting an insufficient supply of combustion air. Burner adjustments must be performed by a trained professional with a necessary instrument to assess the amount of oxygen or carbon dioxide and ppm of carbon monoxide.
It is paramount to ensure that the air supply is sufficient for the combustion process to prevent the possibility of mishaps such as furnace explosion in process plants.
Since our inception in 1983, Rakhoh Boilers have emerged as a globally leading industrial steam boiler manufacturer. With our beginning as a steam boiler manufacturer in Pune, we are presently one of the trusted names as boiler manufacturers and thermal solution providers in over 26 countries worldwide. We manufacture a range of efficient steam boilers, waste heat recovery boilers, thermic fluid heaters, and boiler accessories. We provide the best boiler services like energy audit, steam trap assessment, boiler automation, annual maintenance contract, turnkey solutions, etc.
To learn more about our products and services, visit www.rakhoh.com
- Published in Boiler
An Overview of Functioning and Advantages of Thermic Fluid Heaters
With the increasing demand for heat and temperature control to accomplish process operations, manufacturing units are looking for methods to improve operational performance with optimal efficiency, safety, and minimal downtime. Although steam boilers are a commonly found asset, many manufacturing facilities opt for Thermic Fluid Heaters as a solution to various applications, delivering enhanced temperature control and reliability.
Compared to steam boilers, the demand for Thermic Fluid Heater is relatively low but gradually increasing. Thermic fluid heaters are effective for process heating purposes as it uses highly viscous oil as the heating medium. It operates in a closed-loop with the heat transfer fluid in constant circulation. The components of a Thermic Fluid Heater includes,
- Heater Coils
- Furnace
- Expansion cum Deaerator
- Circulating Oil Pump for High Temperature
- Heat Recovery Units
- Primary and Secondary Fans
- Chimney
- Pollution Control Equipment
- Control Panel for Monitoring
Working Principle of Thermic Fluid Heater:
Thermic Fluid Heaters function in either the liquid phase or vapor phase. In the liquid phase, thermic fluid heaters use a flooded pressure vessel to heat the heat transfer fluid without vaporization taking place in the vessel. It is similar to hot water boilers with a closed-loop system that is, either open or closed to the atmosphere. On the other hand, the vapor phase uses a heater for vaporizing the fluid internally within the vessel. It also uses a flooded heater for vaporizing the fluid externally through a flash drum. The vapor phase with condensing steam provides heat uniformly compared to the liquid phase.
The combustion air enters the burner fan inlet and travels upside between the inner and outer jacket, preheating the air before its ingress in the top-mounted burner. Hot gases pass across the length of the vessel in the first radiant pass. The gases then pass through the inner row of coils in the second convection pass. The third convection takes place as the gases continue back between the inner coil and outer coil. The final pass occurs upwards between the outer coil and inner jacket to the outlet of flue that leads to the fourth convection pass.
Usually, the heater design is similar to the fire-tube boiler or water-tube boiler that is direct-fired by the fuel combustion. Depending on the operation requirements and fluid used, heaters may operate at up to 750⁰F temperature. Generally, large-sized heaters are field erected.
Advantages of Thermic Fluid Heaters:
A thermic fluid heater offers numerous advantages over steam boilers. Some of the significant benefits of thermic fluid systems are as follows:
High Temperature at Low Pressure:
Thermic fluid heaters offer a higher temperature range as they can achieve temperature up to 750⁰F whereas steam boilers operate only up to 350⁰F. Along with high temperature, thermic fluid systems operate at lower pressure, requiring less than 100 PSIG of vapor pressure at 750⁰F compared to steam boilers that require 3,200 PSIG of operating pressure at 750⁰F.
Low Maintenance:
Thermic fluid heaters require minimal maintenance due to the simplicity of circuits. The fluid does not need regular adjustments or additions if checked periodically to ensure preventing any forthcoming issues. Unlike steam boiler systems, thermic fluid heaters do not require blowdown practices, steam trap maintenance, and boiler water treatments.
No Operating Personnel Required:
Due to reliability concerns in steam boilers, manufacturing facilities assign operating personnel in the boiler room with training to control and monitor the boiler operation. Thermic fluid heaters function safely, without requiring any attention from operating team members.
Outdoor Installation:
Since thermic fluid heaters offer indirect heating, they can be easily installed outdoors. Installing heaters away from the actual production area enhances the safety of the process plant.
Central Plant Heating:
Earlier, large and multi-purpose facilities required both the steam boiler and thermic fluid heater for process operations. With developments, today industries utilize heat exchangers along with the thermic fluid heater. Heat exchangers combined with hot oil can produce sufficient steam that is used for sterilization, washdowns, and other operational processes.
Less Operating Costs:
The investment for a thermic fluid heater is less as compared to steam boilers. The advantages of the system lead to reduced operating costs as well.
Benefits of Thermic Fluid Heaters over Steam Boilers:
- No Corrosion or Water Carryover Issues
- Blowdown, Steam Traps, Condensate Return Systems, Feedwater Treatment Not Required
- Low Maintenance without Any Retrofitting Requirements
- High Operating Temperature with Minimal System Pressure
- Single Fluid Required for Heating and Cooling Purposes
Industries Using Thermic Fluid Heater:
- Rubber
- Paint
- Plywood
- Refineries and Petrochemicals
- Paper and Packaging
- Pharmaceuticals
- Textile
- Food Processing
- Chemicals
Thermic Fluid Heater by Rakhoh Boilers:
Rakhoh has been manufacturing Steam boilers, Boiler accessories, Waste Heat Recovery Boilers, and Thermic fluid heaters for more than 38 years. Our thermic fluid heater comes in a compact, horizontal design. The 4 pass design with a proven coil ensures instant heat and automation control maintains thermic fluid temperature. It includes ID and FD Fans, Internal Air Preheaters, an Automation system, etc.
It consists of various grates, an auto-feeding system with a screw feeder, and a Fluidized Bed for optimal combustion. Our Thermic Fluid Heater offers a multi-fuel firing option, optimal energy utilization, reliability, and up to 82% efficiency.
- Published in Boiler
10 Signs That Indicates Boiler Replacement
Industrial Steam Boilers are indispensable for process operations. Boilers generate steam by igniting fuels and heating water. Steam is excellent for processing, heating, drying, and sterilizing purposes in industries as it has exceptional energy holding capacity, is easy to use, and is cost-effective to generate. It can be concluded that steam boilers are unarguably vital equipment in the manufacturing and process industries.
Plant managers take various measures to ensure that the boiler systems operate efficiently by regular maintenance and other practices. Although periodic servicing and maintenance contribute significantly to optimal productivity, there is a certain point in the boiler lifecycle, when it needs replacement. Boiler problems are inevitable and can be prevented or resolved. However, some signs indicate that the boiler needs replacement. They are as follows:
- Low Energy Efficiency:
Steam boilers’ efficiency is rated by boiler manufacturers that are usually higher than its actual efficiency. Yet, by following standard guidelines and regular inspections, process plants can achieve optimal efficiency. With many years down the line, boiler efficiency gradually starts decreasing, which indicates boiler replacement.
- Boiler Takes Longer to Heat Than Usual:
A steam boiler that operates properly can heat within a few seconds. But, if it takes several minutes for heating even after inspection and repair, it becomes necessary to consider replacing the boiler.
- Leakages in Boiler:
Leaks in steam boilers often indicate that it has neared the end of its lifecycle. Boiler leakages are a serious issue that should be addressed immediately. If overlooked, it results in hazardous consequences. Additionally, leakages occur due to broken seals or valves and may be expensive to repair. Usually, plant managers prefer boiler replacement to repairs during severe leakages.
- Increased Fuel Demand:
Optimum combustion is a sign of a properly functioning steam boiler. When the boiler fails to burn fuels completely, it indicates the reducing combustion efficiency of the boiler. The fuel is required on a larger scale because it is partially wasted as unburned and results in increasing the fuel costs of the processing unit. In such cases, a boiler replacement is necessary as there are no reparations for fuel loss.
- Boiler Flame is Yellow Colored Instead of Blue:
The yellow color of the flame indicates the presence of carbon monoxide. It is odorless and therefore, difficult to detect. Installing a carbon monoxide detector helps in alerting the carbon monoxide. Boilers should be examined immediately by professionals as they can result in catastrophic accidents. Carbon monoxide emission is also indicated by black soot.
- Operating Temperature and Pressure:
One of the significant indications of steam boiler issues is its failure to maintain the required temperature and pressure during process operation. The parameter displays the boiler performance by measuring the amount of steam generated at present compared to when the boiler was installed. In such cases, it is advisable to consider boiler replacement for efficient production.
- Boiler Needs Frequent Repairing:
Although periodic repairing and servicing are inevitable for boilers, the constant need to repair some components or others may pose a serious issue. Rather than spending considerably on frequent reparations, it is prudent to invest in boiler replacement to prevent recurring boiler issues.
- Boiler Parts are Not Available:
If the boiler parts are challenging to procure or no longer manufactured, it is a sign that the boiler is outdated. In such a scenario, it is ideal to opt for boiler replacement instead of retrofitting.
- Safety Parameters:
Safety of the operating personnel is a matter of high priority that must be ensured with regular inspection. If the boiler system indicates safety issues during the inspection, it is essential to replace the boiler immediately. Some of the notable signs of safety problems are:
Irregular Starting or Stopping of Boilers: It indicates that the operating controls and safety valves are not operating properly. Such issues need prompt resolution with professional assistance.
High Stack Temperature: Stack temperature exceeding the usual level is a sign of poor heat transfer and unsafe conditions. The primary reasons for high stack temperature are low-quality boiler water, poor combustion, or low water level.
Poor Combustion Tuning: Improper fuel flow and airflow can result in significant pulsations and surges.
- Boiler Age:
Investing in boilers is a decision that impacts productivity for decades. On average, a boiler operates effectively for 20 to 25 years. It is advisable to retrofit the boiler when it crosses 10 to 15 years. However, after 25 years, it would be ideal to consider boiler replacement for enhanced productivity, efficiency, and advancement.
Boiler Replacement or Retrofitting:
Plant managers often consider the benefits between boiler replacement and retrofitting. Although the decision depends on various factors and the condition of the boiler system, boiler retrofitting can boost its efficiency by 10% to 12%. Boiler replacement offers a higher level of efficiency and safety, depending on the boiler. In either case, boiler replacement and retrofitting can considerably increase the efficiency and productivity of boilers.
Since our inception in 1983, Rakhoh Boilers have emerged as one of the leading boiler manufacturers and service providers globally. We have successfully installed more than 3000 boilers in 26 countries worldwide. We provide the best boiler retrofitting and upgrading services along with various other boiler-related services like boiler automation, energy audit, steam trap assessment, annual maintenance contract, etc.
Explore more about our products and services at www.rakhoh.com
- Published in Boiler