Sounds like a weird question? We all know natural gas is a fossil fuel, right? Not so fast.
Natural gas is mainly methane and is used for making energy in power stations, or for heating and cooking in our households. We also use it to make ammonia based fertiliser. Without natural gas many of us would be freezing in winter and eating cold food only. It also produces half as much emissions than its cousin, the much maligned coal.
Natural gas is a fossil fuel because it is the left over from organic matter from millions of years ago.
Therefore, releasing its carbon dioxide emissions into the atmosphere can upset the subtle balance of our atmosphere. Releasing dioxide emissions or the equivalents from renewable fuels is not counted in the greenhouse gas balance, as we assume these gases can be absorbed and renewed, whereas to re-create natural gas made over millions of years would take… well… millions of years.
What are we growing and consuming on a regular basis? Food. Yes, food contains energy – how else do you think our bodies would keep going?
We throw out food on a regular basis, too. Too much of it to be sure.
But here is the good news: we can capture the energy contained in the food and other organic matter we throw out and make natural gas from it.
Soon, all councils in NSW will start collecting the food we throw out together with the green waste we already separately put into a green waste bin. This new collection will be called the Food Organics and Garden Organics (FOGO) collection.
Up to now, we have composted our green waste – and that’s a good thing, as our Australian soils need compost or organic matter. Composting green or garden waste is easy. Adding food waste to the mix makes it a bit trickier, because, if not handled well, food waste can turn quite smelly quite quickly.
When we anaerobically digest food and also the green leafy matter from garden organics, we can harvest biogas. Biogas is the excretion of anaerobe bacteria that digest the organic matter and consists mainly of methane and carbon dioxide, with fluctuations, of course, depending on what’s in the food.
You ask what is anaerobic digestion (AD)? AD is the controlled breakdown of organic matter in the absence of air. Anaerobe bacteria are doing all the work. The process is similar to what happens in our own bodies.
There are various technological approaches to AD, depending on the feedstock you have. The main point is that biogas is a very useful by-product, which can be utilised in various ways.
Biogas can be used as a fuel in a reciprocating engine to generate electricity, as is already happening at many landfill sites with landfill gas. The heat this engine generates can be used to ensure the AD process is always at its best process temperature so the bugs feel really comfy having their dinner.
Carbon dioxide from the engine’s exhaust can also be used to go into a greenhouse to feed the plants, which are happy to receive four times atmospheric carbon dioxide twice a day.
Finally, biogas can be upgraded to biomethane. Biomethane is natural gas, actually it is better than natural gas. Why? Because the methane content in natural gas can fluctuate (between 70 – 90 per cent), but when upgrading biogas to biomethane, we strip the carbon dioxide out of the biogas and get a pretty reliable high methane content of around 96 – 97 per cent.
We can then compress the biomethane and have a renewable compressed natural gas (CNG), which we can run our vehicles on. That way we replace the fossil fuel we currently use.
Once the digestion process is finished, the residue, called digestate, can be composted and used as compost. The AD process causes a mass loss of between 15 – 20 per cent, which is not a bad thing, as we now have energy and less compost to sell.
If you ask most composting companies, their concern with the new FOGO collection is what to do with the additional amount of compost being produced. Whilst Australia has plenty of land that could use the additional compost, the urban amenity market which currently absorbs most compost, is pretty saturated. Other markets need to be established, which costs time and money.
The best thing about digesting our FOGO is the additional value it creates.
Let’s assume you have 100,000 tonnes of FOGO a year. If you compost this amount, you get a mass loss of around 40 per cent, so 60,000 tonnes of compost to be sold at the end. Say a cubic metre of good quality compost sells for $35, then a tonne of compost is worth $70. That puts the value of the compost produced at $4.2 million. Not bad.
Let’s assume the energy value of biogas is 5.6 kilowatt hours for every normal cubic metre, and the typical yield of biogas in food organics (FO) is 150 normal cubic metres a tonne, and for garden organics (GO) it’s 90 normal cubic metres a tonne.
Let’s further assume the FOGO mix is 30/70 – that is, 30 per cent FO and 70 per cent GO. I know it will be different in every situation, but these numbers are in the range and we have to have some basis for the calculations. What makes the calculation more complex is the level of contamination (plastics don’t digest) and the fact that not all FO end up in the AD process as some stick to the coarser GO, and so on.
We also have to take into account the so-called parasitic load of running the AD process and the biomethane upgrade.
You won’t put all the 100,000 tonnes of FOGO through the AD process, as the bugs don’t like wood, for example. We shred and screen the fine from the coarse and only digest the fines, say 50 per cent of the feedstock. The coarse is mixed with the digestate and composted. So, when 50,000 tonnes are digested, they lose around 15 to 20 per cent mass, let’s take 17.5 per cent here. That means 91,250 tonnes is composted, minus the mass loss of 40 per cent (same as above), which means we end up with 54,700 tonnes of compost. At $70 per tonne value that is $3.8 million.
But we also now have the value of the biogas or biomethane. Taking the above numbers, we end up having around 27 million kilowatt hours in energy. If we value that at say 20 cents per kilowatt hour, assuming that’s how much we would pay for the energy, if bought from an energy retailer, then that represent a value of $5.4 million we get on top of the value for the compost, so altogether $3.8 plus $5.4 is $9.2 million. Not bad either, right?
Well, think about it this way: so far we have been throwing this value into landfills.
Okay, I know some landfill gas people will say, but hold on, we are recovering the gas in the landfill, what about that? My point is: if your gas recovery system is good, it may get 60 per cent of the biomethane potential over 40 years. With AD we get 99 per cent of it over four weeks, if you go high-tech, or four months, if you go low-tech. Ever heard of net present value.
Clearly, we have been throwing money away.
Now is a really good time to stop this waste and embrace the coming FOGO opportunity and get into anaerobic digestion. Big time. We can do the same with residual household waste! Let me know if you want to know more.
Let’s not forget the other value we are getting, but always forget to value, is less fossil fuel consumption, less air pollution and a better environment.
By the way, we still need landfill gas capture. Even so-called non-putrescible landfills can generate gas. Just ask Bingo at their Eastern Creek landfill. You just never know what ends up in a landfill.
I would be remiss to not mention a little hiccup. The State EPA’s haven’t yet clarified what the regulations are for digestate. Yes, we can compost some digestate, but what about the liquid phase? Not having regulations is a major barrier to investment.
Please – let’s not reinvent the wheel again. AD in Europe seems to be functioning well.