Tri Harder Part 6 – What about flue emissions and maintenance?

Many, many years ago we moved our power generation from being within the city to being outside the city – driven by air quality, expanding populations and access to coal. But what about air quality from trigeneration systems, should we be worried about it?

I’ve covered a lot about trigeneration from Part 1 of Tri Harder where I talked about the different carbon intensities of grid electricity around the world to Part 5 of Tri Harder where I talked about what might happen to trigeneration systems if the grid gets better.

The next question – what about flue emissions and maintenance?

Lets talk about flue emissions first.

In any combustion process whether it’s coal, natural gas, biogas, wood or even hydrogen there are combustion gases. The type and quality of the fuel, so here are some comparisons.

Flue Emissions for Electricity Emission
CH4 (g/GJ) Nox (g/kg) kgC02/GJ
Coal 1 4.5 90
Petroleum 3 27 70
Natural Gas 1 1 51
Wood 30 0.7 89
Biogas 50

The amount varies dependent upon flue grade, combustion efficiency – these are averages.

CH4 is methane, coal and gas are more efficient in a combustion process compared to wood and petrol but there is always some unused methane in a combustion process otherwise the burners would be 100% efficient. CH4 has a global warming impact 25 times greater than carbon dioxide by weight over a 100 year period.

NOx is really bad stuff. Its what creates acid rain, worsens respiratory diseases, aggravates heart disease and generates ozone. But lets not panic just yet. The NOx emissions from all the trigeneration units being proposed by the City of Sydney is still 0.2% of the total NOx emissions from transport!

Plus, we have some awesome technology from the automotive industry called Selective Catalytic Reduction. Basically, it reduces your NOx by 80-95% through injecting ammonia into the flue gas and then catalyzing the NOx out of the flue air before it goes to atmosphere. These are common place in Australia on trigeneration systems and are needed in most cities to meet the air quality requirements. See here for more info.

One constraint to be aware of if not using catalytic reduction is a maximum load throttle that maybe imposed, this maybe time of day constrained and longevity constrained. It is cheaper than a catalytic reduction unit but may mean that you can’t run the unit for as long as you had calculated.

So yes, we moved power generation out of the city because the coal fired power stations caused a great deal of air quality problems. But with new technology, using natural gas to generate electricity in our cities shouldn’t be an issue. But, and a fairly big but, the more we put into our cities and the potential for an increased amount of electric cars in the future it wont be long before that 0.2% starts to increase and when it does we will be paying more attention to NOx from trigen.

A quick one on carbon emissions, as previously mentioned Natural Gas produces less carbon dioxide that coal in combustion but we need to be careful about the way we talk about biogas being a green gas. Yes, it is renewable if its from a waste stream but it will still produce the same amount of carbon dioxide as natural gas in combustion.

And so, on to maintenance.

This bit will be short and sweet. The engine, absorption chiller, selective catalytic reduction units all require maintenance. This comes at not a cheap cost and isn’t that much cheaper on a smaller unit – you can expect to pay around $50k per year for a trigeneration system. However, maintenance of the engine (like your car engine) is essential if you want it last and when a trigeneration unit has a life of about 12 years you want it to last.

It is also important to factor downtime for maintenance into your operational calculations, the downtime for maintenance can be as much as 5% of operational hours. It may seem like a small amount of time but it can have an impact on carbon and financial paybacks.

One final but important note. If you have reduced your peak demand charge costs by deducting the power generation of the engine from the total peak demand of the building – what you would do normally. Be very careful about when you undertake maintenance, if you exceed the peak demand charge while the engine is off for maintenance you will be heavily penalised – peak demand charges are typically 40% of your total energy bill without exceeding them.

Calculating the exact benefit and cost of a trigeneration system should be done for each individual building as the profile of electrical demand, cooling demand and operational profile will vary the calculations. If you are looking at a trigeneration system and need help please email me at

The next Tri Harder posts will go into more detail on:

Tri Harder The Final Chapter – Why do we have so many trigeneration systems going into office buildings and how do I know if trigeneration is right for my building?

1 Comments on “Tri Harder Part 6 – What about flue emissions and maintenance?”

  1. Pingback: Tri Harder The Final Chapter – Why do we have so many trigen systems and 6 reasons to install one – Simon Wild

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