Comparison of Biomass Hydrolysis and Gasification Processes
As presented in Parts 2 and 3 of the book, the biomass hydrolysis (cellulolysis) path and gasification path are based on different chemical/biochemical transformations and techniques. However, some researchers have attempted to compare these two main paths in techno-economic studies in terms of economic feasibility. The ultimate goal of these studies is to predict minimum ethanol selling price (MESP) using a particular technology. As shown in Section 17.1, this approach has been quite successful for biofuels. Typically, calculations are done for 2000 tonne dry biomass/day plants. These techno-economic analysis studies are generally based on what is known as "nth-plant" economics. The key assumption implied by nth-plant economics is that the analysis does not describe a pioneer plant; instead, several plants using the same technology have already been built and are in operation. In other words, it reflects a mature future in which a successful industry of n plants has been established. In this scenario, artificial inflation of project costs associated with risk financing, longer start-up costs, equipment overdesign, and other costs associated with first-of-a-kind or pioneer plants are ignored. At the very least, these nth-plant economics should help to provide justification and support for early technology adopters and pioneer plants. Recent comparisons of thermochemical and biochemical processes based on nth-plant economics are found in the work of Villanueva Pearls et al. [22] and Piccolo et al. [10]. A 2011 comparison of the two processes is shown in Table 17.5.
This comparison of biomass hydrolysis and gasification paths applies to two ethanol plants consuming roughly equal amounts of biomass (2000 and 2140 dry tonne/day). The expected ethanol productions are 202 and 147 ML/year (53 and 39 Mgallons/year) for biomass hydrolysis and gasification plants, respectively. Even though the production capacities of the two routes are different, MESPs are approximately equal for ethanol produced; $0.95 and 1.05/L ($3.60 and 3.97/L) for biomass hydrolysis and gasification path ethanol, respectively. Then there are certain advantages in the biomass hydrolysis or cellulolysis path when compared to the gasification path, such as the lower capital investment and the revenues expected from exporting electricity generated from burning lignin and solid wastes.