Energy efficiency, environment and emerging low-carbon gases
What this domain tests
This domain links gas utilisation to carbon and energy performance: combustion efficiency, condensing technology, demand reduction, emissions and the introduction of hydrogen, biomethane and LNG. Expect calculations and an understanding of safety and interchangeability limits.
Efficiency and emissions
- Gross efficiency = useful heat output / gross fuel input. Condensing appliances recover latent heat from water vapour and can exceed 90% gross efficiency; non-condensing appliances typically sit around 78-85%.
- Stack loss rises with flue temperature and excess air. Reducing excess air and stack temperature improves efficiency, but only within flame-stability and NOx limits.
- Hong Kong's climate action plan targets carbon neutrality by 2050. Gas engineers contribute through efficient appliances, better controls, heat recovery and switching to lower-carbon gases.
- Whole-life carbon includes embodied carbon in pipes, vessels and equipment, not just operational emissions.
Emerging low-carbon gases
Town gas in Hong Kong already contains a large share of hydrogen, so the network has relevant experience. Hydrogen has a wide flammability range (about 4-75% in air), low ignition energy, a nearly invisible flame and a high diffusivity. It can embrittle some steels and needs different leak detection. Blending hydrogen changes Wobbe index and appliance performance, so injectors and controls must be checked. Biomethane is chemically similar to natural gas and can be injected after upgrading. LNG provides a lower-carbon alternative to oil but is still fossil methane.
Worked example
A 100 kW boiler runs for 2,000 hours per year at 80% gross efficiency. Annual fuel input = 100 x 2,000 / 0.80 = 250,000 kWh. Replacing it with a 92% gross condensing boiler gives input = 200,000 / 0.92 = 217,391 kWh, a saving of 32,609 kWh, or 13% of fuel. Using 0.20 kg CO2/kWh for natural gas, the saving is about 6.5 tonnes CO2 per year. Check that the lower return-water temperature needed for condensing can actually be achieved, otherwise the 92% figure is optimistic.
Traps that cost marks
- Quoting condensing efficiency without verifying low return-water temperature.
- Mixing gross and net efficiency or calorific values.
- Assuming hydrogen can be blended without checking Wobbe index, materials and detection.
- Ignoring NOx and indoor air quality when tuning for efficiency.
- Claiming carbon savings without a baseline and an agreed emission factor.