Biorefineries and platform chemicals: Conversion routes: how the plants work

3.1 Pretreatment and enzymes: the expensive key to cellulose

Without pretreatment, Tolan writes, the enzyme demand for cellulose is too high to be practical. Iogen's answer was steam explosion with a little acid. Milled wheat straw is hit with high-pressure steam and 0.5 to 2% sulphuric acid at 180 to 260 C for 0.5 to 5 minutes, then the pressure is released suddenly. The hemicellulose hydrolyses to xylose, opening surface area; the straw comes out as a dark, muddy mass. Cooking too hard turns xylose into furfural and glucose into hydroxymethylfurfural, losing sugar and making fermentation inhibitors, so Iogen aims to hydrolyse less than 20% of the cellulose at this stage. Tolan compares it to roasting a turkey: the longer the time, the lower the temperature.

Older routes skipped enzymes. Concentrated acid (72% sulphuric, 40% hydrochloric) gives high yields at low temperature but needs about 99.9% acid recovery and exotic materials, which has blocked commercial use. Dilute acid at 180 to 200 C for 1 to 4 hours gives about 50% glucose yield, and Kent cites kinetic models capping it at 65 to 70%, because glucose degrades to HMF and levulinic acid. Plants built this way in wartime Germany and Russia suffered corrosion, low yields and poor returns.

Enzymatic hydrolysis is slow. Iogen runs a 5 to 15% solids slurry at pH 5 and 50 C in 200,000 gallon tanks for 5 to 7 days, reaching 90 to 98% cellulose conversion. The rate falls to under 2% of its initial value within 24 hours, and cellobiose, the glucose dimer, inhibits the enzymes at as little as 5 g/L. On-site enzyme production saves stabilisers and transport.

• Kamm (2006), Vol 1 Ch 9 Iogen's demonstration process · pdf pp. 226-235
• Kent (2012), Ch 33 Biomass conversion · pdf p. 1289
• Kamm (2006), Vol 1 Ch 6 Lignocellulosic feedstock biorefinery: history (Katzen & Schell) · pdf p. 165
• Kamm (2006), Vol 1 Ch 3 Development of biorefineries · pdf pp. 104, 109
• Wittcoff (2004), Ch 14 Carbohydrates · pdf p. 483

3.2 Fermentation is judged on titre, rate and yield

Kent's chapter on industrial biotechnology names the three numbers that define a fermentation: rate (productivity, grams per litre per hour), yield (carbon conversion, grams of product per gram of sugar) and titre (final concentration, grams per litre). Each drives a different cost. Yield sets raw material cost. Rate sets how many tanks you must build. Titre sets how much water you must remove.

Dale and Kim give the arithmetic. With glucose at 0.10 dollar per pound and a 90% yield of lactic acid, sugar costs 0.11 dollar per pound of product; at 50% yield it costs 0.20 dollar. A lower yield also nearly doubles reactor volume for the same output, and separation costs rise even faster because they scale with the volume of dilute broth handled.

Werpy and Petersen set targets for commodity diacids: productivity of at least 2.5 g/L/h and a fermentation cost at or below 0.25 dollar per pound. Their Frye and Holladay chapter in Kamm gives 2 to 3 g/L/h for succinic acid.

Acidity is the hidden cost. Most organisms prefer pH 6 to 8, but organic acids have pK values of 3 to 5, so the broth must be neutralised with lime, ammonia or caustic. The product comes out as a salt that must be converted back to acid, and Werpy, Frye and Holladay note acidification can be up to 30% of total cost. In lactic acid plants, sulphuric acid turns calcium lactate into lactic acid plus gypsum, a low-value by-product to dispose of. Enzymes add another mismatch: cellulases work at 50 to 60 C and pH 3 to 5, most production strains at 30 to 40 C and pH 6 to 8.

• Kent (2012), Ch 30 Industrial biotechnology: discovery to delivery · pdf pp. 1137, 1159-1166
• Kamm (2006), Vol 1 Ch 2 Biomass refining global impact · pdf p. 85
• Werpy & Petersen (2004), Section 9.1 · pdf pp. 31-32
• Kamm (2006), Vol 2 Ch 13 Succinic acid (Werpy, Frye & Holladay) · pdf pp. 823-824
• Kamm (2006), Vol 1 Ch 3 Development of biorefineries · pdf p. 111
• Kamm (2006), Vol 2 Ch 14 Polylactic acid (Gruber et al.) · pdf pp. 838-839

3.3 Chemical catalysis rearranges sugars directly

Not every route needs a microbe. Some of the largest bio-based chemicals are made with metal or acid catalysts acting on sugars, and Werpy and Petersen note that chemistry dominates the second step, turning building blocks into derivatives.

Hydrogenation adds hydrogen to the sugar's aldehyde group. Glucose becomes sorbitol over Raney nickel in batch reactors, batch because food uses demand almost no leftover sugar; Engelhard showed a continuous ruthenium-on-carbon process at near 99% yield. Xylose gives xylitol the same way. Further steps are harder: sorbitol to isosorbide reaches about 76% (90% desired), and hydrogenolysis of sorbitol to propylene glycol gives about 35 lb per 100 lb, against 60 needed.

Dehydration strips water. Pentoses give furfural with commercial yields around 50%, or 85% with improved hydrochloric acid processes. Hexoses give HMF, which in water rapidly goes on to levulinic acid and formic acid. The Biofine process exploits that sequence in two reactors, removing products quickly to limit tar. Its theoretical ceiling is 71.6% levulinic acid by weight of hexose; in practice it converts about 50% of hexose mass to levulinic acid, 20% to formic acid and 30% to tar, and 50% of pentose mass to furfural.

Selective hydrogenation upgrades fermentation acids. PNNL turned succinic acid to gamma-butyrolactone at over 95% yield on palladium-on-carbon at 150 to 175 C and 800 to 2,000 psig, leaving acetic acid untouched, then to 1,4-butanediol at over 95% selectivity. The recurring challenge is catalysts that tolerate the impurities in fermentation broths and prove their lifetime in long continuous runs.

• Werpy & Petersen (2004), Sections 7, 9.11 and 9.12 · pdf pp. 25, 65-68
• Kent (2012), Ch 33 Biomass conversion · pdf pp. 1290-1291
• Kamm (2006), Vol 1 Ch 7 The Biofine process · pdf pp. 173-175
• Kamm (2006), Vol 2 Ch 13 Succinic acid · pdf pp. 829-832
• Kamm (2006), Vol 1 Ch 6 Lignocellulosic feedstock biorefinery: history · pdf p. 162

3.4 Heat instead of microbes: gasification and pyrolysis

Thermochemical routes break biomass with heat. The product depends on temperature, time and oxygen. Kent's table for wood gives fast pyrolysis about 75% liquid, 12% char and 13% gas; slow carbonisation 30% liquid, 35% char and 35% gas; gasification 5% liquid, 10% char and 85% gas.

Gasification runs at about 750 to 850 C. Biomass dries, pyrolyses, then its char reacts with steam and carbon dioxide to give carbon monoxide and hydrogen, while the water-gas shift (CO + H₂O → H2 + CO₂) adjusts the ratio. Overall the process absorbs heat, so either some fuel is burned inside with air, diluting the gas with nitrogen, or pure oxygen is used, which Brown calls extremely expensive, or heat is supplied indirectly. Raw biomass syngas carries tar, up to 10% by weight in some gasifiers, plus ash and char, and Werpy and Petersen identified gas cleanliness as the key barrier.

Fast pyrolysis heats finely ground biomass (under 1 to 2 mm, dried below 10% water) at 100 to 1,000 C per second to about 500 C, with vapour residence under 2 seconds and rapid quenching. It yields up to 75% bio-oil, a dark liquid with 15 to 30% water, pH about 2.5, 37% oxygen and a heating value of 16 to 19 MJ/kg, under half that of fuel oil. It is corrosive, unstable and immiscible with hydrocarbons, so upgrading it to transport fuel was not economic. Small chemical outlets exist: liquid smoke flavouring, pyrolytic lignin as a phenol substitute in resins, and potentially levoglucosan.

So and Brown compared routes to ethanol (1997 dollars): pyrolysis plus fermentation 0.42 dollar per litre, enzymatic SSF 0.34, acid hydrolysis 0.35.

• Kent (2012), Ch 33 Biomass conversion · pdf pp. 1294-1297
• Kamm (2006), Vol 1 Ch 11 Hybrid thermochemical-biological processing · pdf pp. 258-260, 269-275
• Werpy & Petersen (2004), Section 6 · pdf p. 24

See it in the table

Check your understanding

Why does Iogen deliberately hydrolyse less than 20% of the cellulose during steam pretreatment?

A fermentation reaches the same titre and rate but its yield falls from 90% to 50%. What happens to raw material cost per kg of product?

Which statement about fast pyrolysis bio-oil is supported by the sources?