4.1 THE IOGEN CORPORATION PROCESS AS A TEMPLATE AND PARADIGM

The demonstration process operated since 2004 is outlined in figure 4.1. In many of its features, the Iogen process is relatively conservative:

• Wheat straw as a substrate — a high-availability feedstock with a low lig­nin content in comparison with tree wood materials (figure 4.2)12

• A dilute acid and heat pretreatment of the biomass — the levels of acid are sufficiently low that recovery of the acid is not needed and corrosion problems are avoided

• Separate cellulose hydrolysis and fermentation with a single sugar substrate product stream (hexoses plus pentoses) for fermentation

• Cellulase breakdown of cellulose — Iogen is an enzyme producer

• A Saccharomyces yeast ethanologen — relatively ethanol-tolerant and engineered for xylose consumption as well as offering a low incidence of contamination, the ability to recycle the cells, and the option for selling on the spent cells for agricultural use1

In the first description of the process (written in and before July 1999), agricultural residues such as wheat straw, grasses, and energy crops (aspen, etc.) were equally “pos­sible” or a “possibility.”3 By the next appearance of the article in 2006[31] — and as dis­cussed in chapter 2, section 2.6 — cereal straws had become the substrates of choice. Lignin does not form a seriously refractory barrier to cellulase access with wheat straw; this renders organic solvent pretreatment unnecessary. More than 95% of the cellulosic glucose is released by the end of the enzyme digestion step, the remainder being included in the lignin cake that is spray-dried before combustion (figure 4.1).

The Iogen process is viewed as a sequential evolution of the bioethanol paradigm, no more complex than wet mill and dry mill options for corn ethanol production (figures 1.20 and 2.21), substituting acid pretreatment for corn grinding steps, and

adding on-site cellulase generation, the latter mostly as a strategy to avoid the costs of preservatives and stabilizer but possibly also to use a small proportion of the hydro­lyzed cellulose as a feedstock for the enzyme fermentation itself. Salient features of the technology were present in Canadian initiatives from the 1970s and 1980s. The Bio-hol process, financially supported by the Ontario Ministry of Energy and Energy, Mines, and Resources, Canada, opted for Zymomonas mobilis as the ethanologen and had established acid hydrolysis pretreatments for wheat straw, soy stalks, corn stover, canola stalks, pine wood, and poplar wood.4 For both Z. mobilis and S. cere — visiae, pretreated wheat straw had the distinct advantage of presenting far less of a toxic mixture to the producer organism (figure 4.3); methods for removing growth inhibitors from the biomass acid hydrolysates could reduce the effect by >20-fold.

Minimum Inhibitory Concentration (% w/v)

FIGURE 4.3 Growth of Z. mobilis on biomass hydrolysates. (Data from Lawford et al.4)

The Stake Company Ltd. was founded in 1973 to develop and market a process for biomass conversion to sugar streams for both biofuels and animal feeds as well as chemicals derived from lignin and hemicellulose.5 A continuous feedstock processing system was constructed to handle 4-10 tons of wood chips/hr and licensed to end — users in the United States and France.

Before 2004 (or 1999), moreover, more radical processes were examined in detail — including being upscaled to pilot plant operations — for lignocellulosic ethanol. These proposals included those to avoid the need for cellulase fermentations
independent of the main ethanolic fermentation as well as the use of thermophilic bacteria in processes that more closely resembled industrial chemistry than they did the traditional potable alcohol manufacture. Indeed, it is clear that Iogen consid­ered sourcing thermophilic bacteria[32] and nonconventional yeasts during the 1990s.3 The achieved reality of the Iogen process will, therefore, be used as a guide to how innovations have successfully translated into practical use — or have failed to do so — reviewing progress over (mostly) the last three decades and offering predictions for new solutions to well-known problems as the bioethanol industry expands geo­graphically as well as in production scale.