Project Details
Description
In Hong Kong, domestic sectors are consuming huge amount of energy annually. According to the 2017 report presented by the Census and Statistics Department, it is 18.95% of the total annual energy consumption of 340,000x1012 J. The main sources of energy consumption for domestic sectors are electricity (12.45%), and gaseous hydrocarbon fuels including essentially town gas (4.53%) and liquefied petroleum gas (1.97%). The commercial liquefied petroleum gas (LPG) using in Hong Kong consists of 30% propane (C3H8) and 70% butane (C4H10) and is commonly used in the domestic sectors as it is conveniently available, user-friendly, and safely and reliably operated. However, the air pollution problems caused by burning LPG especially the toxic-gas emissions of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOX), and the global-warming-gas emission of carbon dioxide (CO2) are of great concern due to their very significant and negative impacts on human health and the environment. This air-pollutant-emissions problem is much magnified in Hong Kong because of the extremely dense population. In addition, due to the rapid depletion of global hydrocarbon resources there is also an urgent need to seize for alternative green energy sources.
Hydrogen gas (H2) is identified to be a zero-emission fuel as it reacts with oxygen (O2) to form mainly water vapor (H2O) and release large amount of energy. H2 is the lightest fuel of 2 kg/kmol with an excellent calorific value of 150 MJ/kg and is currently using as the fuel for spacecraft propulsion. However, there is a very serious concern in applying H2 as domestic fuel related to its handling and operation because of its very high flammability and explosion natures. Hydrogen gas is highly flammable when burning with oxygen gas even though in small amount. In addition, the very high risks of easy leaking, low-energy ignition, extremely low-temperature vaporization (–252.87°C), explosion and embrittling the metal container should be fully overcome before H2 can be distributed, stored and consumed as a safe and reliable domestic fuel.
We take the challenge to obtain the advantages and overcome the drawbacks in using both LPG and H2 fuels by blending them together in appropriate percentages to modify and improve their properties to achieve the following goals:
• Produce lesser global-warming gas, CO2, because the H2 content of the blended H2/LPG fuel is carbon-free.
• Extra-lean-burning of LPG can be achieved because its narrow flammability limits (including both higher and lower limits) can be extended by mixing with H2. Extra-lean-burning will usually lead to lesser CO, HC and NOX emissions.
• The very high flammable and explosion risks in applying H2 can be overcome as these negative properties are modified by blending with the more stable LPG.
• High leakage risk caused by the strong buoyancy of H2 can be reduced by mixing with the heavier LPG. Hence the potential fire and explosion risks caused by the hydrogen leakage is expected to be eliminated.
• High cost of H2 can be reduced by mixing with the cheaper LPG, therefore the blended H2/LPG fuel is economically sustainable for the commercial and domestic sectors.
Hydrogen gas (H2) is identified to be a zero-emission fuel as it reacts with oxygen (O2) to form mainly water vapor (H2O) and release large amount of energy. H2 is the lightest fuel of 2 kg/kmol with an excellent calorific value of 150 MJ/kg and is currently using as the fuel for spacecraft propulsion. However, there is a very serious concern in applying H2 as domestic fuel related to its handling and operation because of its very high flammability and explosion natures. Hydrogen gas is highly flammable when burning with oxygen gas even though in small amount. In addition, the very high risks of easy leaking, low-energy ignition, extremely low-temperature vaporization (–252.87°C), explosion and embrittling the metal container should be fully overcome before H2 can be distributed, stored and consumed as a safe and reliable domestic fuel.
We take the challenge to obtain the advantages and overcome the drawbacks in using both LPG and H2 fuels by blending them together in appropriate percentages to modify and improve their properties to achieve the following goals:
• Produce lesser global-warming gas, CO2, because the H2 content of the blended H2/LPG fuel is carbon-free.
• Extra-lean-burning of LPG can be achieved because its narrow flammability limits (including both higher and lower limits) can be extended by mixing with H2. Extra-lean-burning will usually lead to lesser CO, HC and NOX emissions.
• The very high flammable and explosion risks in applying H2 can be overcome as these negative properties are modified by blending with the more stable LPG.
• High leakage risk caused by the strong buoyancy of H2 can be reduced by mixing with the heavier LPG. Hence the potential fire and explosion risks caused by the hydrogen leakage is expected to be eliminated.
• High cost of H2 can be reduced by mixing with the cheaper LPG, therefore the blended H2/LPG fuel is economically sustainable for the commercial and domestic sectors.
Key findings
This research study involves the use of LPG blended with hydrogen which is a highly flammable, potentially explosive and light gas. Therefore, the experimental work was started by performing a parametric and safety study. This study was performed using blended biogas/hydrogen fuel which is proven to be a safe blend of fuel. The findings from this study helped to establish the safe limits of hydrogen enrichment within the equivalence ratio range of 1.0 – 3.0 and Reynolds Number range of 300 – 1500 (covering entire laminar flow region). It is found that stable combustion can be ensured up to the hydrogen enrichment of 30% by volume without any unusual combustion behavior. Beyond this limit, it was observed some instability of flame, a significant formation of NOx and significant rise in flame temperature.
Numerical investigations with ANSYS CHEMKIN were performed to study the Laminar Burning Velocity (LBV) of various LPG-H2 blends for range of equivalence ratios. The results showed that the LBV increased with the increment of H2 fraction and it may adversely affect the flashback potential for fuel mixture with high percentage of H2. Through the experimental investigations conducted, the flammability limits (lean-burning limits) of LPG-H2 blends with H2 volume fraction of 0%, 5%, 10%, 15%, 20% and 25% were established for the Reynold Numbers ranging from 600 – 1800. Experimental results agreed with the numerical predictions. The results showed that the lean-burning limit is increased, on average, by 4.0% to 7.2% for every 5% increment of H2 volumetric fraction under different Reynolds numbers. It is due to the higher burning velocity of H2 rich mixtures. A lean-burning limit map was developed for H2 enrichment range from 0% - 25% and Reynolds Number range from 600 – 1800.
From the combustion and thermal characteristics investigations of premixed flames, it was found that overall flame shapes and the characteristics remain almost the same for pure LPG and LPG-H2 blends up to 25% H2 (highest H2 concentration tested). This observation supports the contention that there is a good blending of the LPG and H2 in the fuel jet, even though the physical properties of LPG and H2 are quite different. However, the measurement of flame cone height showed that the cone height reduces with the increment of H2 fraction. It is due to the high burning velocity of H2 rich mixtures. Thermal efficiency measurements showed that the efficiency reduces with the increment of the H2 fraction. It is due to the low volumetric heating value of H2 compared with LPG. Thermal efficiency reduced further with the increasing Reynolds number for all equivalence ratios tested.
The emissions investigations performed showed a general reduction of CO2 and CO emissions within the range of equivalence ratio and Reynolds Numbers as the hydrogen fraction increased. It is due to the reduction in the carbon content in the fuel blend and the enhancement of combustion efficiency by H2 with high LBV and adiabatic flame temperature. The H2 addition, however, negatively affected the formation of NOx. NOx formation gradually increased with increment of H2 fraction due to temperature effect. High flame temperature of H2 rich mixtures enhanced the NOx formation.
The interchangeability analysis revealed that the maximum potential H2 enrichment is limited to 15 vol% considering the heat input needs of domestic cooktop burners. The experimental investigations carried out according to GB 16410-2020, showed that a typical LPG cooktop stove (self-aspirating type) can be operated with LPG-H2 blends with up to 15 vol% H2 safely without any physical modifications. No significant changes to the emissions and the flame stability were observed.
Numerical investigations with ANSYS CHEMKIN were performed to study the Laminar Burning Velocity (LBV) of various LPG-H2 blends for range of equivalence ratios. The results showed that the LBV increased with the increment of H2 fraction and it may adversely affect the flashback potential for fuel mixture with high percentage of H2. Through the experimental investigations conducted, the flammability limits (lean-burning limits) of LPG-H2 blends with H2 volume fraction of 0%, 5%, 10%, 15%, 20% and 25% were established for the Reynold Numbers ranging from 600 – 1800. Experimental results agreed with the numerical predictions. The results showed that the lean-burning limit is increased, on average, by 4.0% to 7.2% for every 5% increment of H2 volumetric fraction under different Reynolds numbers. It is due to the higher burning velocity of H2 rich mixtures. A lean-burning limit map was developed for H2 enrichment range from 0% - 25% and Reynolds Number range from 600 – 1800.
From the combustion and thermal characteristics investigations of premixed flames, it was found that overall flame shapes and the characteristics remain almost the same for pure LPG and LPG-H2 blends up to 25% H2 (highest H2 concentration tested). This observation supports the contention that there is a good blending of the LPG and H2 in the fuel jet, even though the physical properties of LPG and H2 are quite different. However, the measurement of flame cone height showed that the cone height reduces with the increment of H2 fraction. It is due to the high burning velocity of H2 rich mixtures. Thermal efficiency measurements showed that the efficiency reduces with the increment of the H2 fraction. It is due to the low volumetric heating value of H2 compared with LPG. Thermal efficiency reduced further with the increasing Reynolds number for all equivalence ratios tested.
The emissions investigations performed showed a general reduction of CO2 and CO emissions within the range of equivalence ratio and Reynolds Numbers as the hydrogen fraction increased. It is due to the reduction in the carbon content in the fuel blend and the enhancement of combustion efficiency by H2 with high LBV and adiabatic flame temperature. The H2 addition, however, negatively affected the formation of NOx. NOx formation gradually increased with increment of H2 fraction due to temperature effect. High flame temperature of H2 rich mixtures enhanced the NOx formation.
The interchangeability analysis revealed that the maximum potential H2 enrichment is limited to 15 vol% considering the heat input needs of domestic cooktop burners. The experimental investigations carried out according to GB 16410-2020, showed that a typical LPG cooktop stove (self-aspirating type) can be operated with LPG-H2 blends with up to 15 vol% H2 safely without any physical modifications. No significant changes to the emissions and the flame stability were observed.
| Status | Finished |
|---|---|
| Effective start/end date | 1/01/20 → 30/06/22 |
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