Burning hydrogen for heating
In the quest to find a mechanism for providing low carbon heating to combat climate change it has been suggested that one solution would be to burn hydrogen instead of burning natural gas. The advantages and disadvantages are discussed below:
Advantages of burning hydrogen for low carbon heating
When hydrogen is burnt with pure oxygen the end products are heat and water – the big advantage is that no greenhouse gases, like carbon dioxide, are produced. This sounds like the answer to low carbon heating.
However, a number of practical issues arise which prevent hydrogen being supplied now through the existing gas grid as the solution to low carbon heating:
Disadvantages of burning hydrogen for low carbon heating
Many of the problems with using hydrogen for combustion and heating arise from its very light weight: hydrogen is the lightest element in the periodic table with an atomic mass of 1. Hydrogen appears in nature as a gas molecule (H2) with very light weight and therefore floats up into the upper atmosphere. It is not found naturally in high concentration on Earth so it has to be produced and then compressed so that it can be transported to where it is needed.
Hydrogen gas can be produced in quantity using the Steam Methane Reformation process by combining natural gas (methane - CH4) and steam at around 1,000°C. However, this process, like burning methane, releases CO2: so to obtain hydrogen for carbon-free heating from Steam Methane Reformation merely pushes the release of CO2 back up the supply chain. There is also the cost and limited efficiency (around 70%) of the process to be paid for.
Those who support the production of hydrogen from the Steam Methane Reformation argue that most of the CO2 released in the process can be captured and stored away from the atmosphere: Carbon Capture and Storage (CCS). However, this process of CCS is currently in Research & Development because it is expensive and requires additional consumption of methane to compensate for the inefficiencies inherent in the process.
For these reasons it is thought that practical commercial production of hydrogen for heating via the gas grid is likely to be at least ten years away. It is also estimated that production of hydrogen in this way will cost up to 4 times as much as the production of natural gas.
There are also issues in relation to safety and to health.
Safety concerns on burning hydrogen to provide low carbon heating
Hydrogen fuel is a source of safety concern because of its low ignition energy and high combustion energy. There are also explosion risks as hydrogen gas is so light that it has to be stored under very high pressure for a useful amount of it to be stored. Because hydrogen molecules are so small they have a much greater chance of escaping from storage tanks or natural gas pipelines than molecules of methane.
As hydrogen burns with an invisible flame, there would be safety concerns of using pure hydrogen in the national gas grid for the purposes of cooking.
Health concerns on burning hydrogen to provide low carbon heating
There are already health issues arising from the high levels of NOx emissions in major cities like London in relation to asthma and respiratory diseases. While much of the NOx emissions come from the exhausts of petrol and diesel engines, it is estimated that up to 22% of NOx emissions in London come from gas boilers used for heating. This proportion is likely to increase as the UK moves away from using internal combustion engines for transport to using electric vehicles.
Burning methane in pure oxygen produces CO2 and H2O. However, methane is normally burnt in air (which is 78% nitrogen) and some of the ferociously active oxygen atoms combine with nitrogen in the air to form NOx, while most of the oxygen atoms combine with carbon atoms, which are more reactive than nitrogen, to form CO then CO2.
Burning hydrogen in pure oxygen just produces H2O. However, hydrogen would normally be burnt in air and some of the ferociously active oxygen atoms combine with nitrogen in the air to form NOx. There are no carbon atoms for the oxygen atoms to combine with, so a higher proportion combine with nitrogen from the air to form NOx. For this reason burning hydrogen in air produces up to six times as many NOx emissions as burning methane in air. There is therefore a seriously increased health risk of burning hydrogen for heating as compared to burning natural gas.
Hydrogen from electrolysis of water
An alternative means of producing hydrogen is from electrolysis of water. This has the major advantage of producing H2 without emitting CO2 (or any NOx). The disadvantage is the cost of electricity incurred in separating H2O into H2 and O2. As the cost of electricity varies over each 24 hour cycle it can be possible to employ electrolysis only in off-peak times of night when electricity is cheapest.
However, this is a very inefficient route to achieving heating when a well proven and highly efficient mechanism already exists for arranging heat transfer without using combustion.
Alternative means of providing heating without any emissions
There is a well proven way of providing heating which emits no CO2, no NOx and no other emissions either, because it does not involve combustion: heat pumps are already widely used in Sweden, Canada, France, Germany, Italy and many other countries.
Heat pumps provide an elegant solution of how to provide low-carbon heating to meet climate change objectives without side effects.
In order to avoid the need to upgrade radiators within existing buildings, high temperature heat pumps can be used in direct substitution for gas boilers.
Up to 22% of NOx emissions in London come from gas boilers used for heating
See: Air Quality in City of London: a guide for public health professionals prepared for the Mayor of London. This states that, "The direct sources (as opposed to external sources produced outside of London) of NOx in Greater London in 2008 are presented in image 2 below. This shows that road transport contributed 46% and gas boilers 22% of the total NOx emissions".
Burning hydrogen in air can produce up to six times as much NOx as burning methane in air
See: H2 Emission Potential Literature Review by E4tech for BEIS, April 2019 page 26, paragraph 4.4, Emissions from end-use of hydrogen.
23,500 people in UK suffer early deaths because of NO2 pollution each year
See: BBC report on claim that 40,000 UK lives a year are cut short by air pollution as reported by the Royal College of Physicians. The number of early deaths attributed to NO2 is 23,500 according to a 2015 report from Defra. The precise number may be disputed, but, for the sake of comparison, the number who die in the UK each year from road traffic accidents is around 1,800.
Over half a million premature deaths a year across Europe have been attributed to pollution from particulates and NOx emissions.
"Grey Hydrogen" is used to mean hydrogen derived by steam methane reformation – with CO2 emitted during in its generation: not low carbon hydrogen.
"Blue Hydrogen" is used to mean hydrogen derived by steam methane reformation – but with Carbon Capture and Storage to prevent the release of CO2 into the atmosphere.
"Green Hydrogen" is used to mean hydrogen derived by electrolysis – with no emissions of CO2 in its generation, using green electricity.