Kame Khouzam
Queensland University of Technology Research Concentration in Electrical Energy School of Electrical and Electronic Systems Engineering
2 George St., Brisbane, Queensland 4000, Australia Tel. 61-7-3864-2483, Fax. 61-7-3864-1516
E-mail. k.khouzam@qut.edu.au
Abstract
The disposal of waste is a problem for all developed nations with the amount of waste tending to increase in proportion to the affluence of the population. Though, it is becoming increasingly important to conserve scarce resources by recycling, the potential to convert waste into fuel and energy can be realised. A study into the feasibility of an energy from waste facility in south-east Queensland indicate that energy generated via combustion of municipal solid waste of high calorific value offer multiple economic and environmental advantages. Emissions resulting from incineration must be monitored and controlled in order to meet environmental standards.
1
INTRODUCTION
A number of factors in recent years have been pushing policy makers and energy experts worldwide to consider alternative means to generate energy. Energy from waste is one of an array of solutions to the increasing demand for energy as well as to the increasing amounts of waste produced by urban living. The conversion of waste into fuel and energy solves a number of interrelated objectives. The implementation of fuel from waste plants reduces the existence of potential environmentally hazardous situations. Energy from waste schemes also provide a useful contribution to waste management strategies. In addition, they can contribute to reduction in dependence on fossil fuel and associated environmental problems.
In the year to June 1995 the city of Brisbane generated close to 800,000 tonnes of waste material which was directed to transfer stations, material recovery facilities, and landfill sites, and hardfill sites. Included in this total was 321,000 t of household refuse and 309,000 t from commercial and industrial operations. The Brisbane City Council (BCC) diverted 57,000 t of material mainly glass, newspaper and steel products -into recycling. It is worth adding that since 1990 the BCC has reduced the amount of waste going to landfill from 890,000 t to about 630,500 t. The BCC hopes that by the year 2000 it will achieve a 50% reduction in the volume of material consigned to landfill.
The objective of this study is to investigate the economic viability of an energy from waste facility in south-east Queensland for the purpose of electric power generation.
2
NATURE AND COMPOSITION OF WASTE
The feasibility of energy recovery through incineration depends primarily on the properties of refuse. The process can be most economical if the combustion is autothermic, ie. if combustion is sustained without addition of auxiliary fuel once ignited. The properties that determine the suitability of the refuse are the calorific value, moisture content, and combustible versus non-combustible content. The most suitable source of combustible waste for incineration comes from commercial and industrial sources. Table 1 gives a classification of the waste collected by BCC in 1995. The data shows that paper, cardboard, plastic, wood and textile constitute a large percentage of combustible refuse of high calorific value. These refuse amount to a total of 333,964 tonne per year.
Table 1. Composition of waste directed to BCC landfill in 1995 (Brisbane City Council).
Waste Type Weight
(tonne) Percent (%) Moisture Content (% wt) Ash Content (% wt) Calorific Value (kJ/kg) Paper and Cardboard 190,901 30.28 33.50 1.7 18,000 Textiles 24,519 3.89 29.60 3.0 21,000 Glass 19,266 3.06 - - -Plastics 56,044 8.89 7.50 0.4 39,000 Green waste 75,000 11.90 76.50 2.7 20,050 Food waste 126,100 20.00 76.50 2.7 20,050 Timber 62,500 9.91 15.00 1.0 16,000 Metal 31,525 5.00 - - -Others 44,645 7.08 - -
-The output energy of the plant can be estimated using the amount, composition and calorific values of the waste, the efficiency of the incinerator and the heat rate of turbine.
3
INCINERATOR SYSTEMS
There are five basic types of incinerators: mass burn, modular mass burns, refuse derived fuel (RDF), refuse derived fuel plus coal, and fluidised bed-burning.
Mass burn incinerators are large burning facilities designed for use with municipal solid waste with virtually no processing. Modular incinerators are usually smaller in scale and can burn solid waste. This type offers flexibility in use and design. Refuse derived fuel incinerators typically utilise shredded waste from which heavier, non-combustible items such as glass and metal are removed. The volume reduction for RDF system would therefore exceed that of a comparably sized mass burn systems. RDF is frequently burned in standard or modified utility burners, typically mixed with coal. The less commonly used design is fluidised-bed incinerators, which typically burn a waste stream that has undergone at least some pre-processing and may simultaneously burn other wastes such as sewage sludge.
The process of incineration involves a number of processes, namely combustion control, pollution control, and ash residue handling. In addition, energy recovery or co-generation requires a boiler and a turbine-generator system.
3.1
Combustion Control
Modern incinerators are equipped with computerised combustion control systems designed to allow adjustment to the rate at which waste is fed into the incinerator and the amount of air in the combustion chamber. By controlling the combustion conditions, the operator can maintain high efficiency of combustion, maximise volume reduction, and reduce the formation of products of incomplete combustion such as dioxins.
3.2
Air Pollution Control
Pollution control is an essential part of the incineration process to meet the increasingly stringent emission controls. Different configurations and air pollution control measures are used depending on the nature of the gas being produced and its effect on the environment. Virtually all incinerators are equipped with particulate control devices, ranging from wet scrubbers to electrostatic precipitators and fabric filters. New facilities also incorporate acid-gas scrubbers to remove hydrochloric acid and sulfur dioxide. In a few cases, nitrogen oxide controls are used.
3.3
Ash Residue Handling
3.4
Boiler
The boiler makes use of heat released and converts it to steam for thermal applications and for generation of electricity. Because efficiency varies continuously with loads and stack draft, boilers are normally designed and selected to operate at maximum efficiency when running at their rated output. A multiple of boiler units, each having less capacity should be used to increase efficiency during seasonal variations. Each unit will be fired at full capacity only when required and fluctuations of load would be met by firing fewer boilers longer. Modular boilers also allow rapid response and low heat-up and cool-down losses. Preheating of primary air will reduce cooling effect and inside the combustion chamber and increase efficiency.
3.5
Turbine-Generator
Two types of turbine plant are suitable for low-grade fuel: the back-pressure steam turbine and the extraction-condensing steam turbine (Rohrer 1996). The advantages and disadvantages of both systems are given in table 2.
Table 2. Comparison of back-pressure and extraction-condensing steam turbine systems.
Type of plant Advantages Disadvantages
Back-pressure steam turbine • High fuel utilisation • Simple plant design
• Limited flexibility in design and operation Extraction-condensing steam
turbine
• Highly flexible in design and operation
• Expensive design • High cooling-water
demand
4
INCINERATION RESIDUALS
Waste that enters an incinerator’s combustion chamber exit in a number of the following forms (Denson et
al, 1990):
• As combustion gases, such as carbon dioxide, water, nitrogen oxides, sulfur dioxide and hydrochloric acid. These gases exit via the stack except to the extent that some may be removed by air pollution control.
• As emissions of particulate material, such as oxides, salts, silicon or aluminium. These are lightweight particles that are borne out of the chamber along with combustion gases. This may be small enough to escape the air pollution control devices.
• As fly ash, made up of particulate material of oxides or salts light enough to be borne upward with the gases but heavy enough to fall out as the gases cool before leaving the stack or large enough to be captured by the air pollution control devices.
• As bottom ash, which is solid material composed of non-combust waste such as glass and metal that passes through the combustion chamber on the grates, and is usually conveyed to a water-filled pit for quenching.
Depending on the composition of the waste and the combustion process, the actual volume reductions may vary tremendously. A reasonable rule-of-thumb estimates are 80 to 90% reduction by volume and 65 to 75% reduction by weight. Other residuals which are by-products of the incinerator are:
• Contaminated liquid effluent which may be generated by the boiler or scrubber and discharged with or without treatment.
• Quench water used to cool the ash as it exists the furnace. This may contain high levels of salt and heavy metals dissolved in the ash.
Proper design can minimize the need to discharge the water and other liquids. Water recycling and the use of scrubber effluent may result in no discharge. Reduction in weight will have an effect on ash disposal charges.
5
AIR POLLUTION CONTROL SYSTEM
Air pollution control systems are designed to reduce or eliminate the pollutants present in the refuse. Table 3 gives a summary of the potential hazardous chemicals and the method used to control their emissions (Wheelabrator, 1989).
Table 3. Pollutants associated with municipal solid waste incineration.
Pollutant type Pollution Control
Light hydrocarbon waste Incineration and energy recovery is advantageous if not chlorinated. (chlorination would result in tube corrosion). Sulfur compounds Sodium hydroxide scrubbing is a traditional method. This is
usually done before incineration. Nitric and hydrofluoric acid
mixtures
Batch neutralisation with limestone is a traditional method resulting in calcium fluoride and calcium nitrate sludge.
Carbon monoxide Found to be lower than coal.
Anaerobic odours Pond chlorination which oxides the odour compounds and control bacteria. Fabric filters can remove up to 90% of organic
emissions. Hydrogen Chloride and Hydrogen
Fluoride
Can be suppressed using calcium-compound additives. Sophisticated acid-gas scrubbers can remove 90% of these compounds.
Sulfur dioxide Advanced acid-gas scrubbers can remove more than 60%. Trace metals in flue gases Fabric filters can remove up to 90%.
Fly ash Sorting and elimination of waste material which contain high
levels of lead and cadmium will reduce the toxicity of fly ash.
Other contaminants which exist in the waste and must be eliminated or controlled during the process include lead, mercury, dioxin compounds, furans and PCBs.
To ensure that emissions from the incineration remain clean, monitoring systems must continuously sample and analyse the emissions of the multitude of chemicals and contaminants. The capital cost of the pollution control and monitoring system can be as high as 30% of the total cost of the incinerator and its running cost is known to be high.
6
ECONOMIC ANALYSIS
This section presents the basic economic concepts required to calculate the long-term costs of the plant. It must be noted that long-term projections of future costs naturally brings a variety of uncertainties. Life-cycle analysis accounts for all revenues and expenditures that come into play over the lifetime of the plant. The main cost components are:
• Capital expenditures: include construction of the plant, air pollution control, ash disposal facility, cranes, trucks and waste sorting equipment. These costs are recouped over a number of years.
• Recurring costs: include operation and maintenance, air pollution control and monitoring, labor, fuel, insurance and residue disposal. These costs are paid as they arise.
The expected revenues of the plant will come from sale of energy, tipping charges and sale of recyclable and ferrous materials. In total, the economic performance will depend on the combined performance of the operating and maintenance costs and the performance of the revenues.
Due to the ever present fact of inflation, the cost of different components is expected to change. A levelized net present value can be expressed as:
V
PV
r
i
i
r
N=
+
−
+
+
(
)
(
1
)
1
1
1
(1)where; V = initial year value, r = discount rate, i = inflation rate, N = number of years and PV = present value. Table 4 gives the assumptions made in this study including the finance of the project.
The total construction cost including inflation over three years of construction time is calculated at $97.394 million. The net interest that must be paid to bond holders during construction is $14.609 million. Based on a 4% the bond issuance cost is estimated at $4.667 million which brings the total construction cost to $116.67 million (1999). The proceeds from the bond issue can be placed in interest-earning accounts until the expenditures occur. Construction cost repayment are calculated as equal annual instalments over 20 years at a rate of 10%. Operation and maintenance expenses are $5 million (1996) per year at a rate of inflation of 4%. Ash disposal is estimated at $55 per tonne (1996) at an inflation of 4%. Sales of electricity is $0.04 per kWh and increase at 4% per year. No assumptions are made for the sale of recyclable or ferrous metals. The tipping fees are estimated after subtracting the annual revenues from the running costs. It is found that tipping fees has to be inflated at a rate of 0.61% annually. A 20 year plant life is estimated during which all expenses must be repaid. No additional taxes or incentives are assumed.
Table 4. Economic assumptions and financing of the incinerator plant.
Item Cost General capital Construction cost Inflation rate Interest rate Discount rate Bond issuance cost
$90 million (not including cost of site) 4.0% 10.0% 10.0% 4.0% General recurring Incinerator life Operation and maintenance Ash disposal cost Incinerator capacity Capacity factor
Ash fraction by weight Waste volume reduction Waste weight reduction
20 years
$5 million per year $55 per tonne ash
800 tonne per day (only paper and cardboard, plastics, timber and textiles are considered here)
84% 28.45% 90% 75% Revenues Electricity price
Electricity price inflation Net generation
Minimum tipping fee
$0.04 per kWh 4.0% per year
678 kWh per ton of refuse incinerated
calculated on the basis of running cost - revenue of energy sales
6.1
Net Present Value
The net present value (NPV) is calculated using:
NPV = - Bond payment - Operation and Maintenance cost - Disposal cost + Electricity sales + tipping revenues (2)
The NPV for the project is estimated at $28.576 million indicating that the investment is economically viable.
6.2
Internal Rate Of Return
The internal rate of return (IRR) is used to check whether the investment is paying back itself. This can be done by iteration to calculate the discount rate which gives a zero value of NPV. In this analysis IRR is found to be 12.79%.
6.3
Payback period
Payback period (PBP) is used to estimate the length of time needed to make the investment pay itself back. This can be done by making the NPV zero through the iterative process. The PBP of the investment is 13.41 years.
6.4
Sensitivity Analysis
Analysis is performed to see the effect of small changes on NPV. The most sensitive variables are tipping fees, sales of electricity, and capital cost. The investment, however, will still be viable if only one variable changes by ±19%.
Levelization methods can be used to determine the annual per-ton tipping fees that have the same present value. In constant terms the tipping fee is $61.83 per ton. If the tipping fee is decreased by 20%, the NPV will become negative making the investment not viable.
7
SITING
One of the most complex and controversial step in devising a solid waste management plan is site selection. The important factors for selection are environmental and economical. Environmental factors reflect the physical characteristics of surrounding air, ground water and surface water. Environmental factors also require consideration of vulnerable groups within the population. Considering all factors being equal, a less expensive site will then be preferred. Economic criteria must take into account the direct expense of the site and any necessary modifications including easement for high voltage power lines. Effects on adjacent property values may have to be acknowledged.
8
CONCLUSION
The paper presents a study into the feasibility of an incinerator facility in south-east Queensland for the purpose of power generation. Solid waste can be burned to produce electricity or steam for sale, but energy revenues alone will not nearly cover the full costs of construction and operation of the plant and disposal of ash. Tipping charges will help cover the operating costs and raise profit. The economic analysis reveals that the project is economically viable and yields a pay back period of less than 14 years. The proposed plant will have two generating units rated at 10 MW each with an average overall conversion efficiency of 10%. A modular type incinerator can be used to handle a total of 2 x 400 tonne of waste per day.
Although the study included the cost of the air pollution control it may be advantageous to use a computer package to estimate the amount of pollutants and contaminants associated with the plant. The ultimate decision to carry out the project is dependent on the availability of funds, policies, incentives, social acceptance and environmental issues.
9
ACKNOWLEDGMENTS
This project has been proposed by Mr. Bill Croft of Acer-Wargon Chapman. The computational work in this paper was performed by L.J. Wong, W.T. Gan, and T.G. Seah. The Brisbane City Council provided the data for the study.
10
REFERENCES
Brisbane City Council, Business opportunities in waste, Waste Management Branch, Brisbane, Queensland.
Bullock P. (1994), Energy from municipal solid waste: Discussion paper 4, Department of Primary Industry and Energy. Croft B. (1996), Generation of electric power from the incineration of south east Queensland municipal waste - issues
paper, Acer-Wargon Chapman - QIDC, Brisbane, Queensland.
Denson R.A. and Ruston J. (1990), Recycling and incineration: evaluating the choices, Island Press, Washington. Jungmann G. (1993), Pollutant emissions reduced by retrofitting waste incineration plants, ABB Review 2/93, pp.15-19. Keong W.W. (1994), Municipal incinerator operational and economic analysis, Nanyang Technological University, Singapore.
Rae G.W. (1994), Waste to energy: the new frontier?, Engineering Science and Education Journal, pp.105-111. Rohrer A. (1996), Comparison of combined heat and power generation plants, ABB Review 3/96, pp.25-32. Wheelabrator Technologies Inc (1989), Annual report, New technology keeps air clean, USA.
APPENDIX: INCINERATOR PLANT DATA
Technical assumptions for a proposed incinerator plant for electricity generation in south-east Queensland.
General Area served Financing Brisbane City Bonds issue Capacity Storage pit Nominal capacity Availability 8,000 cubic metres 800 tonnes per day 84%
Refuse handling
Weighbridges
Refuse discharge bays Refuse cranes
3 x 40 tonnes capacity each 8
3 x 10 cubic metres overhead grab cranes
Furnace design - combustion
Type of system Incinerator Boiler Operation Feed system Grate design Combustion temperature Auxiliary fuel
Waste volume reduction Waste weight reduction Cooling system
Mass burning with integral water-wall boilers 2 x 400 tonnes per day
2 x 4 pass steam
24 hours per day, 365 days per year 3 overhead refuse cranes with ram feeder reciprocating
1000 °C Natural gas 90% 75%
Closed loop treated water
Steam boiler design
Type Steam output Pressure Temperature Steam condensation Cooling system 2 x 4 pass 2 x 77 t/h 43 bar 415 °C
Air cooled condensers with 8 fans each Closed loop treated water
Turbine generator
Type Capacity Customer
Condensing turbine generator 2 x 10 MW
SEQEB
Flue gas handling design
Type of equipment Chimney
2 x reactor for flue gas treatment
1 concrete with ceramic brick inner-lining
Electrostatic precipitator design
Type of equipment Separation efficiency
2 x 3 zone electrostatic precipitators 99.5%
Ash handling design
Type
Cooling system
2 pusher-type ash extractors, semi-dry vibrating conveyors Closed loop treated water
Scrap metal recovery
Type
Materials recovery
2 x magnetic separators Ferrous, aggregate
