Compressed natural gas vehicles: Why they are just as bad

Compressed natural gas (CNG) has been heralded as “the future of fuel”. Market analysts Technavio predict that the market share of compressed natural gas will increase by USD $7.66 billion from 2021 to 2026. They cite an “increase in the demand for cleaner fuel” as a reason for CNG market growth and describe it as “an eco-friendly technology”. These claims are misleading.

Unfortunately, compressed natural gas is not actually a green alternative fuel. CNG-powered vehicles are just as bad for the environment as their petrol or diesel counterparts.

There are other practical problems connected with compressed natural gas. Converting an existing vehicle carries risks, such as possibly damaging the vehicle or invalidating the warranty. There is also a scarcity of CNG fuelling stations, not just in South Africa but in the wider continent and even globally.  This makes them a poor choice for the typical motorist who wants to avoid any worries over fuelling up.

What is compressed natural gas?

Compressed natural gas, or CNG, is exactly what the name suggests. It consists of natural gas (which itself is mainly methane) that is compressed and stored under high pressure. In this compressed condition, it takes up much less space than natural gas in its ordinary state.

The most common method for compressing natural gas is the ‘diaphragm compressor’. These are complex machines, actually combining two systems: a gas compression system and a hydraulic system with a motor-driven crankshaft. The gas is pumped into a series of increasingly small chambers where its volume is steadily reduced.

They are called diaphragm compressors because the two systems are kept separate by metal diaphragms. This means that the hydraulic fluid does not come directly into contact with the gas. Gas enters the system at standard atmospheric pressure, but by the end of the process, it occupies just one per cent of its former volume.

The difference between CNG and liquefied natural gas

As the name suggests, liquefied natural gas (LNG) is in liquid form. LNG producers take natural gas and cool it in giant fridges to -161°C. This process changes it from its gaseous form to liquefied natural gas. This makes it much denser, taking up 1/600 of its original volume. The gas industry can then transport LNG in shipping containers rather than through pipelines.

Both liquefied natural gas and compressed natural gas are originally formed from natural gas. In its normal form, natural gas is mostly methane. There are also small amounts of ethane, butane, pentane and propane.

Unlike with compressed natural gas, there is no compression involved in creating liquefied natural gas – just extremely low temperatures. But, reaching these sub-zero temperatures is a very energy- and carbon-intensive process. Norwegian analysts Rystad Energy found that the greenhouse gas emissions connected with creating and transporting liquefied natural gas are ten times greater than for natural gas going through pipelines in its natural state.

Another difference between compressed natural gas and liquefied natural gas is the density. The density of LNG is roughly 0.41kg/litre to 0.5kg/litre. This is around half the density of water. Compressed natural gas has a density of around 0.7kg/litre. In both cases, the exact density varies slightly depending on factors like temperature, pressure and the exact composition of the original gas.

Common uses of CNG

It is possible to use compressed natural gas for power generation, water heating and air conditioning. But, it is more commonly used as a substitute for petrol or diesel to fuel vehicles.

There are currently around 23 million vehicles in the world that run on natural gas. This number includes both CNG- and LNG-powered vehicles. Roughly six to seven litres of compressed natural gas are enough to drive 100 kilometres. This compares favourably with petrol, which would need 10 litres to cover the same distance.

The popularity of natural gas as a vehicle fuel varies a great deal in different parts of the world. In 2018, Latin America had over five million natural gas vehicles, compared to less than two million in Europe. Compressed natural gas is also a very popular fuel in India. Over eight per cent of passenger vehicle sales there are CNG-powered.

In South Africa, compressed natural gas vehicles are much less popular. Most motorists are unfamiliar with the idea of running a vehicle on anything other than petrol or diesel. Logistics company BridCam Distributors was the first to trial a CNG-powered truck on South Africa’s roads in August 2022.

The problems of converting to CNG

Some cars are available to buy with a compressed natural gas fuel system. However, in South Africa, there are very few vehicles for sale that run on CNG or LNG. Most cars that run on CNG have been converted from petrol.

Technically, it is possible to convert any petrol car to CNG. Rather than modifying the way the existing engine burns fuel, you add the compressed natural gas system as an additional fuel source. This means the petrol engine stays in place and the car can switch between types of fuel. This is why CNG cars are sometimes called ‘bi-fuel’ or ‘dual fuel’.

You need to add a cylindrical CNG tank to store the gas at a pressure of around 200 bar, and a refilling port so that you can refuel the vehicle from outside. Conversion to CNG also requires many other components, including a pressure regulator and a filter to protect the engine from any impurities in the fuel. These components are often available as part of a ready-made ‘CNG conversion kit’.

Advertising for these kits makes CNG conversion look easy. But, there can be problems. If you are retrofitting a kit, you need to ensure that it is compatible with the model of the car. So-called ‘universal kits’ are a possible solution, but they are less safe than a kit specifically designed for your vehicle. Installing the wrong kind of kit can also reduce the life of your engine.

You should also look carefully at the terms of your vehicle warranty. It may be invalidated when you convert the car. Older cars are generally not compatible with CNG kits at all.

CNG fuelling stations: few and far between

The fuel tank and fuel regulator for a car running on compressed natural gas tend to be bigger than a conventional fuel tank. This means sacrificing some boot space, but this and the fuel efficiency of CNG allow you to go further between refuelling. However, refuelling a car with compressed natural gas is not necessarily easy.

Globally, a lack of CNG fuelling stations is one of the reasons why these vehicles are not in more widespread use. The national picture is similar. As of July 2020, CNG fuelling stations in southern Africa were few and far between. CNG fuelling stations are concentrated in specific areas in Gauteng, Mpumalanga, Free State and KwaZulu-Natal. For this reason, compressed natural gas is an impractical option for the typical private motorist.

The use of compressed natural gas in South Africa has historically focused on fleet vehicles. South Africa’s first CNG fuelling station opened in Johannesburg in 2014 for the use of the city’s taxis. Taxi drivers who had converted their vehicles to CNG found the dual-fuel aspect essential for coping with the scarcity of CNG fuelling stations. One taxi driver told news reporters: “If it’s long distance, I make sure I use petrol before the gas. On my way back, I use the gas because I already know I’m going to come to Johannesburg.”

Johannesburg was also one of the first cities in sub-Saharan Africa to run a fleet of buses on CNG. The municipal bus operator has its own in-house CNG fuelling station at the Braamfontein bus depot. However, ordinary drivers in South Africa do not have reliable access to CNG fuelling stations.

Why compressed natural gas isn’t really green

Many sources claim that compressed natural gas is a greener alternative to petrol or diesel. It is true that compressed natural gas results in fewer tailpipe emissions than conventional engines. This is because natural gas is a relatively clean-burning fossil fuel. It produces less carbon dioxide and lower levels of other pollutants when it combusts. Unfortunately, this is not the full story.

Carbon dioxide emissions at the point of combustion only represent part of CNG’s climate impact. Natural gas is mostly methane, which is a much more harmful greenhouse gas than carbon dioxide. Using methane as a fuel means extracting it, processing it and transporting it. And these processes provide many opportunities for it to leak into the atmosphere. This is disastrous for the climate because methane has more than 80 times the global warming potential of carbon dioxide, at least for the first 20 years after it reaches the atmosphere.

There is international recognition of the danger methane poses to the climate. Over a hundred countries have signed the Global Methane Pledge. This means that they will take voluntary steps to reduce global methane levels to at least 30 per cent below 2020 levels before 2030. But, despite widespread acknowledgement of the problem, methane concentrations in the atmosphere are still increasing. This jeopardises the goal of the Paris Agreement to keep global warming below 1.5°C over pre-industrial levels.

Ultimately, to achieve net zero emissions the world will need to drastically reduce fossil fuel use. Depending on the mitigation technology available, it may be necessary to phase out fossil fuels entirely. Converting vehicles to CNG simply swaps one fossil fuel for another.

South Africa’s doubtful CNG strategy

Compressed natural gas may already be popular as a fuel in many countries, but this does not make it a good choice for South Africa. Unfortunately, South Africa’s current Green Transport Strategy (GTS) mentions natural gas as a “potential transition fuel”. The GTS describes it as an option for the minibus taxi industry and city bus systems. It says that the Department of Transport should push hard to communicate “the benefits and cost-effectiveness of CNG relative to fossil fuels”. This is misleading because CNG is itself a fossil fuel.

The GTS admits that for compressed natural gas to become a viable transport fuel in South Africa, “security of CNG supply is crucial”. Yet, South Africa currently has no CNG reserves available other than from Mozambique. It also has a limited gas distribution network. Pivoting to natural gas would make the country more reliant on an increasingly volatile import market. And investing in the infrastructure required could be a costly mistake. It could lock the country into gas dependence just as the rest of the world moves away from fossil fuels and towards renewables.

What are the vehicles of the future?

Unstable petrol and diesel prices have been causing stress for South African drivers in recent years. This uncertainty is likely to continue. In January 2023, data from the Central Energy Fund showed that motorists should prepare for yet another jump in prices. This rise was partly fuelled by the reopening of China’s economy after a long period of COVID-19 restrictions. Many other factors feed into changing pump prices, including global oil prices, supply chain logistics, fuel taxes and the strength of the rand.

Electric cars are a better option in South Africa. They may have a higher sale price, but when you consider the entire lifespan of the car, they work out cheaper. This advantage of electric vehicles is even more pronounced when you consider the trend for South African drivers to keep the same car for longer. Fuel costs for electric cars are much lower and you save money on repair and maintenance too.

Crucially, electric cars are also compatible with South Africa’s emissions reduction targets. As the country moves away from coal, the electricity available from the grid will gradually become greener.

South Africa already has the most advanced electric car market in Africa. The first ultra-fast 150kW chargers reached the country in August 2022. They are part of a growing network that will make electric vehicles the most convenient choice for most drivers.

However, it is worth remembering that less than a third of South African households own a vehicle at all. Car ownership correlates strongly with wealth. A genuinely just transition will also require investment in active travel infrastructure and public transport. That said, for the section of the population who drive private cars, electric is definitely best.