In mid-January, VTT, the research institution based in Helsinki, announced an “Emission-Free Pulping” research program, a five-year €15 million research project to “significantly reduce biomass burning and increase the product yield of wood material used for pulping from approximately 50% to around 70%.”
On the surface, the first objective sounds backwards and the second has already been achieved (it’s called thermomechanical pulping (TMP)). However, I think it’s a little more complicated than that! Let’s look at these two goals in the light of history, science and engineering.
Biomass Burning
The industry has spent the last few decades significantly increasing biomass burning and reducing fossil fuel use, using residual biomass such as bark, sawdust and harvesting residues. Not only does this lower GHG emissions (since biogenic CO2 is considered part of the natural short-term carbon cycle), but as recently as 20 years ago, when heavy oil was cheap, it was common practice to put bark in landfills, allowing it to decompose anaerobically to methane, which has a global warming potential 27 to 30 times higher than that of CO2. We don’t want to go back to doing that!
Many older biomass power boilers, however, were designed as a way to get rid of a waste material, most of which was quite wet. It was common practice to use wet debarking and store the bark outdoors where it got wetter due to rain and snow in northern climates. Fossil fuel was added to the boiler as a supplementary fuel to make sure this wet fuel could be consumed. Today, dry debarking is more common and some facilities dry their solid biofuel with waste heat prior to combustion, and/or grind it to a dry powder to use in a lime kiln. This takes some investment, but reduces emissions of both fossil and biogenic CO2. Furthermore, with the global Net-zero by 2050 target, solid biofuel currently used by the forest products industry will likely become more expensive as other industry sectors compete for it to lower their emissions. It’s a good time to make sure their use by the forest product sector is as energy-efficient as possible!
70% Yield
Let’s review the history of pulping. Stone groundwood, with a yield near 100%, was first developed in 1843, before chemical pulping, which was first patented in 1854. Of course, groundwood was not much use for making paper by itself; a typical recipe for newsprint up to 1960 was 70% groundwood and 30% chemical pulp. Researchers tried to find lower cost recipes with a higher overall yield. The first wave of solutions occurred in the 1960s to 1980s, when “alphabet” semichemical pulps, mostly based on sulphite and bisulphite chemistry, such as HYS, VHYS, SCMP, BCMP, TCMP and CTMP, were developed. The yield of these pulps typically ranged from 70% to 94%, and, of course, contained a lot of hemicellulose and lignin. High yield helped the economics by using less wood per tonne of pulp, but there were three disadvantages: (1) less strength than chemical pulps, (2) yellowing with age, driven by oxidation of the lignin component and (3) lack of a recovery cycle like that of the kraft process to recover the inorganic chemicals and burn the organic portion, meaning the pulping liquor was discharged to the environment. Ultimately, the development of TMP, which has a yield of 97-98%, strong enough for making paper on its own without a reinforcement pulp, was the big winner. When secondary treatment of mill effluent became compulsory in the 1990s and newsprint demand started to plummet, any newsprint mill that had not invested in TMP was one of the first to be closed, due to higher production costs, higher labour costs (to produce two types of pulp instead of just one) and higher environmental treatment costs.
But let’s look again at the objective mentioned: increase pulping yield to 70%. Wood is typically 25–30% lignin and the rest is cellulose and hemicellulose. Chemical pulping is designed to remove the lignin and hemicellulose removal is collateral damage. But what if only the lignin could be removed? This would deliver a yield of … about 70%! Of course this would not be easy, since some hemicellulose is removed simply by exposure to hot water, and some of the lignin is chemically bonded to the cellulose! But this is 2024, not 1854. We know a lot more about the chemistry of lignocellulose. Instead of using the hammer of sulphur-based chemistry, could it be possible to design some cleverer chemistry to retain the hemicellulose and remove the lignin?
Meanwhile, there are other ongoing projects around the world to make significant strides in improving the energy efficiency of the kraft process. Does weak black liquor have to be at such a low concentration? Can membrane technology replace some of the energy-intensive work of the evaporators? Can high-temperature heat pumps be applied to the process to save energy?
Now those are some ideas worth spending research money on!
Martin Fairbank, Ph.D. Martin Fairbank has worked in the forest products industry for 31 years,
including many years for a pulp and paper producer and two years with
Natural Resources Canada. With a Ph.D. in chemistry and experience in
process improvement, product development, energy management and lean
manufacturing, Martin currently works as an independent consultant,
based in Montreal. He is also an author, having recently published
Resolute Roots, a history of Resolute Forest Products and its
predecessors over the last 200 years.
Martin Fairbank Consulting
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