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A Different Kind of Biorefinery

Readers will have noticed the boom in discussions for biorefineries as potential auxiliaries to pulp mills, generally based on some kind of waste from the mill as a feedstock to produce diesel fuel or perhaps some marketable organic chemicals. Biorefineries are normally proposed for pulp mills rather than paper mills, simply because a pulp mill handles vastly more not-marketable organic material than does a paper mill. Most of the organic matter in a paper mill is fiber, which has a much higher value as a product than for its energy content.

Some proposals even drop pulp manufacture, and suggest using a former mill site for a biorefinery because of local availability of wood and utilities such as boilers, maintenance shops, and effluent treatment. This makes sense.

In spite of significant taxpayer contributions to R&D and pilot plants in the field for several years now, I have yet to hear of a viable stand-alone business unit based on biorefining wood or wood wastes. (If you know of one, please contact me at Neil@McCubbin.ca.)

In most cases, organic matter proposed for use as the biorefinery feedstock is being removed from a stream such as black liquor or hog fuel, which normally would be available to contribute to mill steam production. Because most North America mills still burn oil, coal, or gas to raise some their process steam, I find it very hard to see how it can make engineering or thermodynamic sense to convert the wood waste to biodiesel, while buying fossil fuel to replace the lost steam-generating capacity.

Many of the publications on biorefineries focus on the interesting, and often innovative, technology involved, but seem to forget that in the end, energy cannot be created by any biological or chemical process. It can only be changed in form, and all changes involve some losses. Isaac Newton figured that out a few centuries ago, and high school physics teachers try to get this basic fact into teenage heads every year, although not always successfully.

Of course, legislators can make any business profitable by a mixture of tax breaks and subsidies. The recent multibillion dollar windfall in the form of the black liquor fuel tax credit in the United States is one of the most spectacular examples ever of taxpayer largesse contorting pulp industry economics. This has been a short-term affair, but who knows what governments might do next.

Perhaps we should be looking at some alternatives to the present biotech proposals.

While the pulp mill “wastes” that current biorefinery proposals usually rely upon have alternate uses, mostly as fuel to generate steam, several waste streams inside the mills are currently useless. A vast amount of energy is contained in liquid effluents and stack gases, including paper machine or pulp dryer vents, lime kilns, and boilers. Efficient mills already recover all the energy in these streams that the mill can use. (If your mill is inefficient in this regard, then recovering this energy will probably be a much better investment that any exotic new technology.)

The problem with such discharge streams is that the energy is available at temperatures too low to be useful in a mill. None of the multiple studies of using heat pumps to recover energy have evolved into economically attractive projects, primarily because the capital and equipment maintenance costs have outweighed the obvious value of the recovered energy.

However, some industries can use relatively low temperature energy, particularly in cold climates. To me, the most interesting is back on the farm.

Today’s food industry transports fruits and vegetables thousands of miles, from warm regions to cold ones, at considerable expense, particularly in winter. Many of these products can be grown in greenhouses, but this seems to be less profitable than growing crops in warm climates and then trucking the harvest to far-off markets.

Greenhouses operate at temperatures well below pulp mill waste streams, and it would be relatively simple to heat them with warm water from a mill. A few relatively hot effluents would suffice, rather than the whole effluent stream. Hot enough water also can be generated easily from dryer vents and some stack gases. The only question is economics.

In addition, plants grow best in a carbon dioxide–rich atmosphere. I am told that some greenhouse operators even purchase carbon dioxide. Pulp mill stacks, particularly lime kilns, carry lots of carbon dioxide.

Waste-activated sludge from the effluent treatment plant also generates useful soil supplement for agriculture, and presumably could be used in greenhouses.

I have not seen any serious analysis of the application of pulp mill waste heat to greenhouse cultivation. I cannot see any major technical difficulties, but an engineering analysis of the costs and benefits would be needed to determine whether it would be profitable. None of the technology would be new. It would require a multidisciplinary team with pulp mill, heat transfer, and agricultural knowledge to put it all together. Not as sexy as a biorefinery using exotic enzymes, but perhaps less risky and more profitable.

Effectively, this would be using the pulp mill waste heat to produce marketable biomass (food), which typically retails for several dollars per pound, whereas diesel sells for well below a dollar per pound (excluding taxes).

Instead of being at a climatic disadvantage relative to South American mills, the mills in cold North American and European countries would have the advantage of their climate-boosting selling prices for vegetables, etc.

The whole concept also is sustainable and genuinely “green,” so it would gain some marketing support from that angle, and help the industry improve its image.

Does anyone know of a serious study of such a biorefinery? If not, perhaps one of our more innovative pulp and paper companies will undertake one.


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