Chemical Engineering July 2012 - 42

Model predicted steam generation
Engineering Practice
itability under changing market and
energy conditions.
A good rule of thumb is to begin a
profitability study at the lowest set of
process conditions attainable and then
work upwards until a positive result
can be obtained. Projects that require
extreme conditions to give a positive
return aren't worth the time involved.
If it's profitable under minimum conditions,
it's likely to be profitable
throughout. It's a handy means for
eliminating less-promising projects.
Most model packages permit data
export directly into a personal spreadsheet
already containing your project
cost macros, which greatly facilitates
economic analysis.
Using the model
EPA solid waste heating values [2]
ranging from 5,040 to 5,865 Btu/lb
were model tested to determine the
potential profitability of the example
synthetic fuel from waste process.
Model tests utilize the heat content
of EPA landfill fuels up until 2005 (the
year the EPA data tabulation ends), as
indicated in Figure 6. Although greater
temperatures than 1,600°F increase
yield, they also increase emissions.
Values above 1,900°F cause disassociation
of CaSO4 formed during sulfur
dioxide removal from the fuel, and
2,000°F and above causes the NOx
concentration to go up exponentially
in the combustion reaction.
Figure 5 plots three projected steam
margin lines for a low-heat content
waste fuel (4,466 Btu/lb). The lowest
line represents arbitrary steam pricing
of $6.95 per 1,000 lb, based on a 2002
commercial contract. The model identified
a 46% gross profit using this fuel.
However, the model profit margin decreased
with additional steam generation.
Low fuel-heating values restrict
steam generation and demonstrate the
need for a higher energy content fuel.
The inflation-adjusted line demonstrated
a satisfactory rate of return
using the same fuel. If attainable,
steam contract pricing should be set
at a rate that helps reduce pay out.
Fuel blending
Table 3 indicates a typical spreadsheet
calculation that develops fuel energy
content for the simulation. Heating
Tons waste combusted per hr: 24 - 30 Temperature: 1,500°F Methane feed, lb/h: 1,480
Methane production
100
90
80
70
60
50
40
30
20
10
Profitability
HHV
4466
6651
7480
8351
9265
10221
Table 4
Steam
profitability
(1465)
(3701)
5,040
4,466
by fuel heat contentm, HHV - Btu/lb
Landfill heat values
Gross profit
44.60
50.72
51.18
51.57
51.96
52.28
Steam margin
adjusted for
inflation
Shredded tire fuel blending
Gross profit
31.71
38.29
39.41
40.46
41.46
42.43
(1480)
2005 6,651
5,865
5,305
5,103
(370)
4,000 5,000 6,000 7,000
Fuel heating value, Btu/lb
8,000
9,000
10,000
FIGURE 6. The model-predicted steam generation by fuel heat content (HHV-Btu/lb)
suggests the best steam and methanol generation production points as a function of
fuel heat content and a 30-ton/h fuel limit. The model-calculated product quantities at
1,500°F are converted into proit margins (curved dashed lines) help the engineer assess
project viability (Source: Heat Content from Municipal Waste in the U.S., http://
www.epa.gov/msw/msw99.htm)
value is based on the standard heats
of combustion for the elements and
blend composition.
A solid waste heating value of 5,865
Btu/lb was selected for 2005 [2] from
EPA's 1989-2005 Solid Waste Heat
Content chart to represent the model
fuel base. The model fuel-base composition
matches the EPA determined
heating value.
The base heating value is EPA's
2005 [2] reported land-waste heat
content to which scrap tire rubber
was added.
Shown below is the chemical composition
of tire-grade rubber:
CH3
|
(CH2-C=CH-CH2)n
|
S
Scrap tires are assumed to contain
high moisture.
The model-blended fuel contains
2.38% sulfur, 14.67% moisture and
has an ash content of 6.41% as shown
in Table 3.
The mix is a simple ratio. For carbon
content, the 2011 blend is:
C: = 0.5 * 0.48 + 0.5 * 0.5797
= 0.5297
42 CHEMICAL ENGINEERING WWW.CHE.COM JULY 2012
Combustion
Add new fuel compositions and the
updated heating value to the Furnace
segment of the model, based on blending
computations:
Calculate the fuel fed to the furnace
at mid-range conveyor belt speed, 'S'
as a function of belt width 'w' and fuel
density 'd':
fuel = S (ft/h) * w (ft2 wide) * d (lb/ft3)
Correct the blended fuel heating value
(Lhv) for the energy lost from fuel
moisture and hydrogen content.
The heating values of Figure 6
were determined by fuel blending.
Calculated heat content appears next
to each point with blend composition
below in parenthesis. The dashed
lines represent the model-predicted
pre-tax profits, adjusted for inflation.
The upper dashed line represents
30 tons per hour fuel consumption.
The lower dashed line demonstrates
the combined effect of 20% less
throughput and a $3.56/ton fuel increase,
resulting in a 10% profitability
loss.
Fuel moisture for the tests ran between
13% and 16%, depending on
blend composition.
(10%)
(9%)
End EPA heat
content chart data
Methane feed, lb/h
(33%)
9,265
(40%)
(29%)
8,351 (30%)
(24%)
7,480 (20%)
(17%)
60
50
40
30
20
10
10,221
(50%)
Product, lb/h (x 1,000)
Gross profit, %
http://http:// http://www.epa.gov/msw/msw99.htm http://WWW.CHE.COM
Chemical Engineering July 2012

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