Combustion optimization in boilers is a central element in improving energy efficiency in systems that use biomass, contributing to a more sustainable and economical industrial operation. Sugarcane biomass, for example, stands out as a renewable and abundant energy source, but the challenge of maximizing its combustion efficiency persists. One of the main factors influencing this performance is the precise control of the air-fuel mixture.
The role of oxygen analyzers in optimizing combustion in sugarcane biomass boilers.
It is in this context that Fuji Electric's zirconium oxide oxygen analyzers (models ZKMA/B + ZFK8) play a fundamental role. These instruments offer a high-precision solution for monitoring and optimizing combustion, allowing for fine adjustments to the air-fuel ratio. The result is more efficient combustion, which not only maximizes energy generation but also significantly reduces fuel consumption and pollutant emissions.
The in-situ oxygen analyzer, based on the zirconium oxide principle, is specifically designed to measure O2 levels directly in furnaces, ovens, or boilers. This continuous, real-time measurement is crucial for automatically adjusting excess air in combustion, ensuring the process occurs under ideal conditions. By maintaining the proper balance between air and fuel, the analyzer contributes to a more complete and efficient combustion, resulting in substantial operational gains and significant cost reductions.
In summary, using an oxygen analyzer is not just a technical choice, but a smart strategy to boost energy efficiency, reduce expenses, and promote more sustainable operating practices in the industry.
Zirconium oxide (ZrO2)
The operation of the in situ oxygen analyzer is based on the property of zirconium oxide, also called zirconia, which conducts oxygen ions when heated.
This analyzer obtains the measurement of O₂ concentration by detecting the electromotive force generated by the difference in O₂ content between air and the sample gas.
Fuji Electric's oxygen analyzer utilizes zirconia oxide cell technology, which operates on the principle of ionic conduction. The cell generates a voltage proportional to the oxygen concentration in the combustion gases. This precise reading allows for automatic adjustment of the amount of air supplied to the boiler, ensuring that combustion occurs under ideal conditions.
The zirconia oxide oxygen analyzer is widely used to monitor oxygen concentration in combustion processes. The operating principle of this analyzer is based on the electrochemical properties of yttria-stabilized zirconia, which becomes conductive to oxygen ions at high temperatures.
Operation principle
Zirconia oxide sensor
- The main sensor consists of a ceramic tube made of yttria-stabilized zirconia oxide. This tube acts as a solid electrolyte.
- At each end of the tube, there are porous platinum electrodes that allow the passage of oxygen ions.
Oxygen concentration difference
- One side of the zirconia tube is exposed to the combustion gas (with a variable oxygen concentration), while the other side is exposed to a reference gas (usually ambient air, with a known and constant oxygen concentration).
- Due to the difference in oxygen concentration between the two sides, a potential difference (electrical voltage) is generated between the electrodes.
Generating the signal
- The potential difference generated between the two electrodes is proportional to the logarithm of the ratio of the oxygen concentrations on the two sides.
- This electrical signal is then amplified and converted into a direct reading of the oxygen concentration in the combustion gas.
Operating Temperature
- The zirconia oxide sensor needs to be heated to a high temperature, usually around 600 to 800 °C, so that the ceramic material becomes conductive to oxygen ions. This heating is maintained by a heating element integrated into the sensor.
Benefits:
- Quick Response: Real-time measurement allows for rapid adjustments to the combustion process, increasing efficiency.
- Precision: The high sensitivity of the zirconia oxide sensor ensures accurate measurements, even in harsh environments.
- Durability: Designed to withstand high temperatures and harsh conditions, the sensor offers a long service life.
Impact on combustion efficiency
Combustion efficiency is directly related to the amount of oxygen present in the exhaust gases. Substoichiometric combustion (low oxygen) can result in the formation of carbon monoxide (CO) and soot, while superstoichiometric combustion (excess oxygen) leads to wasted energy due to the burning of excess oxygen, reducing thermal efficiency.
With the implementation of the ALUTAL/FUJI Electric oxygen analyzer, it is possible to adjust the combustion air rate in real time, maintaining the oxygen level within an ideal range (generally between 2% and 4% for biomass). This control results in more complete combustion, increasing the boiler's thermal efficiency and reducing pollutant emissions.
Basis for calculating combustion efficiency
To calculate combustion efficiency and optimize air-fuel control in a boiler, it is essential to understand some key concepts and use specific formulas. Combustion efficiency is generally expressed as the proportion of energy released by burning fuel that is converted into useful heat.
Basic concepts
- Fuel: Biomass, such as sugarcane bagasse.
- Lower heating value (PCI): The amount of heat released by combustion.
complete of a fuel unit, without considering condensation of
Water vapor in the combustion products. - Excess air: The amount of air supplied is above the amount theoretically
necessary for complete combustion.
Important parameters:
- Excess air fraction (E): The ratio of excess air supplied in relation to
stoichiometric air (minimum amount of air required for combustion)
). - Oxygen (O2) measurement: The concentration of oxygen in the combustion gases,
measured by the oxygen analyzer. - Exhaust gas temperature (Tgases): The temperature of the gases after
combustion, which affects efficiency.
Formula for excess air
Where O2 is the oxygen concentration measured in the exhaust gases (in %).
Thermal efficiency of combustion (ηc)
Losses may include:
- Heat loss through exhaust gases.
- Heat loss due to moisture in the fuel.
- Heat loss due to excessive ventilation.
Heat loss in exhaust gases
Where:
- Cp is the specific heat of the combustion gases.
- Tamb is the ambient temperature.
Combustion efficiency (ηcomb):
Total losses include losses due to exhaust gases, excessive ventilation, and other factors.
Overall efficiency (ηglobal):
Where:
- ηc is the thermal efficiency.
- ηcomb is the combustion efficiency.
Practical application with the oxygen analyzer:
- Using the oxygen analyzer, monitor O2 continuously.
- Adjust the airflow to minimize E, without compromising safety.
- Monitor exhaust gas temperatures to assess losses.
- Use the O2 and Tgases values to calculate and optimize efficiency.
Calculation example
To calculate the combustion efficiency of a boiler, let's use a practical example. Suppose we have a boiler that uses biomass (sugarcane bagasse) as fuel, and we need to calculate the combustion efficiency based on the measured data.
Example data
- Lower Heating Value (LHV) of the fuel: 15 MJ/kg
- Exhaust gas temperature (Tgases): 250°C
- Ambient temperature (Tamb): 25°C
- Oxygen level (O2): 5%
- Specific heat of combustion gases (Cp): 1.005 kJ/kg°C
- Excess air: Calculated based on the measured oxygen level.
Steps for the calculation
Calculation of excess air (E)
Replacing O2 = 5%
E = 0,3125 (or 31.25%)
Heat loss in exhaust gases (Qgases)
The heat loss in the exhaust gases can be calculated using the formula:
Replacing the values:
Qgases = 226.125 kJ/kg
Combustion efficiency (ηcomb)
Combustion efficiency can be calculated by considering heat losses:
Replacing the values:
ηcomb = 98,49%
Result:
The combustion efficiency of the boiler, in this example, is approximately 98.49%. This means that 98.49% of the energy available in the fuel is converted into useful heat, while the remainder is lost, mainly through exhaust gases.
Practical calculations of earnings and savings:
To quantify the gains and savings provided by using an oxygen analyzer, consider the following parameters in a sugarcane biomass-fired boiler:
- Boiler Thermal Capacity: 100 MW (Megawatts)
- Current Combustion Efficiency (without the analyzer): 82%
- Efficiency with the Analyzer: 85%
- Average Cost of Sugarcane Biomass: $30/ton
- Annual Biomass Consumption (without the analyzer): 500.000 tonnes
Biomass economy:
First, we calculated biomass consumption with and without improved efficiency:
- Current Combustion Efficiency (without analyzer): 82%
- Biomass consumption with current efficiency:
- 500.000 × 0,82 = 410.000 Ton.
- Efficiency with the analyzer: 85%
- Reduction in biomass consumption (with improved efficiency):
- 500.000 × 0,85 = 425.000 Ton.
- Biomass consumption with current efficiency:
Therefore, the annual biomass savings would be:
- Annual Savings: 75.000 × $30/ton = $2.250.000,00 per year
- Biomass savings: 500.000−425.000 = 75.000 Ton.
Energy gain
In addition to direct savings in biomass consumption, energy gains should also be considered. With a 3% improvement in efficiency, the boiler generates more
Useful energy can be generated from the same amount of fuel, which can translate into higher production or a lower need for auxiliary fuels.
Conclusion
Adopting Fuji Electric's oxygen analyzer to optimize combustion in boilers using sugarcane biomass can provide significant gains in combustion efficiency, resulting in considerable fuel savings. Based on the calculations presented, annual savings can exceed several thousand reais per year, in addition to contributing to the reduction of pollutant emissions and the sustainability of operations. Therefore, implementing this technology represents a strategic investment for companies seeking to optimize their energy processes and reduce operating costs.
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