Reduction of Ammonia and Tar in Pressurized Biomass Gasification Page: 4 of 12
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Data evaluation and modeling by multivariate data analysis
The variables that were investigated were listed in Table 2. The table provided the
detailed information about the 10 descriptors for operating parameters as well as 2 responses
for tar or ammonia. The PLS method was employed to treat the problem involving such a
large number of variables. The PLS analysis quantitatively associated the formation of
ammonia and tar with the operating parameters. Two-component PLS model was found to be
significant both for tar and for ammonia according to cross validation. Figures 5 and 6
compared the known values of tar and ammonia with the calculated values by the PLS model.
The data points in Figure 5 covered gasification tests of sawdust, and sawdust mixed
with cardboard, plastic or textile wastes. The two-component model accounted for 62% (50%
for the first component and 12% for the second component) of the variance for the carbon
conversion and the tar level.
The data points in Figure 6 accounted only for the experiments for sawdust as
feedstock. For feedstock with wastes, the determinant factor became the fuel-N content, as it
was shown in Figure 4a. In order to better explain the scattered data points in Figure 4b, the
experiments with wastes were excluded. The two-component model accounted for 76% (63%
for the first and 13% for the second component) of the variance for the nitrogen conversion
and the ammonia level.
The results of the quantitative PLS analysis will be discussed below. The details of
the PLS model are available for interested readers.
In the PLS analysis the significance of the descriptors was graphically present by
combined loading plots (Figures 7 and 8). On the plots the significance of the variables was
visualized by the distance to origin along the PLS axes. The significant variables located
away from the origin. The points in the opposite directions indicated that these variables were
The amount of tar was negatively associated with the fuel-carbon conversion (Figure
7). Increasing equivalence ratio and all the temperatures resulted in decreasing amount of tar.
The temperatures above the feeder (T8) and the freeboard temperature (T5) seemed to be of
more importance. The observations were in accordance with those found by Kurkela et al.,
Gil et al. and Narvaez et al. (Kurkela et al. 1992, Gil et al. 1999, Narvaez et al. 1996).
Comparing with ER and the temperatures, the pressure and the gas residence time were of
secondary importance since they were along the PLS2 that accounted for 12% of the variance,
comparing with 50% for PLS 1. High pressure or long residence time favored the reduction in
tar formation. The impact of residence time could easily be understood, since long residence
time in the gasifier helped tar cracking. The influence of the pressure and residence time
might also be caused by the fact that the pressure, the residence time and the temperatures
were associated. The particle size of sawdust was not found to have significant effect on tar
formation. Do clear difference was obtained when applying the PLS analysis to the
experiments with or without wastes. Usually more tar could be observed in the producer gas
when plastic was added to the feedstock. This indicated that additions of synthetic materials
such as plastic and textile wastes were not found to have significant impact on tar formation.
The change in tar amount was more likely to be attributed to the decrease in equivalence ratio
with addition of plastic.
The effect of fuel-N on ammonia formation was present in Figure 4a. The influences
of the other operating parameters were illustrated in Figure 8. Ammonia formation and fuel-N
conversion increased with increasing temperatures and ER, and decreasing residence time.
The reason was that the relatively low temperatures in our gasifier were not sufficient to
convert the fuel completely. As a result, high temperatures, particularly high freeboard
temperatures (T5) and the temperatures above the feeder (T8), as well as large ER values,
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Wang, W. & Olofsson, G. Reduction of Ammonia and Tar in Pressurized Biomass Gasification, article, September 19, 2002; United States. (digital.library.unt.edu/ark:/67531/metadc782933/m1/4/: accessed November 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.