AN ECONOMIC EVALUATION FOR SMALL SCALE THERMAL PLASMA WASTETO-ENERGY SYSTEMS
1
I.J. van der Walt1, J.T. Nel1, D. Glasser2, D. Hildebrandt2, L. Ngubevana2
The South African Nuclear Energy Corporation SOC Ltd. (Necsa), Elias Motsoaledi Street Ext., (Church
Street West), Pelindaba, Brits Magisterial District, North-West Province, South Africa.
2
University of South Africa,Chemical Engineering Department, Cnr. Christiaan de Wet Road & Pioneer
Avenue, Florida, Gauteng, South Africa
Abstract: A combined small scale plasma-gasification system, developed by Necsa, and a Fischer-Tropsch
syngas system, developed by Wits and Unisa, is being developed in order to determine the economic viability of
such a system. A techno-economic model was developed and different scenarios will be discussed to describe
the influence of different waste types and industry-specific influences.
Keywords: Plasma gasification, plasma economic viability, small scale Fischer Tropsch.
Background
Experimentswith various types of biomass proved
that when different feed materials are gasified by
athermal plasma, the product mixture can be
changed by manipulating the plasma conditions in
order to generate a good quality syngas. The
compressed syngas is purified from any impurities
and in one integrated process Fischer-Tropsch (FT)
synthesis can be used to synthesize solid (wax),
liquid and volatile hydrocarbon products. Coal
gasification via the FT synthesis process is a wellknown process and economically viable on a large
scale[1]. Small scale portable plasma/FT systems
are being envisaged for use in rural areas in South
Africa. Economics of scale usually favours big
systems, but the need arose for application of micro
scale systems that are situated closer to the user, or
in rural areas. These systems could range in size
from 1 to 100 ton of biomass feed material per day
(tpd).In order to commercialize these smaller
systems the limiting factors for downscaling needs
to be determined.It has been found that heat losses
and the expensive nature of recycle loops are
responsible for low efficiency in small systems.
However, by intelligent system integration these
losses can be reduced and this can consequently
reduce the size of a plant that can still be
economically feasible.
It is well-known that the scale of operation
determines the economic viability of a
plant.Therefore the design of a small system must
be simplified and optimised to determine the lower
limit. A techno-economic model was developed to
evaluate different scenarios in order to determine
the minimum plant size that will still be
economicallyviable. According to this model a
1 tpd waste plant for organic or municipal waste is
not feasible, but for medical waste, a positive
internal rate of return (IRR)can be achieved. On the
other hand a 100 tpd waste plant will be viableeven
if only electricity is produced.
Plasma gasification
The process of plasma gasification of municipal
and other types of waste is not new [2-5].
Commercial systems are available worldwide and
many plasma research projects are currently being
conducted to cater for the treatment of various
types of waste.Unfortunately, the different waste
types differ significantly from each otherin respect
of physical form, moisture content, ash content,
waste quantities, waste toxicity and conventional
treatment cost. These differences largely influence
the plasma reactor design, the feed system and offgas treatment.For this reason one could not apply a
single design for all waste treatment cases. The
advantage of the plasma gasifier is the concentrated
high energy source that is injected into a relatively
small reactor. Although the reactor`s mean
temperature is still 1000 – 1300ºC, the turbulent
action by the plasma torch increases the heat
transfer as well as causes the waste to pass through
the 2000 to 6000ºC plasma tail flame (Figure 1).
Solids
feeder inlet
Plasma inlet
Reagent inlet
Figure 1: Turbulent action inside the plasma
gasification reactor
Solid particles
feeder inlet
Plasma inlet
Reagent inlet
Figure 2: Particle trajectory inside the plasma
gasification reactor
In addition to this unique process, the off-gas exits
the reactor through a high efficiency self-cleaning
quench probe in order to form syngas (CO and H2)
and small quantities of CH4 and CO2. This very
high quench rate of the plasma product gas
prevents the formation of longer carbon chain
products and toxic products. Typicallythe
composition of the gas product is 22% H2, 23%
CO, 1.2% CH4, 2.8% CO2, 0.5% acetylene, 0.2%
C2H6 and 0.2% O2in 50% N2 from the plasma feed.
One of the most common final products that are
produced by the plasma waste-to-energysystem is
electricity from syngas. This is the product that
could most easily be used by society. A typical
plasma gasification system would consist of the
items as indicated in Table 1.
Determination of the total cost must however,
include establishment costs (Table 2). The cost
contribution of these items is determined by adding
a certain percentage to the capital cost in Table 1.
These ratios are obtained by evaluating the
sophistication and complexity of the plant. Typical
percentages are given in Table 2.
Table 1: The different components of a plasma
gasifier
Plasma gasifier components
Solids feed system
Shredder
Gasification reactor
Power Supply
Torch
Quench system
Refractory
Filters
Scrubber system/s
Utilities
PSA for enriched air
Argon supply
Flare
Control System PLC plus programming
Power generator
Table 2:Establishment costs associated with the
capital cost of the plant
Cost item
%
12
Installation costs
9
Instrumentation
10
Valves and piping
15
Electrical
equipment
and
connections
10
Buildings Incl. offices, stores, etc.
5
Yard improvements
4
Service Facilities
5
Engineering and Supervision
5
Construction Expenses
4
Contractors Fees
2
Land
10
Contingency
Although the production of electricity could benefit
a small rural community, the selling value is low
compared to the price of fuel. For this reason the
synthesis of hydrocarbon fuel was investigated
using a combination of plasma gasification and
Fischer-Tropsch.
Fischer-Tropsch(FT) synthesis
The FT process was developed almost a hundred
years ago in Germany and commercialised by
companies such as SASOL, to produce
hydrocarbon compounds for example synthetic
crude oil, waxes or gases. These products can be
refined in order to produce products such as
synthetic diesel, paraffin, petrol, methane and
propane[6].
After scrubbing, the moist gas is compressed to a
pressure of 4 to 10 bar and dried before it is passed
through a gas clean-up trap where activated carbon
removes any traces of sulphur. The clean and
sulphur-free syngas is now passed through the
catalyst bed where the long-chain hydrocarbon
synthesis starts. To increase conversion efficiency,
more catalyst bed reactors could be installed in
series. Each reactor could be fitted with
condensers/separators where a specific product
range is extracted.
E+F
1
4
E+F
10
18.2
E+F
100
94.3
E = electricity only, E + F = electricity and fuel
After the last FT reactor some un-converted gas
along with gaseous hydrocarbon products could be
used in a gas generator, generating electricity.
In order to calculate the economic viability of the
fuel production system the real selling price of the
commercially available diesel was used. The
reasoning was that a company, land fill site or a
farmer who operates this system will produce fuel
and electricity for his own consumption and
therefore saves the real commercial selling price.
The current commercial diesel price is ~R11.50. In
addition to fuel, the wax could be sold to a refinery
for ~R24.50/kg.
A typical syncrude composition could be 40% wax,
a 50% diesel fraction and a 10% light fraction.
The typical FT system would consist of various
components as indicated in Table 3.
Table 3: The different components of a FT system
FT components
Syngas compressor
Gas clean-up reactor
FT reactor
Wax collection
Liquid knock-out pot
Gas collection
Product storage
Heater
Heat exchangers
Although the equipment cost for the FT system will
be added to the gasifier cost, the additional cost
items will be similar for the FT system than for the
plasma gasifier and will therefore be calculated for
the combined cost.
Economic analysis
Two scenarios could be considered namely a
system that generates only electricity and a system
that generates electricity and mainly liquid fuel.
Table 4 presents theequipment cost for three size
systems namely 1, 10 and 100 ton waste feed per
day.
Table 4: Estimated equipment cost for a plasma
Waste-to-Energy systems
Plant
Output
Equipment cost
tpd waste
1
10
100
R/million
E
E
E
2.9
11.6
60
In order to calculate the economic viability of the
electricity system, the electricity cost in South
Africa was taken to vary between R0.70 and
R1.4/kWh. For the purpose of this paper the lower
cost will be used.
If a scaling factor of 0.65 was used, a mark-up of
30%, a 12.76% discount factor, a 28% tax rate, 6%
inflation and 8% interest, the techno-economic
indicators were calculated and are presented in
Table 5.
Table 5:The techno-economic indicators for
different size plants
Planttp Input
IRR
NPV
Payback
d
R/t
%
R/Mil
Years
-6000
13.8
0.5
7
1E
-200
15.8
3.5
6
10E
0
38.6
197.5
2.1
100E
-5000
15.5
1
7.3
1EF
0
32.1
46.3
3.4
10EF
0
66.2
853
1.3
100EF
E = electricity only, EF = electricity and fuel
The fixed and running costs were taken into
account when the data was calculated.
The input column indicates the savings earned that
a client would pay to dispose of his waste. For
municipal waste this can be R800/ton and for
medical waste R20 000/ton. This column was
modified to get anIRR of >13%. The high values in
the input columns for the 1 tpd systems indicate
that this size system must be used for waste types
that has a high disposal cost when using
conventional methods. Chemical waste, medical
waste and pharmaceutical waste are examples of
these.
The IRR is generally higher for the electricity-andfuel plant if compared to the electricity only plant.
The IRR for the 100 tpd electricity plant is 38.6%
lower than for the same size plant generating and
selling both electricity and fuel. The net present
value (NPV) after 10 years is also significantly
higher in the latter case. Consequently the payback
period for these plants is also shorter.
After performing a sensitivity analysis it was clear
that the product selling price, the negative cost for
feed and the discount factor was very sensitive to
change. The capital cost on the other hand was not.
A few viable scenarios will now be discussed. The
first scenario is for the medical waste industry.
From various sources the cost for disposing of
medical waste may vary between kR20 - kR40/t. If
the conservative case (kR20/t) is taken and this
waste is converted in a 1 tpd system and only
electricity is produced then the IRR of such a
system is 99.8 % with a payback in 1.8 years. This
1 ton-per-day system could be operated on the
premises of a hospital and can save a hospital R7.3
million/a. The plasma gasification gives the
advantage that the metal ‘sharps’(needles, etc.) as
well as other hazardous waste could be disposed of
in a very safe manner and no secondary toxic
compounds are generated in the process.
The second scenario is the application of a 10 tpd
system for organic material (biomass) on a farm or
sawmill. In this application the farmer purchase a
system, feed it with typical biomass waste that is
normally produced on the farmand then produces
enough electricity to cater for his needs as well as
producing fuel for the farm implements. The farmer
will pay his system off in 3.4 years by saving on
electricity and fuel cost. In current terms, his plant
will be worth R46.3 mil after 20 years.
The third scenario is the well-known application of
a plasma gasification system on a land fill site.
According to the calculations (Table 5) the
electricity option is already lucrative. If the option
is exercised to produce fuel as well, the IRR could
almost be doubled.
Conclusions
Plasma gasification is a viable option for waste
treatment. The advantages that plasma gasification
offers outweighs most other conventional
technologies in that the system has a relatively
small footprint, can treat unsorted waste and can
produce good quality syngas without extremely
toxic by-products.
Plasma gasification systems connected to either a
generator for electricity production or to a FT
system for fuel production can be economically
viable. Depending on the waste type and the
conventional cost of disposal, the plasma
gasification system can be economically viable
even on a small scale such a 1 tpd. For application
on a land fill site or on a farm the small
conventional cost of disposal causes the minimum
size to increase in order to render an economical
process. For these types of applications at least a
10tpd system is proposed.
References
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Purdue University, 2007.
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Technology for Industry
http://www.inentecchemical.com/.
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http://www.pyrogenesis.com/
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division
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Alter
NRG
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http://www.westinghouse-plasma.com/.
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