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Thursday February 3 2022

Biogas vs Natural Gas – how do they compare?

For many years, natural gas has been used as a fuel to heat and power homes and businesses across the world. However, as we get a better understanding of the impact fossil fuels have on the planet, more sustainable alternatives are being produced.

Biogas production is, by contrast, a carbon neutral process and can even be carbon negative with certain feedstocks. With its waste-to-energy credentials, it offers a greener source of gas for heat, power, and fuel for heavy goods vehicles compared with natural gas. And with the government’s Green Gas Support Scheme (GGSS) now underway, production will increase, meaning more UK homes, businesses, and heavy goods vehicles will soon be powered by biogas.

But what is the difference between biogas and natural gas and how does it compare as a fuel source?

In this article, we explore the similarities and differences between the two, starting with composition.

Natural gas

Both natural gas and biogas are formed through the decomposition of organic matter. However, the location and speed at which this takes place are worlds apart.

Natural gas is generally the product of the decomposition of marine microorganisms deposited over the past 550m years, which became buried by mud, silt, and sand on the seabed to create an anaerobic environment.

Over time, and with the addition of heat and pressure, the organic matter decayed into the gaseous hydrocarbons which constitute natural gas.

Natural gas is made up primarily of methane, but it also contains other hydrocarbons such as ethane, butane, pentane, and propane.

Biogas

The primary component of biogas is also methane, although biogas also contains a high level of carbon dioxide, sometimes as much as 50 per cent.

In its unprocessed form, biogas also contains a range of impurities such as nitrogen, hydrogen, hydrogen sulphide, and water vapour. It may also contain Siloxanes, volatile organic compounds (VOC) in different quantities, depending on the feedstock used.

This crude form of gas can be used to heat and power local facilities such as external farm buildings, or a factory, but it is not of high enough quality to export to the national gas grid.

To do this, the biogas must be upgraded to create biomethane. Biomethane is biogas with the carbon dioxide and impurities removed to create a near-pure form of methane.

Biomethane is essentially the same as natural gas in composition. So much so, it is often given the name renewable natural gas (RNG).

Production

As already touched upon, natural gas is formed under the seabed by the breakdown of organic matter in anaerobic conditions.

Extreme heat from the earth’s core and huge amounts of pressure from rocks above are also integral to the formation of natural gas.

The same process also creates other fossil fuels such as coal and oil.

Creating natural gas takes millions of years and as a result, it is not a sustainable form of energy. The world’s reserves of natural gas will likely run out within the next century.

Couple that with the energy intensive extraction process, and it is easy to see why natural gas is not the most environmentally friendly of fuels.

Biogas, on the other hand, is manmade in a carbon neutral or even carbon negative process, which takes a fraction of the time.

Biogas is created in a process called anaerobic digestion. This essentially reproduces what happens beneath the seabed for natural gas – the decay of organic matter in the absence of oxygen, using heat and microbes – but in a controlled environment. It takes place in the reactor of a biogas plant, also known as an AD plant.

Generating biogas is a quick process – unlike the formation of natural gas – and it utilises organic waste products such as food waste, wastewater biosolids, natural manures, and slurries, as well as purpose-grown energy crops such as maize and sugar beet, making it fully sustainable.

Relative cost

Biogas

There is a large number of costs associated with producing biogas. First of all, there is capital expenditure (capex) needed to build the plant.

Depending on size, this can be anywhere from £1m to £15m.

Then there is the commissioning cost, which is the time the plant owner takes to learn how to run the plant at its most efficient. Here the plant won’t be running at full capacity, but all input costs such as feedstock and staff salaries are high relative to the return. This usually lasts around three to six months.

From here on, there is the operational expenditure – the ongoing costs for keeping the plant running.

Finally, for plants that produce biomethane for the national gas grid, there is the cost of upgrading the biogas and exporting it.

Biomethane currently sells for around one Euro per litre, and with the Green Gas Support Scheme (GGSS) incentivising the production of biomethane, a return can be made on the investment of building a new biomethane plant.

Natural gas

As a commodity, the price of natural gas is determined by supply and demand. This makes it more volatile than manmade fuels like biomethane.

The three main influencing factors on natural gas prices include:

  • The amount of natural gas production
  • The level of natural gas in storage
  • The volumes of natural gas imports and exports.

Other factors that have a bearing on price include:

  • Variations in weather
  • Levels of economic growth
  • The availability of alternative fuels
  • Political instability.

As a rule, if more gas is available, the price goes down. When demand goes up, such as when we move towards winter, so does the price.

Natural gas is measured in therms. One therm equals 100 cubic feet of gas. Therms tend to fluctuate from around 150p to 200p in the UK.

Environmental impacts

Biogas

The production of biogas is a carbon neutral process as it creates no net increase in carbon emissions into the environment. With certain feedstocks, the process can even be carbon negative.

This is because rather than creating new emissions, it releases the carbon that was originally sequestered from the atmosphere and stored in the organic feedstock and uses that as a source of fuel.

Although the process creates methane, this is captured and used to generate heat and power, or as a heavy goods vehicle fuel in the form of Bio-Compressed Natural Gas (bio-CNG) or Bio-Liquified Natural Gas (bio-lng).

If the organic matter that makes up the feedstocks for anaerobic digestion was left to decay naturally, that methane would be lost to the atmosphere.

Also, as the process is carried out by anaerobic bacteria, it doesn’t require a large energy input to drive it, unlike other forms of energy production.

As the composition of biomethane is almost identical to that of natural gas, it can be injected directly into the 7,600km UK National Transmission System (NTS) gas pipeline network without any modifications to the gas pipeline infrastructure or end-user appliances, such as gas cookers, boilers, and fires.

This ability to seamlessly integrate with natural gas to help fulfil energy demand peaks instantaneously at any time (winter or summer, day or night) is invaluable to helping overcome the challenges with other renewable energy sources such as wind and solar, which are constrained by climate patterns.

The ability to process organic waste into a valuable energy source also makes biomethane the obvious choice for heating and heavy vehicle transport fuel – the two most difficult sectors to decarbonise in the race to net-zero.

This means that it is not just the method of production that makes biomethane greener than natural gas. It further adds to climate mitigation by replacing fossil fuel usage and reducing the overall carbon footprint and emissions of energy provision in the UK.  

Natural gas

Unlike biogas, natural gas has a number of negative environmental impacts, which are based on the following three areas:

  • Extraction
  • Transportation
  • Burning and consumption.

Drilling and extraction

There are many environmental issues with drilling for and extracting natural gas. Firstly, it can have a detrimental effect on local habitat and wildlife populations, in some cases even destroying local ecosystems. It can pollute streams, rivers, and land, and cause erosion. Fracking can cause local earthquakes.

Transportation

Building the infrastructure needed to transport natural gas from gas wells to power stations is also disruptive and environmentally damaging. Laying the gas pipelines can lead to the loss of vital habitat, and gas leaks from pipes can pollute groundwater sources, causing a major environmental incident.

Burning and consumption

Of all the fossil fuels available, natural gas is the least environmentally damaging because it burns more cleanly than other types of fuel. However, that doesn’t make it environmentally friendly.

Combustion still emits methane, a powerful greenhouse gas, as well as lowers air quality, meaning it is still a source of damaging pollution.

Efficiency

Biogas

The efficiency of biogas production is dependent on a range of factors. However, these can be divided into two types – biological and mechanical.

Common biological impacts on efficiency include:

  • Changes to feed loads or intervals
  • Organic overload, where there is more organic matter in the reactor than the microbes can break down
  • Hydraulic overload, where the incoming water rate exceeds the system’s flow rate
  • Ammonia inhibition, where nitrogen-rich feedstocks create a high concentration of ammonia, reducing microbe activity
  • Hydrogen sulphide inhibition, where a build-up of hydrogen sulphide reduces microbe activity
  • Reduced temperature
  • Lack of micronutrients such as selenium, copper, and cobalt.

Common mechanical impacts on efficiency include:

  • Reduced activity volume due to a build-up of material in the reactor
  • Loss or reduction of mixing due to a damaged or faulty mixer.

To guard against these impacts, regular biological monitoring of an AD plant must take place. By monitoring factors such as temperature, pH, total solids, fatty acids, ammonia, and others, the plant can be kept at its most efficient, producing better quality biogas and better financial returns for the owner.

Natural Gas

Once the infrastructure required to extract natural gas and transport it to a power station is in place, it is an efficient source of fuel. It is the cleanest burning hydrocarbon, producing around half the carbon dioxide and one tenth of the air pollutants compared to coal when burned to produce electricity.

It is also abundant, with around 50 years’ worth of known natural gas still available, and potentially other fields yet to find.

As it is considered a more environmentally friendly fuel than other fossil fuels, many believe it to be a useful transition fuel as we change to more sustainable sources.

However, natural gas is still a pollutant releasing greenhouses gases when burnt, so it is likely to be phased out altogether over the coming decades.

For advice on how to produce biogas and biomethane more efficiently, get in touch.