Building a bright future for biogas

Methane gas is produced when we send organic waste to landfill and is a strong greenhouse gas (28 times the greenhouse gas intensity compared with carbon dioxide). Biogas projects capture this methane gas and can also divert organic waste away from landfills to reduce methane emissions and produce clean energy.
While many countries have policies and renewable energy targets that support a strong biogas sector, this remains a fledgling industry in Australia. However, in response to growing pressures to decarbonise our energy systems, interest in biogas is growing, primarily because of its flexibility. Biogas can be combusted to provide heat, electricity or both, and can also be upgraded to produce higher value energy sources including renewable natural gas (RNG), which is essentially the same composition as natural gas, or renewable hydrogen.  

To explore this further, GHD recently undertook a technology review of RNG and hydrogen production from biogas, including comparison of the processes and effectiveness. These opportunities were compared against a base case of cogeneration using biogas (for electricity and heat generation).

Biogas is produced when bacteria digest organic matter in the absence of oxygen through the process of anaerobic digestion. Biogas typically contains around 60% methane (CH4), 40% carbon dioxide (CO2) and trace gases, including hydrogen sulphide and nitrous oxide. 

In Australia, biogas is typically used as a fuel to generate heat through biogas boilers to satisfy onsite heat demand, or for power generation via Combined Heat and Power (CHP) engines, also known as cogeneration. Power from CHP can be used behind the meter to offset onsite demands with any excess energy exported to the grid as a revenue stream. Based on the typical variance between power import costs (often retail) versus export tariffs (often wholesale), maximising the use of power behind the meter improves project viability.

Biogas can also be purified into RNG, which can substitute for fossil-derived natural gas.

The purification of biogas to RNG is a multi-step process, with the core gas purification process being carbon dioxide removal. There are a number of commonly used technologies available to do this, including absorption/scrubbing, pressure swing adsorption and membrane separation.

As a further step up the value-chain, RNG made from biogas can be used as a feedstock to produce renewable hydrogen. Hydrogen can be produced from natural gas through the process of Steam Methane Reforming (SMR) which typically relies on fossil-derived natural gas as a feedstock (this is how most hydrogen is currently produced around the world). Using RNG in this process instead of natural gas enables the generation of renewable (green) hydrogen via SMR. Other emerging processes include the Hazer process, which converts biogas into hydrogen and graphite products using an iron ore catalyst. 

The process for converting natural gas into hydrogen (using SMR) includes:

1. Reforming: where steam and natural gas are combined and react under high pressure and temperature to form a syngas composed of carbon monoxide and hydrogen.
2. Water gas shift: to improve the hydrogen yield, carbon monoxide and steam are reacted to form hydrogen and carbon dioxide.

From our analysis it’s clear that further stimulation of the biogas sector is necessary if Australia hopes to challenge the scale of development currently underway in other countries and enable biogas to contribute in a meaningful way to the decarbonisation of our gas network.

In parts of Europe and North America, renewable energy targets, policies and subsidies have led to established RNG markets supported by mature technologies. Experience in Denmark, Germany, and Italy shows that favourable RNG policies can drive significant growth in biogas and RNG projects.

In Australia, the market for RNG is still emerging and supporting policies do not currently exist, which detracts from the commercial viability of these projects

To further consider the feasibility of biogas utilisation, we developed a case study for a conceptual organic waste-to-energy facility which incorporates anaerobic digestion of mixed organic feedstocks comprising food waste, biosolids, brewery residues and meat processing waste. Our concept assumed that digestate from the process (the residual solid material left over from anaerobic digestion) would be combined with garden organics and composted. Three variations to the concept were developed based on the three biogas utilisation options presented above (cogeneration using biogas as the base case, RNG production and renewable hydrogen production through SMR).

The case study demonstrated that using biogas for cogeneration is already feasible, particularly where there is a behind the meter demand for power and/or heat. Using biogas to generate RNG could present exciting short-to-medium term opportunities to decarbonise Australia’s gas grid, and could be expected to be competitive with cogeneration with the right policy drivers and as Australia’s experience with biogas purification improves. In the longer term and as the emerging market for hydrogen continues to grow, biogas could play a role in the production of renewable hydrogen, which could decarbonize a number of energy sectors.