Partnerships, Future, and Emerging Technologies - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 409

bed media becoming sticky which in turn leads to agglomeration and defluidization
of the bed media.
Sulfur that gets converted into H 2 S and alkaline earth metals such as Ca or Na
can be significant catalyst poisons for both the tar reforming catalyst and the fuel
synthesis catalyst. Depending on their concentration in the feedstock and the
sensitivity of the catalysts being utilized, they may need to be reduced or eliminated
from the syngas stream by water scrubbing or catalyst guard beds. Although both of
these techniques are well-proven technologies, they do add cost and complexity to
the process.
Economics
The thermochemical conversion process has not received the same degree of focus
as has the biochemical conversion process, and therefore there are less references in
the literature on the economics of the process. Similar to the biochemical conver-
sion process, probably the best public source of production cost numbers is NREL.
Dutta et al. published a case where a fully loaded production cost of $2.05/gallon
(2007 dollars) could be achieved based on technology demonstrated at the pilot
plant for a 2,000 tonnes/day commercial plant for an nth plant case [ 43 ].
Given these two independent technology approaches for producing cellulosic
ethanol, the logical question is how they compare. Several studies have looked at
this particular question, and one study [ 44 ] specifically did a rigorous comparison of
these two technologies based on 2007 reported numbers. Table 19.1 provides some
direct comparison numbers for the two processes based on the latest reported results
from NREL referenced above.
As can be seen from the values in Table 19.1 , the economics of the two processes
are very similar. The thermochemical process has a slightly lower MESP (5 %
lower) but a higher required capital investment (22.1 % higher). The thermochemi-
cal process does have slightly higher yields since the lignin portion is utilized for
fuel production as well. Average return on investment is almost identical.
An important point of distinction is that the biochemical results are presented for
corn stover, whereas the thermochemical results are for pine. The authors of the
referenced studies choose the feedstocks that tended to give the best performance
and economics for their conversion technology. Although conversion economics do
not exist for pine feedstocks for the biochemical conversion process or corn stover
for the thermochemical conversion process, poorer conversion economics would be
expected for these cases due to lower carbohydrate content for the pine feedstock
for biochemical conversion and higher ash content for the corn stover feedstock for
thermochemical conversion. This illustrates the earlier point that there does not
appear to be a clear superior conversion technology in terms of yields and/or
economics. Hence, the best approach is to best match the conversion technology
to the predominant feedstock. Since feedstocks tend to be local, the best conversion
technology choice will most likely be feedstock dependent or regionally specific.