This is a transcript of an episode of Public Address Science which was originally broadcast on Radio Live, 26th May 2007, 2 pm - 3 pm.
You can listen to the original audio version of the programme by clicking on the 'Play the audio for this post' link at the top of this page or the 'Audio' button at the bottom of this page.
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There's been a lot of talk about biofuels recently, but what exactly are they? Well, essentially they're the end product of nature's own solar collector. Plants absorb energy from sunlight, and then store this energy by turning water and carbon dioxide into hydrocarbons. By employing an appropriate chemical process, such as burning these hydrocarbons, the original solar energy can be re-released in a concentrated form that we can use.
The neat thing, of course, is that burning these plant-derived hydrocarbons only releases the same amount of carbon dioxide as the plants themselves absorbed from the atmosphere in the first place. So, from this perspective, biofuels can be said to be carbon neutral, because it's theoretically possible to manufacture and use them without adding to the overall amount of carbon dioxide in the atmosphere.
However there is, admittedly, a potential problem. Because the energy in raw biomass isn't necessarily in a form that can be easily used -- then additional energy is required to harvest the biomass and process it into a usable form. Not to mention the energy input that's also required in terms of planting, tending, and fertilizing a biofuel crop.
This leads to something called the energy yield ratio. In simple terms, this is the ratio of the chemical potential energy stored within the fuel, in comparison to all the energy input required to bring the fuel to the point of end use. The higher the energy yield ratio, the more energy you get out of the fuel in comparison to the energy input required during processing.
Some biofuel crops (notably bioethanol from corn in the United States) have an energy yield ratio which is only slightly greater than one -- in other words: absolutely pitiful. But it's important to realize that this is by
no means the case for all biofuels. And it's certainly not the case for biofuels in this country.
You might be surprised to discover that (according to the Energy Efficiency Conservation Authority) in New Zealand we already get nearly ten per cent of our total energy input from biofuels -- in the form of the burning of firewood and waste biomass in households and industry. Now, of course, this doesn't translate to ten per cent usable energy output because -- for various reasons -- biofuels are currently consumed in a rather inefficient manner [in 2006, a publication from the National Centre for Climate–Energy Solutions stated that "bioenergy currently provides about 5% of New Zealand’s total primary energy supply"].
But if we could economically make use of more biofuels for simple purposes, such as household and industrial heating, then it would have two important results. Firstly, it would displace the consumption of some of the fossil fuels which are currently used for this purpose; and, secondly, it would displace consumption of electricity used for heating -- thus freeing up both electrical energy and our electrical networks for other purposes.
It turns out that an interesting and relatively new manufactured biofuel could help us achieve just that. Wood pellets have been on the market for a few years now, and have the potential to become an important part of New Zealand's energy mix.
I talked to George Estcourt, a scientist at Scion (one of New Zealand's Crown Research Institutes) who studies the production of wood pellets in New Zealand. According to Scion's research, wood pellets are among the most cost-effective form of heating in New Zealand -- far cheaper than coal, electrical resistance heating, or gas. I asked George if wood pellet biofuels have any other advantages to offer.
George Estcourt
Yes,
the wood pellet stove is basically designed around the wood pellet. The problem with the normal log fireplaces is that they're tested (to reach their standards of, say, how many grams particulate per kilogram of wood, and that type of thing) [in a] lab using the absolutely correct firewood, moisture content, and so forth.
But as soon as you take it home, and put it in your house, and then you start using a bit of this, a bit of pine, a bit of tea-tree -- and you really don't know what the moisture content is -- your emissions can be ten times [or] a hundred times worse than what they were in the lab. But with your pellet stove, really nothing is going to change, so you're always going to [achieve] high quality results.
The wood pellets themselves are pure wood, which means that there's no other material in there that could cause poor emissions. And [of course] the other thing -- when you get contaminants -- they generally become ash. Anything that can't be burnt ends up in your ashtray. Wood pellets have an ash content of [only] around 0.5 per cent
Interviewer
Right -- and, of course, in the sort of systems you're talking about, the wood pellets are stored in a hopper, and then they're automatically fed through into the combustion space as required. So it's also much more convenient than a conventional log-burner.
Now I think there's a few myths around about the inclusion of dangerous chemicals in wood pellets. Would you mind giving a description of how wood pellets are made, and telling us what they're actually made from?
George Estcourt
The two main feedstocks for wood pellets are either wet sawdust or dry shavings. [The] wet sawdust comes straight off the sawmill bench.
Dry shavings [are] produced once you've sawn [and] dried your timber, and then you're basically dressing the timber to put the nice smooth surface on it. The shavings [that] come off are [a] waste product, and one of the best materials to make wood pellets.
The wood waste -- such as the sawdust and the wood shavings -- have to go through a hammer-mill, and that breaks the material down to a very fine particle size. [If] the material [is] not already dry, like the dry shavings, it will have to be dried. Wet sawdust is about 50 percent moisture content -- [so] you need to drive all that water off and get it down to about 10 per cent moisture content.
Once it's all down at that 10 per cent then it goes through a pelletizer, and this is when the hammer-milled product, which is a very fine-looking sawdust, is pushed through a die. So you're pushing the material through the die, it's heating up, the lignin that's within the wood-fibre softens -- and I think the wood pellet gets up to about 90 or 80 degrees Celsius while it's being formed -- and that lignin holds the wood material together as it's cooling down.
It's a natural glue that's within the wood fibre. There's no additives -- it's pure 100 per cent sawdust or dry shavings.
Interviewer
Okay, so just to recap: the wood pellets are made from waste sawdust and waste wood shavings, and they're held together by a natural glue that already exists within the wood itself.
Now because the wood pellets are made from waste products they'll inherently have a very high energy yield ratio. Have you actually calculated or measured it?
George Estcourt
Obviously when you're producing it from a dry shaving, the only
energy that you're really using is the electric motor doing the hammer-milling and running the pelletizer itself, so there's very little energy required.
We have people having to dry the sawdust [as well]. One of them was using gas to dry the sawdust. [But] he's [now had] a wood pellet furnace made big enough, so that he's actually burning his own wood pellets to dry the sawdust material. [So he is] using a product that [he has] produced [himself]. [He's] not using fossil fuels any more -- so again it would have quite a high [energy yield] ratio.
Interviewer
At the moment, as you've said, the pellets are made from waste wood. But what happens once your market grows larger than the waste wood supply, and you have to use virgin sources? Does the price go through the roof at that point?
George Estcourt
At the moment there is an abundance of sawdust in the North Island. We think that the industry could [increase] maybe ten-fold before you'd have to think about actually growing crops to make wood-pellets.
What we're hoping [and] trying to encourage [is] more processing of New Zealand logs within New Zealand -- rather than just exporting them. And if you can encourage more processing in New Zealand, and keeping the logs here, then we get more sawdust and wood waste [to turn into pellets]. So there's a double benefit if you try [to] get more processing done in New Zealand.
Interviewer
Right, which all sounds very good.
Now, exotic wood crops are -- so we're told -- produced very sustainably in New Zealand. Do you think it would be important to have legislative controls on any biofuel imports to ensure that they've also been produced in an equally sustainable manner?
George Estcourt
I think it would
be helpful to have limitations on what we bring in. [But] on the wood pellet side I can't see that ever happening. We get a lot of emails from companies -- mainly in Europe -- wanting to know if we can export wood pellets to them.
So I think there is [a greater] possibility that we will be exporting our biofuels out of New Zealand.
So a simple manufactured biofuel like wood pellets -- which makes use of sawdust and shavings that would otherwise be discarded -- will provide clean and efficient heating in domestic or industrial situations, and could make an increasingly important contribution to New Zealand's energy mix.
Next week we look at another big biofuel that could also be manufactured from New Zealand's exotic forests.
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- Read more about bioenergy at Scion.
- Visit the EECA website to find out about energy sources and usage in New Zealand.
- Read the Warm Homes Technical Report to find out about home heating methods and fuels in New Zealand.