Section A (Answer ALL questions in this section; Total Marks: 80 marks)
There are FIVE COMPULSORY short answer questions in this section A.
Question 1 (17 marks)
Table 1 shows some excerpts from a recent newspaper.
Table 1
Excerpt 1
Excerpt 2
Country R’s 5th waste-to-energy incineration plant, when completed in 2020, will generate $500 million in revenue over a 25-year period for its Builder S, Mr. Project Manager said at the plant's groundbreaking ceremony yesterday.
The plant will be able to treat 800 tons of municipal waste per day and generate some 20 MW of green electricity, Mr. Project Manager said.
A factory here, which uses microbes to turn rotting food into energy, has won international acclaim for its eco-friendly work.
… Chief Executive Office Mr. CEO said that the factory converts 12.5 tons of food waste each hour into 3MW of energy, enough to light 5,000 four-room apartments…
a. What is the technique used by the factory in Excerpt 2 to generate energy? (1 mark)
b. Based on Excerpts 1 and 2, it can be concluded that food waste is able to generate more energy per unit ton of waste as compared to municipal waste. Explain why. (6 marks)
c. In Singapore, there has been a rising trend in using other waste treatment methods as an alternative to landfills. State TWO reasons why. (2 marks)
d. Name THREE other forms of waste treatment. (3 marks)
e.Table 2 shows the waste disposal tonnage (in thousand tons) at authorised disposal sites in Singapore over ten years.
Table 2
Year
Landfills
Incineration Plants
Total Waste Disposed
1998
958.1
1,884.10
2,842.20
1999
756.2
2,036.30
2,792.50
2000
357
2,440.20
2,797.20
2001
251.3
2,550.90
2,802.20
2002
204.3
2,421.30
2,625.60
2003
193.8
2,311.20
2,505.00
2004
219.6
2,263.00
2,482.60
2005
270.1
2,278.60
2,548.70
2006
234.5
2,329.10
2,563.60
2007
187.3
2,379.50
2,566.80
The lifespans of landfills and incineration plants are dependent on the waste disposal rates. Since year 2000, there has been a considerable decrease in the total waste disposed as shown in Table 2. Explain why. (5 marks)
Question 2 (22 marks)
a. Describe how energy can be harvested from incineration of municipal solid waste. (4 marks)
b. An incineration plant receives 100 tonnes of solid waste per day. Table 3 shows the partial composition of the waste.
Table 3
Element or compound
Weight of element or compound (tonnes/day)
Carbon
?
Hydrogen
?
Water
24.2
Bound water
20
Inert
?
Using some or all of the information and assumptions provided, answer the questions below.
The percentage reduction in mass for the incineration process is 71%
The grate residue is made up only of unburned carbon (5%) and inert material (95%)
Dry air is fed into the incinerator in excess
Total amount of water vapour produced is 57.7 tonnes/day
The process produces 88 tonnes/day of carbon dioxide and 1.96 tonnes/day of carbon monoxide
All oxygen in waste exists as “bound water” inside the combustible material
Furnace temperatures are controlled to limit the reaction of nitrogen with oxygen to form NOx. Only nitrogen gas is produced.
i) Calculate the weight of inert sent to the incinerator daily. Give your answer in tonnes/day and to the nearest ONE decimal place. Show your workings clearly. (2 marks)
ii)Calculate the weight of hydrogen sent to the incinerator daily. Give your answer in tonnes/day and to the nearest ONE decimal place. Show your workings clearly. (2 marks)
iii)Calculate the weight of carbon sent to the incinerator daily. Give your answer in tonnes/day and to the nearest ONE decimal place. Show your workings clearly. (6 marks)
Atomic mass of carbon: 12 g/mol
Atomic mass of oxygen: 16 g/mol
c) A mixture of municipal solid waste has a heating value of 1800 MJ/tonne. The solid waste was sent to an incineration plant which has an energy conversion system with a capacity of 500 tonnes/d. The schematic of the incineration system is shown in Figure 1. The efficiency factors for boiler, steam turbine and electric generator are 80%, 50%, and 95%, respectively. Assuming the mechanical equivalent of heat is 3600 kJ/kWh, estimate:
i) the amount of electrical power produced in kW (6 marks)
ii) the overall energy recovery efficiency (2 marks)
Show your workings clearly.
Figure 1
Question 3 (15 marks)
Food waste collected from a city and sludge from a wastewater treatment plant are mixed together and fed into an anaerobic digester.
The digester requires the feedstock to have a C/N ratio of 20 to 30 to function optimally.
Table 4 provides information about a mixture (Feedstock 1) that was fed into the digester during a test run.
Table 4
Component
Mass (kg)
Amount of nitrogen (% by mass)
C/N ratio
Sludge
10
5
6
Food waste
10
0.6
40
a)The anaerobic digestion process can be split into 4 stages. Name the stage at which complex organic compounds are broken down into simpler compounds. (1 mark)
b)Explain what would happen if the C/N ratio in the feedstock is too high. (2 marks)
c)The digester did not function well when Feedstock 1 was used. Calculate the C/N ratio for Feedstock 1 and explain why Feedstock 1 was not suitable for anaerobic digestion. Show your workings clearly. (6 marks)
d)A new feedstock, Feedstock 2, is to be prepared by changing the mass of food waste used. Table 5 shows the information for Feedstock 2.
Table 5
Component
Mass (kg)
Amount of nitrogen (% by mass)
C/N ratio
Sludge
10
5
6
Food waste
X
0.6
40
Calculate the range of values of “X” such that the digester can function optimally. Give your answers to ONE decimal place. Show your workings clearly. (6 marks)
Question 4 (17 marks)
a) Name TWO methods for landfill operation. (2 marks)
b) Proper education is important to run a landfill. Describe FIVE types of skills / knowledge required for good landfill management and operation. (5 marks)
c) A simplified landfill diagram is shown in Figure 2. Identify the key features of the landfill labelled X, Y, and Z. (3 marks)
Figure 2
d)Four sites (A, B, C and D) were surveyed as possible locations for a new landfill. Table 6 shows the different conditions of the four sites.
Table 6
Site
A
B
C
D
Distance from nearest residential area
10 m
1000 m
1000 m
1000 m
Soil type
Clay
Clay
Loamy sand
Loamy sand
Hydro-geologic conditions
Groundwater located more than 20 m below surface
Groundwater located more than 100 m below surface
Groundwater located 20 m below surface
Groundwater located more than 100 m below surface
Identify the most suitable landfill site based on the information given and state THREE reasons to justify your choice. (4 marks)
e) Solid waste collected from a town is compacted and buried in a landfill. Use the information in Table 7 to calculate the volume of compacted waste generated over 20 years. Assume that the amount of waste generated yearly remains the same for 20 years. Show your workings clearly. (3 marks)
Table 7
Amount of waste generated yearly (kg/year)
1,000,000
Density of uncompacted waste (kg/m3)
125
Waste compaction ratio
3.2
Planned lifespan of landfill (years)
25
Question 5 (9 marks)
a) A town intends to build a new incinerator to treat its municipal solid waste. The incinerator has an expected lifespan of 12 years. The cost estimates are given in Table 8 below.
Table 8
Incinerator construction cost
$7,500,000
Operation and maintenance costs per year
$1,000,000
The town chooses to pay the loan back in 12 annual instalments at an interest rate of 6.125%. How much will this incinerator cost the town every year? Show your workings clearly. (5 marks)
b) Name FOUR factors that should be considered when determining the potential of recovery of energy from wastes. (4 marks)
Section B (Total Marks: 20 marks)
There is ONE COMPULSORY essay question in this section B.
Describe the environmental and/or operational issues in using the following for energy recovery from waste.
i) landfill (10 marks)
ii)incineration (10 marks)