Taking energy from waste without the carbon

It sounds impossible, but it's not. Bill Butterworth reports on a technology which turns waste into compost, and uses it to grow biofuels

Energy from waste is, as a matter of basic logic, bad for the environment because it burns things. And burning produces carbon dioxide.

Energy from waste is good, of course, from the perspective that it is an alternative to burning fossilised fuels to generate heat or electricity. Energy from waste is sustainable in the sense that it burns waste, which keeps on coming. For waste, it is hailed by its advocates as the final solution.

But there is a somewhat problematical residual ash. And, worst of all, it is still incineration. It pumps pollutants and large quantities of carbon dioxide back into the atmosphere.

How much greenhouse gas? Well, the research is plentiful and remarkably fickle with a range of answers depending on the system, where the measurements started and finish, and who funded the research. The truth is that, by calculation, burning a tonne of typical municipal solid waste (MSW) must, if it is burned in excess air to avoid the production of carbon monoxide, somewhere along the line, yield 1.4 tonnes of carbon dioxide.

What alternatives have we? Well, maybe not enough. But there are some bright lights within current vision.

In terms of common sense logic, what is needed is some way of extracting the energy from waste without actually burning it or, at least, without releasing more greenhouse gases.

There is not yet a technology available which will do that by a single step, direct route. And there probably never will be. The nearest we have got so far is pyrolysis or gasification. This, it is widely thought, still has some way to go before the technology is good enough for immediate, wide use. In principle, it can produce methane gas, which can be used to drive engines (which of course burn the gas to release carbon dioxide in their exhaust).

But there is a technology which steps round these difficulties by locking up the carbon, and using what is called photosynthetic carbon capture and storage (PCCS) as the link to produce liquid biofuels. The basis of this is to compost these wastes (which locks up the carbon) and then use the nutrients in the compost to grow crops.

If these crops are oil seeds, it is then possible to produce biodiesel and bioheating oils. Alternatively, if the crops use wheat or maize, the fuel can be bioethanol. These fuels, of course, may or may not have not come from waste via composting. If they come from crops grown with compost, then the energy equations do actually stack up.

Carbon capture
The energy in the fuels has come from the sun via the green leaf of the crops. What the chlorophyll in the green leaf does is allow the crop to capture carbon dioxide from the air and water from the ground to make large, carbon-based molecules.

Hence the process name, photosynthetic carbon capture. The S comes from storage, and that involves building up organic matter in the soil from leaves and roots. This is a major plus in that, to produce one tonne of bio-diesel (at least this amount will come from one hectare of crop), there will be at least 18 tonnes of dry matter per hectare of crop, which will have taken at least 28 tonnes of carbon dioxide out of the atmosphere (and put 16 tonnes of oxygen back in). Couple that to the maybe 18 tonnes of carbon dioxide which would be released from the waste (now used to make the compost) if instead it had been burned. Composting plus PCCS locks up a net, after burning the biofuels, of more than 43 tonnes of greenhouse gas per hectare of land so cropped.

Follow the arithmetic? Well, if you don't, look at the common sense. Composting does not burn things, and it can grow crops such as oil seed rape. Growing the crop allows the green leaf to take carbon dioxide out of the air and put oxygen back in.

That is where fossilised fuels came from in the first place.

So, will composting and growing crops do the job? In principle, yes. But in practice, this is too simplistic. On-farm composting does give an entrée into PCCS and biofuels but it has, up to now (and by historical regulation) needed a high degree of source separation (which is expensive and does not work very well). But there is a new technology which can cope, both technically and within the law, with a much lower level of cost and resource input into source separation.

MBT and MBS
There has been much talk of mechanical biological treatment (MBT) plants. MBT can mean anything involving mechanical and biological treatment. At one extreme, it can mean shredding and composting on farms. Mostly, though, MBT refers to large, sophisticated complexes involving separation, production of compost and possibly some form of EfW.

There is, however, a new breed of these plants. New Earth Solutions prefer to use the label of MBS - mechanical and biological sterilisation - simply because the main objective is to produce the maximum of very safe materials which can be recycled to land to produce energy crops.

The claim is that from 80% to almost 100% of MSW (whole municipal solid waste) can be safely recycled to land. MBS uses the highest available technology to give the best available safety with on-line process monitoring direct into Environment Agency offices. The firm has also cracked odour control based in part on technology which is old, proven and really does work.

Energy crops
The key issue, now, in waste management is really in greenhouse gas production - not in a process but in the full cycle. Energy crops producing biodiesel and bioethanol are apparently attractive but not if the crops are grown with mineral fertilisers.

Mineral nitrogen fertiliser uses large amounts of electricity in its manufacture.

Recycling nearly all MSW to land uses the energy locked up in it, adds the photosynthetic bonus from growing crops and can deliver liquid fuels for driving the engines on which our economy depends. Dest separation becomes unnecessary (and it has always been expensive and cannot be depended on to deliver 100% separation).

Defra has already shelved making a decision on allowing MBT plants to avoid source separation. It might be a good time to review the matter again. Logically, if the output is to an appropriate standard, and the standard is adequate, who cares whether the feedstock was source separated or not?

Putting it bluntly, if we are to have a future world at a reasonable temperature and still ride in aeroplanes, drive a car, and heat our homes, then in the medium term we are going to use biofuels.

These will only avoid global warming from too much greenhouse gas if the crops are grown from composts made from wastes. Where MBS scores is in emphasis on recycling as much waste as possible to land to produce crops and preferably energy crops and by using best available technology to produce safety and odour control.

We are, at least, now moving in the right direction. That direction cannot be a
final solution. Sustainability can only come from continually moving everything we produce and consume around in safe circles.

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