Rajan Wilcox
The equivalence ratio is an important parameter in the gasification process, directly influencing efficiency. It is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio, which is the ideal ratio of air to fuel that burns all fuel with no excess air. In the context of diesel fuels, the equivalence ratio specifically refers to the ratio of the fuel mass flow rate to the air mass flow rate in a diesel engine. This ratio is important for maintaining the stoichiometric combustion process and can impact the performance and emissions of the engine.
| Characteristics | Values |
|---|---|
| Definition | The equivalence ratio is the ratio of the actual air–fuel ratio to the stoichiometric air–fuel ratio. |
| Formula | The equivalence ratio Φ is defined as the ratio of the fuel mass flow rate to the air mass flow rate divided by the same ratio at the stoichiometry of the reaction considered. |
| Significance | The equivalence ratio is an important parameter in the gasification process since it directly influences efficiency. It also dictates the performance of a gasifier. |
| Stoichiometric Mixture | The stoichiometric mixture is when there is exactly enough air to completely burn all of the fuel. |
| Rich Mixture | Ratios lower than stoichiometric (where the fuel is in excess) are considered "rich". Rich mixtures may produce more power and burn cooler. |
| Lean Mixture | Ratios higher than stoichiometric (where the air is in excess) are considered "lean". Lean mixtures are more efficient but may cause higher temperatures, leading to the formation of nitrogen oxides. |
| Lambda Sensor Range | The typical lambda sensor range is between 0.2 to 1.8. |
| Upper Limit for NG-Diesel Dual Fuel Engine | The equivalence ratio is between 1.2 and 1.3 (rich). |
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What You'll Learn
Air–fuel ratio (AFR)
The air-fuel ratio (AFR) is a critical parameter in the combustion process, determining combustibility, energy release, and the formation of unwanted pollutants. It is the mass ratio of air to fuel, where fuel can be solid, liquid, or gaseous. AFR is particularly important in internal combustion engines and industrial furnaces, where it is a key consideration for anti-pollution measures and performance tuning.
The stoichiometric mixture, often abbreviated to "stoich", occurs when there is exactly enough air to completely burn all the fuel. Ratios lower than stoichiometric, where there is an excess of fuel, are considered "rich". Rich mixtures are less efficient but may produce more power and burn at lower temperatures. Conversely, ratios higher than stoichiometric, where there is an excess of air, are considered "lean". Lean mixtures are more efficient but can lead to higher temperatures, potentially resulting in the formation of nitrogen oxides.
The AFR can be monitored using an air-fuel ratio meter, also known as an air-fuel ratio gauge, air-fuel meter, or air-fuel gauge. This device reads the voltage output of an oxygen sensor, sometimes called an AFR sensor or lambda sensor. The lambda sensor range is typically between 0.2 and 1.8, with higher values indicating a very lean equivalence ratio.
In certain situations, such as in a gasifier, the term equivalence ratio is used to refer to the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio. This ratio is an important design parameter, influencing the performance of the gasifier. For example, in the gasification of biomass, the equivalence ratio typically falls between 0.2 and 0.3, with a lower value potentially resulting in incomplete conversion of char into gases.
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Stoichiometric combustion
The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. This is typically expressed as a ratio of around 14.7:1, meaning for every gram of fuel, 14.7 grams of air are required. However, this ratio can vary depending on the specific fuel blend and operating conditions. For example, in diesel engines, the stoichiometric air/fuel ratio is typically around 14.5:1 or 14.6:1, but can range from 18 to 70 depending on the operation point.
The stoichiometric air-fuel ratio is important because it determines whether a mixture is combustible, how much energy is released, and how much unwanted pollutants are produced. Ratios lower than stoichiometric, where there is an excess of fuel, are considered "rich". Rich mixtures are less efficient but may produce more power and burn cooler. On the other hand, ratios higher than stoichiometric, where there is an excess of air, are considered "lean". Lean mixtures are more efficient but can result in higher temperatures and the formation of nitrogen oxides.
To achieve stoichiometric combustion, the amount of air supplied must be carefully controlled. If insufficient air is provided, unburned fuel, soot, smoke, and carbon monoxide can be released, leading to pollution, lower combustion efficiency, and potential safety hazards. On the other hand, if too much air is supplied, it can result in incomplete combustion and the formation of excess nitrogen oxides. Therefore, precise control of the air-fuel ratio is critical for optimising engine performance and minimising emissions.
The equivalence ratio is a related concept that is used to characterise combustion processes. It is defined as the ratio of the actual fuel-oxidant ratio to the ratio of fuel-oxidant for a stoichiometric process. In other words, it compares the actual fuel-oxidant ratio to the ideal stoichiometric ratio. The equivalence ratio is an important parameter in gasifier design and the gasification process, influencing efficiency and reaction temperatures.
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Rich and lean mixtures
The equivalence ratio is an important parameter in the gasification process, directly influencing efficiency. It is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio. The stoichiometric mixture is when there is exactly enough air to completely burn all the fuel. This is often abbreviated to 'stoich'.
The air-fuel ratio (AFR) is the mass ratio of air to fuel in the combustion process. The AFR determines whether a mixture is combustible, how much energy is released, and how much pollutant is produced. Ratios lower than stoichiometric, where there is an excess of fuel, are considered 'rich'. Rich mixtures are less efficient but may produce more power and burn cooler. Ratios higher than stoichiometric, where there is an excess of air, are considered 'lean'. Lean mixtures are more efficient but may lead to higher temperatures and the formation of nitrogen oxides.
For precise AFR calculations, the oxygen content of combustion air should be specified because of different air density due to different altitudes or intake air temperatures. The AFR can be monitored with an air-fuel ratio meter, or air-fuel gauge, which reads the voltage output of an oxygen sensor.
In a typical air-to-natural gas combustion burner, a double-cross limit strategy is employed to ensure ratio control. This method was used in World War II. The strategy involves adding the opposite flow feedback into the limiting control of the gas. This assures ratio control within an acceptable margin.
In a combustor, the amount of air supplied is determined by the stoichiometric amount of air and its excess air coefficient. In a gasifier, the air supply is only a fraction of the stoichiometric amount. The equivalence ratio dictates the performance of the gasifier.
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Lambda sensor range
Lambda sensors, also known as oxygen sensors, are used to monitor the air-fuel ratio of an internal combustion engine. The air-fuel ratio is the ratio between the mass of air and the mass of fuel in the air-fuel mix at any given moment. This ratio is important because it determines whether a mixture is combustible, how much energy is released during combustion, and how much unwanted pollutants are produced.
The lambda sensor works by reading the voltage output of the mixture. The output voltage signal range of a properly functioning oxygen sensor is up to ~950 mV. When oxygen content in the exhaust is low and the engine runs with an enriched mixture, the sensor will generate a high voltage signal of 0.65-1V. Conversely, when the oxygen content is high and the engine runs lean, the sensor will generate a lower voltage signal.
The lambda sensor's operating range is typically between 0.9 and 1.1. When the engine is warming up or idling, it is important to maintain an equivalence ratio of 1 to ensure the catalytic converter can function properly and reduce the vehicle's emissions. At higher values of L, the mixture becomes leaner, leading to a drop in engine power. If L exceeds 1.3, the mixture becomes impossible to ignite and the engine misfires.
The lambda sensor's performance can be affected by its placement in the engine. In early injection systems, the sensor was placed as close to the engine cylinders as possible to heat up from the exhaust gases. However, this slowed down the regulation process due to the feedback mechanism. Modern sensors are often placed near the catalytic converter, which can impact the voltage amplitudes of the sensor readings.
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Gasification process
The equivalence ratio is a crucial parameter in the gasification process, influencing the efficiency of the process. Gasification is a technological process that can convert any carbonaceous (carbon-based) raw material, such as coal, biomass, or even plastic waste, into fuel gas, also known as synthesis gas or syngas. This process takes place in a gasifier, typically a high-temperature vessel where oxygen or air, and steam come into contact with the feed material, resulting in a series of chemical reactions that produce syngas and ash or slag.
Syngas, composed primarily of carbon monoxide (CO) and hydrogen (H2), is highly flammable and can be combusted to generate power. It can also be used as a hydrogen source in fuel cells or converted into synthetic fuel through processes like the Fischer-Tropsch synthesis. This synthesis technology, pioneered in the 1920s, involves synthesizing hydrocarbons from carbon monoxide and hydrogen, producing a petroleum substitute. The Fischer-Tropsch process can convert syngas into synthetic biofuels, with the potential to be fine-tuned for specific engine requirements, making it a "designer fuel."
The equivalence ratio is defined as the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio. Stoichiometry refers to the ideal ratio of air to fuel that ensures complete combustion without excess air. In a gasifier, the air supply is typically a fraction of the stoichiometric amount, resulting in air-deficient situations. The equivalence ratio dictates the performance of the gasifier, and maintaining the appropriate ratio is essential for efficient gasification.
The gasification process offers advantages such as the ability to utilize a wide range of feedstocks, including sustainable wood sources, and the potential for lower emissions of atmospheric pollutants. During World War II, for example, wood gas generators were used to power motor vehicles in Europe due to petroleum shortages. Additionally, hydrogen-enriched syngas produced through gasification can be used to make gasoline and diesel fuel, and carbon dioxide can be efficiently captured from syngas, preventing greenhouse gas emissions.
In summary, the gasification process involves converting carbonaceous materials into syngas through high-temperature reactions in a gasifier, and the equivalence ratio is a critical parameter that influences the efficiency of this process. The resulting syngas has various applications, including combustion, fuel cell use, and synthetic fuel production through processes like Fischer-Tropsch synthesis.
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Frequently asked questions
The equivalence ratio is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio. It is generally used for air-deficient situations, such as those found in a gasifier.
A stoichiometric mixture is when there is exactly enough air to completely burn all the fuel. This is often abbreviated to "stoich".
The equivalence ratio dictates the performance of the gasifier. For example, in pyrolysis, which takes place in the absence of air, the equivalence ratio is zero. If the equivalence ratio is too low, the char may not be fully converted into gases.
The equivalence ratio for a typical NG-diesel dual-fuel engine is 1.2-1.3 (rich).
