Chemical Engineering July 2012 - 41

Tons
waste/h
50
Adj. inflation
$8.58/1,000 lb
40
2002 rate
$6.95/1,000 lb
30
20
10
5,000
10,000
15,000
Steam flow, lb/h
FIGURE 5. The model-predicted steam low and margins (combusting 4,466 Bt/lb
solid fuel) illustrate steam-pricing strategies applicable to the prototype
Basis
Split
C
H
S
O
N
H2O
TABLE 3. BLENDED FUEL FOR MODEL EVALUATION
EPA 2005 Tires 2011 Blend
0.5
0.5
Waste Tires Composition H Btu/lb-mole Moles
0.48
0.5794 0.5297
0.0538 0.1558 0.1048
0.0011 0.0464 0.0238
0.26
0.0034 0
0.135
Ash/solids 0.068
Total, lbs 1.001
0.1584 0.1467
0.0601 0.0641
1.000 1.001
89,674.82
12,887.32
3,024.80
0.1300
0.0017
0.0441
0.0520
0.0007
20,000
25,000
primary air in our HYCO-methanol
model process to the same 18% excess
air conditions reported in the literature
[5] to see if our NOx emissions
would be comparable to a one-millionBtu/h
pilot waste combustor.
Model results ranged from 269 ppm
to a low of 178 ppm, denoted by the red
squares in Figure 4. The Model NOx
results obtained as a function of temperature
also appear in red. Predicted
emission data correlate well with previous
literature pilot studies.
Calculated SO2 levels for the model
ranged from 24.5 to 74.08 ppm for
the blended fuel tests. In the 1,600°F
range, the model predicted SO2 was
88.32 ppm. The literature [5] reports a
level of 82 to 91 ppm for the two combustors
tested at 1,550°F-1,600°F.
Model operation between 1,500°F
and 1,600°F with 18% excess air
matches the literature claim for control
of NOx and good absorption
of SO2.
Calculated data that are close to
literature values suggest the model
material and energy balance relationships
are reasonably accurate and representative.
Proper directional trends
in any model are essential.
105,586.94 0.0969
105,587
Avg mol wt: 10.3
Density:
HHV:
560.5°F and the second methane preheater
operating at 1,142°F.
Results
Five steam-to-carbon ratios were
tested -1:9, 2:2, 2:7, 3:2 and 3:7. Table
1 indicates the predicted water-gas
shift conversions (xm).
Methane conversion increased, as
expected, with higher ratios. However,
methane conversion above 2.7 was
nominal. Greater ratios diverted additional
steam to the HYCO reaction,
which increased the condensate recycle
burden. The increased methane
conversion at higher steam/carbon
ratios did not offset the resulting loss
of salable contract steam at our arbitrary
30-ton/h fuel limit.
Product flows are often sensitive to
small changes in feedrates or other
47.7
10,221
Btu/lb-mole
lb/lb-mole
lbs/ft3
Btu/lb
boundary conditions. Model identification
of these break points early on can
lead to better-recommended prototypical
operating points.
When fuel heat content is not available,
as in this example, it can be estimated
from the chemical composition of
the fuel components using traditional
thermodynamic relationships. Use a
suitable spreadsheet for calculating
model fuel composition, as in our rubber
blending computation of Table 2.
The heat content for tire-grade rubber
is approximately 15,500 Btu/lb,
which compares favorably with our
estimated 15,640 Btu/lb.
Using the literature
Emissions data on SO2 and NOx may
be found in the professional literature.
In our example, we set furnace
Economics
Green projects are capital intensive
and can take years for investment
recovery. As government regulations
continue to play an increased role
in national economies, new energy
sources are needed to sustain growth.
There is a vast untapped reservoir of
man made waste, lying in the ground,
that may provide the solution to the
nation's energy appetite.
A 2010 study [1] has found that, in
comparing emissions from landfills
versus municipal waste combustion
using the U.S. Environmental Protection
Agency's life cycle assessment
(LCA) model for the range and scenarios
evaluated, waste combustion
outperforms land filling in terms of
greenhouse gas emissions regardless
of landfill gas-management techniques.
Better
methods are needed to evaluate
alternative energy sources and
processes that can increase competitiveness
in a global market. A good
process model can predict unit profCHEMICAL
ENGINEERING WWW.CHE.COM JULY 2012 41
6.3 9.3 12.8 16.2
19.4
21
Not recommended
$9.00/1,000
Profit, %
http://WWW.CHE.COM
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