Electricity generation with biogas


"It’s not rocket science, says Niwa’s Rupert Craggs.  Recently he and a few others demonstrated how easy it is to produce electricity from the biogas that comes off farm effluent ponds, using a converted generator.

“We plugged in two fan heaters, three spotlights and a 3 phase motor – and they ran for over two hours.  At full throttle it generates 13kW, we were running it at nine or ten.”

Such technology is not novel, he says.  “Most large domestic wastewater treatment plants capture biogas.  For example, they do it here in Hamilton and in Auckland and Christchurch.  There are also farm-based technologies but these have not been cost effective.  What we’re trying to show is that farmers can do it themselves.  It’s that whole number 8 wire thinking.” 

The aim of the FoRST grant which is funding the study is to show that there is a energy resource in wastewater and that it doesn’t require a sophisticated, expensive digester to recover it.

“We’re focusing on using simple pond-based digestion technology and off-the-shelf equipment - that anyone with a small amount of mechanical skill can convert to run on biogas.”

Originally from the UK, Rupert was at UC Berkeley in San Francisco, working on nutrient and energy recovery from wastewater treatment ponds.  When he arrived here nine years ago there was little interest in energy recovery from ponds.

“It’s never really been considered because energy has been cheap in New Zealand. So why bother. And on dairy farms only 20 per cent of the waste is captured, so 80% of the resource is out in the paddocks. But as electricity becomes more expensive, and cows are fed on feed pads or put on wintering pads, you’re going to collect more of that waste and it starts to make sense.

The three-year project has shown that farmers can easily and cost-effectively convert biogas emissions to power.

“We were surprised how easy it was to convert the generator to run on biogas, and are particularly grateful to Entec Services Ltd, Auckland for their assistance.”

Master’s student Stephan Heubeck, originally from Germany, has worked on the project for two years and with NIWA engineering mechanic Mark Smith, converted the generator in under two hours.

A diesel generator can be used as well, and is particularly suited for larger instalations.”

Over the last few years NIWA has measured biogas coming off treatment ponds, looking at how quality and quantity vary with season.

“The biogas contains 70 to 80 per cent methane, which is strong enough to be used in a generator, with little or no removal of impurities like hydrogen sulphide.”  He was quite impressed that the recent test undertaken at Dexcel’s Scott Farm showed that the generator could be run for more than two hours using biogas collected under a small (25m²) trial cover – “and this is the middle of winter, with no waste going into the pond.

The next phase of the research is to do further tests, and monitor a large-scale trial.  “We want to show how easy and simple it is to operate for on-farm electricity generation. And how much time a farmer will have to put into running it. We are also going to trial a low-cost cover, because that and the generator are the two major costs.”  Quite a few of the farmers we’re talking with say they’re thinking about getting a generator anyway, for when the power goes down.  So then they’re just looking at the cost of the cover.”

Cost is a consideration, he says. “It comes down to economies of scale.  If you only have 150 cows, and you’re only collecting 10 per cent of the waste, you don’t have enough of a resource.  At the moment it looks like you need a herd size of about 1000 cows, but if waste from feed pads or wintering pads is collected, it becomes more economical for smaller farms.”

Existing farm anaerobic ponds could be converted, but NIWA’s research has shown that these ponds can be a third smaller and be more effective.  “And that reduces the cost of the cover needed by a third as well.  “Farms which irrigate the effluent are increasingly being required to put in storage ponds for deferred irrigation – these have a retention time of one to two months, and could easily be designed to promote anaerobic digestion and biogas capture.”

Producing power from biogas has many advantages, he says.  “The neat thing about anaerobic digestion is that you don’t lose the nutrients. So after you recover the energy, you’re still left with a nutrient rich effluent which you can irrigate.  And you’re helping reduce greenhouse gas emissions from open ponds by twenty-fold. 

As power prices go up, and the value of these advantages is realised, the technology will become increasingly attractive.”