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Energy In Biogas

Energy in Biogas

How much energy is in biogas?

If you are planning on using your biogas to create heat, power and/or cooling, then you really will need to know how much energy is in the biogas. This is a key question to ask.

The energy in biogas is impacted but the waste feed materials in the anaerobic digest and the type of digester you are operating. So, here is some typical biogas energy information.

One cubic meter of natural gas, which is almost entirely methane, typically has a higher heating value of 39.8 MJ per normal cubic meter, or Nm3, and lower heating value of 35.8 MJ per Nm3.

If your biogas is dry and has around 60% methane, it will have a lower heating value of 21.5 MJ per Nm3.

What is Normalised biogas volume, Nm3 ?

We all know that hot air rises, as air temperature temperature increases, its density decreases. Gases such as methane and boigas are able to expand and contract with different pressures and temperatures just like air.

To accurately describe the amount of energy in biogas, the biogas temperature and pressure are corrected to “Normal Temperature and Pressure”, or NTP. NTP is 20 C or 298 degrees Kelvin and 1 atmosphere pressure or 101.3 KPa. 

Of course, this is different to Standard Temperature and Pressure, or STP, but this Normal Nm3 makes good sense as 20 C is an ambient temperature we can relate too.

What does this mean? If you are in a cold region with an ambient temperature of 10 C and the biogas in your digester is under a slight pressure above atmospheric, then 1 m3 of your biogas will have slightly more energy than “Normal”. However, in a country like Australia, ambient temperatures are often around the 20 C, so your actual biogas volume will be close to “Normal”.

Lower and Higher Heating Values?

When the methane in biogas is combusted to provide heat, power and cooling, it will create carbon dioxide and water. This water will be hot, above 100 C and be in the vapour phase. If the water vapour is not condensed into a liquid, then it will leave the combined heat and power generator or boiler as a vapour at the lower heating value, LHV.

If the water vapour is condensed into a liquid using a heat exchanger or economiser, then more energy is harnessed as the energy in the water phase change from a vapour to a liquid is captured, as the higher heating value, HHV.

The higher heating value, HHV, is 1.1 times greater than the lower heating value, LHV, for methane.

Many combined heat and power generators, particularly those utilising biogas, do not harness the biogas moisture condensation energy, so please check that the LHV is used. 

Does water in biogas reduce its energy potential ?

Yes. Biogas can contain up to 10% water and this can reduce its energy potential and reduce the life expectancy and efficiency of a combined heat and power generator. Please consider drying your biogas.

Now that you know about how much energy is in your biogas, what would you like to do with this energy? Producing power is a great place to start.

How much power, electrical energy, can you produce from biogas?

It is surprisingly easy to calculate approximately how much power you can produce from biogas. Many people ask us this question.

Multiply:

the volume of biogas you will consume in 1 hour (in N m3) 

by the energy (in MJ/m3 LHV) 

by 1000 (convert MJ to KJ) 

divide by 3600 (number of seconds in 1 hour)

and divide by the electrical efficiency of the generator.

If you have 10 m3 of biogas to consume per hour at 60% methane, and the ambient is 20 C and the digester producing the biogas is at atmospheric pressure, the biogas will have a LHV of 21.5 MJ. You would anticipate a smaller generator with 25% electrical efficiency, then you will produce 15 KW over the hour.

If your operations require heat, power and cooling and you have a source of biogas, then we can size up a suitable cogeneration or trigeneration to suit your requirements.

Your knowledgeable biogas experts

If you have any questions regarding biogas, its use or its creation, give Inoplex a call on 0448 307 282 today. We are experts when it comes to biogas and cogeneration systems across Australia, so get in contact now.