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Use of Municipal Solid Waste for Integrated Cement Production.

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Dev. Chem. Eng. Mineral Process. l4(1/2), pp. 193-202, 2006.
Use of Municipal Solid Waste for Integrated
Cement Production
W.H. Cheung, K.K.H. Choy,D.C.W. Hui, J.F. Porter
and G . McKay*
Department of Chemical Engineering, Hong Kong University of
Science and Technology, Clear Water Bay, Kowloon, Hong Kong
A Waste Reduction Framework Plan was initiated by the Hong Kong Environmental
Protection Department in 1998 to set out a I0-year programme and a set of targets
for waste reduction. By combining cement production and power generation, waste
incineration has improved its competitiveness. The co-combustion of Municipal Solid
Waste (MSW) is a novel and highly integrated design combining cement and
electricity productions (Co-Co process). By carefully integrating these three
processes, energy generated from MS W co-combustion can be used eficiently for
cement production and electricity generation. In this process, MSW is incinerated in a
rotary kiln with additionalfuel followed by the secondary combustion unit ( X u ) and
extra fuel is burnt in the SCU to raise the combustion temperature to 1200°C. Flue
gas exiting porn the SCU will then enter a calcinations scrubber for acid gas
removal. The flue gas then enters into a waste heat boiler f o r steam generation. A
pilot plant is being designed and will be constructed to test this design.
Introduction
Due to rapid growth in the economy and population in the last decade in South China,
electricity and cement demands have been increasing rapidly. These two processes
require an enormous amount of energy and generate huge amounts of solid and
gaseous wastes. Due to the population growth, the amount of municipal solid waste
(MSW) generated is increasing rapidly to a level that landfilling is no longer feasible,
or suitable for handling all the materials. For example, in Hong Kong, there are three
landfill sites for 6.8 million people; the construction of the three sites cost US$750
million, the total area of the 270 hectares is worth US$lOOO million as agricultural
land and over US$3000 million as prime real estate. The environmental problems in
terms of gaseous emissions and leachate containment and treatment are also very
significant and expensive [I].
* Author for correspondence (kemckayg@ust.hk).
193
W.H. Cheung, K.K.H. Choy, D. C. W Hui, J.F. Porter and G. McKay
In Hong Kong, the amount of MSW transported to landfills has increased steadily
at a rate of 5% per year since 1985. The landfill sites will be filled up in 10 years if
this growth continues. A Waste Reduction Framework Plan was initiated by the HK
EPD in 1998 to set out a 10-year programme and targets for waste reduction, Various
waste reductionlmanagement processes are proposed, one of these is co-combustion.
However, co-combustion alone may not be attractive enough without waste recycling
in the front end.
In the 1970s there was a significant move towards the use of incineration for
MSW treatment. However, due to lack of knowledge and understanding relating to
emissions problems and poor incinerator design and operation, these early MSW
incinerators developed a bad environmental reputation [2-51. Due to the limited
economic benefits of separation and recycling [6, 71 and significant improvements in
incinerator design [8], resource recovery in the form of heat and power production has
again gained favour in the past 20 years [9-121. The benefits of modem incineration
plants are:
0
The volume and mass of the MSW is significantly reduced (85-90% by volume).
8
The carefid selection of incinerator site locations reduces transportation costs.
0
8
0
The waste reduction is immediate and not dependent on long biological reaction
times.
The energy sales from heat recovery offset the operating costs.
Air emissions can be controlled to achieve the strict BATNEEC incinerator
guidelines.
However, conventional MSW incineration does have the following drawbacks:
All the MSW is incinerated, whereas some components are more valuable for
recycling [ 13, 141.
Poor incineration practices and MSW components containing chlorine may lead to
highly toxic dioxin and furan emissions, sulphur components lead to SOX
emissions.
Controlling metal emissions is problematic, these include arsenic., cadmium,
chromium, copper, lead, and mercury.
Expensive primary fuels are frequently required to achieve the high combustion
temperatures.
There is 10-15% by volume of ash produced for landfill disposal.
Limited thermal efficiencies in converting the heat energy to electrical power.
Industries in Hong Kong have been continuously moving north to the Pearl River
Delta region in the last two decades, particularly those that are labour intensive and
environmentally sensitive. The cement industry is one of these examples. At present,
there is only one cement manufacturing facility remaining in Hong Kang but it is
facing intensive competition from other cement producers in the nearby regions. To
improve the competitiveness and reduce the production costs, the company has been
actively involved in looking for opportunities in which cement production facilities
I94
Use of Municipal Solid Wastefor Integrated Cement Production
and knowledge can be applied to achieve a reduction in production costs and identify
alternative raw materials. Currently, fly ash and gypsum (a waste product from flue
gas desulphurisation) from local power stations and supplementary fuel derived from
waste marine oil and lubricants have been used in cement production. Hence, by
combining cement production and power generation, waste incineration has improved
its competitiveness dramatically. The present study investigates the application of
MSW incineration as an integral component of a cement manufacturing facility, and
discusses how several natural features of the cement production process combine
synergistically with MSW incineration to minimise and even take advantage of its
drawbacks. The integrated process currently under construction at pilot-plant scale is
described, followed by a discussion on how this novel process achieves optimum
waste minimisation. Finally, we summarise the benefits of a full-scale integrated
cement production facility with MSW incineration.
Process Description
This novel process consists of several units including Material Recovery and
Recycling Facility (MRRF) and a Co-Combustion System including MSW Rotary
Kiln; Secondary Combustion Chamber; Precalciner; NOx Reduction; Energy
Recovery System; Carbon Adsorption System; and Baghouse Filtration. The overall
schematic diagram of the process is shown in Figure 1.
Figure 1. The schematic diagram of the co-combustion process.
I95
W.H. Cheung, K.K.H. Choy, D. C. W. Hui, J.F. Porter and G. McKay
The MSW received will be screened by the material recovery processes from the
MSW Reception and Handling Facility (MRRF) through a series of unit processes,
namely, a classifying trommel, density separation and magnetic separation. The
classification of the undersize material is to remove mainly the non-combustibles
from MSW. Most of the undersize materials dropped through the 50 mm trommel
screen are debris comprising mainly food, and other wastes of heavier solid pieces
such as household batteries, the significant source of heavy metal contaminants in
MSW. These batteries, and materials containing batteries such as mobile phones, in
the heavier fraction will be picked up and removed from the undersize debris. The
oversize materials will undergo a series of thermal treatment operations in the
Co-Combustion (Co-Co) Pilot-Plant Treatment Process. This material enters the
Co-Co Treatment Units for the total hydrocarbons destruction of combustibles into
basic gaseous combustion products. The combustion gas is then vented through the
Precalciner for neutralization of acidic gas and chloride with an excess amount of
calcined alkaline material for scrubbing. NO, reduction is implemented at the ducting
of the Precalciner. The adsorption material carrying neutralized products containing
sulphate and chloride will be extracted from the gas by the Cyclone System, and will
be fiuther cooled with the Lime Cooling Process through an arrangement of multiple
staged high-efficiency cyclones systems. The gas is to be further cooled down at the
Heat Exchanger for downstream treatment at the Baghouse Filtration before venting
to the atmosphere. In addition, the gas is polished in a Carbon Adsorp1:ion System
when necessary. The neutralized chloride material is further extracted downstream of
the Lime Cooling Process for collection, sampling and analysis for further handling.
All calcined material in excess is again re-charged to the Precalciner to exploit the
neutralisation effect in this closed system. Cleaned gas is vented to the atmosphere.
Process Simulation Study
Material and energy balances are two of the most important elements in the process
design. All equipment designs and costings will be based on the results from the
material and energy balances. Flow rate and temperature are the two typical
parameters used in the material and energy balance calculations. The basic principle
for both material and energy balance is simply based on the following equation:
In - Out
-t Generation
- Consumption =
Accumulation
Jl)
The overall process simulation, including mass and energy balances, was conducted
by Microsoft EXCEL. The process flow diagram is shown in Figure 1. The data for
the simulation study consisted of three components, namely, physical composition,
chemical composition, and energy content of MSW.
(0 Physical Composition of Municipal Solid Waste (MS W)
Physical composition is the term used to describe the individual components that
make up a solid waste stream and their relative distribution, usually based on percent
by weight. The municipal solid waste is mainly divided into two types - domestic
196
Use of Municipal Solid Wastefor Integrated Cement Production
waste, and commercial plus industrial waste. Typical data from the Environmental
Protection Department (EPD) on the distribution of MSW in Hong Kong [ 151 is that
putrescibles, paper and plastics are the major components constituting about 76% of
MSW, representing about 33.1%, 26.7% and 16.6% respectively. Other minor
components include textiles (3.2%), metals (3.0%), glass (3.1%), bulky waste (3.5%)
and woodhattan (4.3%); the average moisture content for MSW is 28%.
(ii) Chemical Properties of Municipal Solid Waste (MSW)
The feasibility of combustion depends on the chemical composition of the municipal
solid waste. Determining the elemental composition of MSW by ultimate analysis is a
key factor for the detailed design of the MSW combustion plant, and helps confirm
the accuracy of the material and energy balances of the MSW gasification process.
The ultimate analysis of a MSW component typically involves the determination of
the percent of carbon (C), hydrogen (H), oxygen (0),nitrogen (N), sulphur ( S ) and
ash. Because of concerns over the emission of chlorinated compounds, e.g. dioxins,
during combustion, the determination of halogens is often included in an ultimate
analysis. The results of the ultimate analysis are used to characterise the chemical
composition of the organic matter in MSW. The average chemical composition of
municipal solid waste is estimated. The major elements are carbon (43.9%), oxygen
(32.1%) and ash (17.1%) accounting for around 93% of MSW in Hong Kong. Other
elements include hydrogen (5.6%), nitrogen (1.1%) and sulphur (0.3%). For material
and energy balance calculations of the MSW gasification process, the difference is
assumed to be 0.8% of chlorine present. This figure has been used to simulate the
formation of the chlorinated compounds, such as dioxins, during co-combustion.
(ti0 Energy Content of Municipal Solid Waste
After estimating the elemental composition of the MSW, the MSW energy content
can be determined. Typical data for the energy content for the MSW components are
reported in Table 1. Using the physical components described in the previous section,
the total energy content of the MSW is estimated. This is also shown in Table 1 where
the energy content values are on an as-discarded basis. The energy content value of
MSW (moisture content 28%) in Hong Kong is 13143 kJkg (5974 Bhdlb), and
compares well with the typical value of 5000 Btu/lb [16].For a moisture content of
the MSW of 30%, then the energy content of the MSW becomes 8465 kJkg.
(iv) Basis of the Simulation Study
The simulation study has two major input streams, namely the MSW to be treated,
and the cement raw material, limestone, as the scrubbing agent. The output streams
from the pilot simulation system will be the recovered materials from the sorting
process, the thermodynamic data for the export of excess energy, the cleaned flue gas
from the Co-Co Treatment Process, and the stabilised material from the residue
handling operations. Technical data is given in Table 2. The calculations and
simulations are based on the throughput of 4800 t/day of MSW.
197
W.H. Cheung, K.K.H. Choy, D.C. W.Hui, J.F. Porter and G. McKay
Table 1. Energy content of Municipal Solid Waste in Hong Kong in 2003 (adapted in
partfrom Tchobanoglous et al. [16]).
Physical component
MS W (kg)
Lower heat value
Total energy
(kJhk9
(k.lhg)
Bulky waste
3.5%
6,900
153
Glass
3.1%
200
7
Metals
3.0%
700
19
Paper
26.7%
16,500
4105
Plastics
16.6%
32,700
6103
Putrescibles
33.1%
4,200
1600
Textiles
3.2%
18,300
510
WoodRattan
4.3%
16,500
645
Others
6.5%
0
0
Total
100%
13,143
Heat value of MSW
13,143
Heat value of MSW (moisture content 30%)
8,465
Table 2. Basis for the simulation study.
Basis
Data
Daily MS W Throughput
4800 Vday
Clinker Production Rate
6000 t/ddy
Temperature of MS W Kiln
950°C
Temperature of SCC
1200°C:
Flue gas temperature (at stack exit)
16OoC
Results and Discussion
(4 Dioxin Emission Minimisation
Under the conditions of 1200"C, greater than 4 second residence time, rotary kiln
turbulence and tangential burner swirl turbulence, the fuel bum-out will be extremely
high. Thus, minimising the available organic carbon for the downstream reformation
I98
Use of Municipal Solid Wastefor Integrated Cement Production
synthesis of dioxins [B]. The MRRF will also reduce the chlorine content of the MSW
feed by the removal of PVC. In addition, there will be scrubbing of HCl in the
precalciner in which the scrubbing ratio is more than ten times greater than that in a
normal flue gas scrubber. The HCl will form calcium chloride which then participates
in clinker-forming reactions. Since the temperature in the precalciner becomes very
high, there will likely be some dissociation reactions and some chlorine may be
rejected. The cement clinker-forming reactions are too complex for simulation and
prediction. Consequently, quantitative data on this aspect of the project will only be
available after the pilot-plant tests are completed. Finally, there will be a carbon/lime
injection system for the cooled flue gas (200T) immediately prior to the baghouse
filter.
Based on the dioxin content measured from conventional MSW incinerators with
similar baghouse filters [8], and operating in the combustion temperature range 850 to
1150'C, the current system (even without the other dioxin minimization factors)
should reduce the dioxin emission level to well below 0.08 ng TEQMm3.
(ii) Energy Savings
The energy benefits of the MSW co-combustion process mainly come from
generation of electricity and production of high-temperature raw feed material
(clinker) of the cement production plant. This is achieved by utilising the waste heat
energy from the flue gas, while the electricity will be supplied to drive the cement
manufacturing process.
According to the calculated material and energy balances in the MSW Co-Co
simulation program, the total heat energy of the flue gas coming from the Secondary
Combustion Chamber (SSC) is 2,553,000 MJ/hr. The hot flue gas will be used to heat
up 420 tonnes per hour of cement raw meal (limestone) from 25 to 950'C in order to
produce 250 tonnes per hour of clinker material for the cement manufacturing
process. The limestone is brought into contact with the flue gas at 1200°C from the
SCC, bringing about decarburisation, or calcination, of the limestone. The total
energy consumption on the precalciner system was 673,500 MJ/hr, and 26.4% of flue
gas energy can be utilised in order to provide all the energy required in the precalcination process, thus saving US$12,555,000 annually.
It is proposed that the remaining waste heat energy (1,640,000 MJ/hr) from the
flue gas is used to generate electricity for the Co-Co process and the cement
production process. If the electricity generation efficiency of the steam turbine is 11%
[17], and the energy content (55% moisture) of the MSW in Hong Kong is around
7560 kJ/kg, then the MSW Co-Co pilot plant will generate 912,000 kWh (38MW)
electricity per day, and the annual electricity generation will be 300,960,000 kWh.
According to the local power company, the charge for electricity is US$ 0.10 per kWh
unit. Hence, the potential annual revenue from the electricity generation system is
US$28,553,000.
The other possible saving is by the use of hot exit gas from the Clinker Cooler
(850°C) and Cement Kiln (1 100°C) to the MSW SSC. Coal is being used as a source
of fuel for the MSW combustion process in SCC, in order to achieve the hightemperature condition (1200°C) of the exit flue gas. In this case, the amount of coal
could be reduced from 45,630 kg/hr to 25,195 kg/hr. On average, the coal costs
I99
WH.Cheung, K.K.H. Choy, D.C. W.Hui, J.F. Porter and G. McKay
around US$20 per tonne in Hong Kong, therefore, the potential saving from the
utilisation of hot gas from the cement process is US$4,150,000. However, the waste
oil (MARPOL) can be used as the fuel source for the Co-Co process instead of using
coal. The disposal cost for MARPOL given by the Environmental Protection
Department [18] is US$60/m3. In the simulation study, it was found that the process
can utilise 16,797 kg/hr of MARPOL instead of using fuel oil as the fuel for
combustion. Assuming that the density of the MARPOL is 850 kg/m3, then the annual
revenue from utilising the MARPOL is around US$9,585,000. The results of the
above savings are summarised in Table 3.
Table 3. The amount of energy savings with various scenarios.
Energy Saving (Scenario I )
Savings US$ (per year)
Energy for calcination process
12,555,000
Power f o r process
28,553,000
Coal saving
4,150,000
Total
45,258,000
Energy Saving (Scenario 2)
Savings US$ (per year)
Energy for calcination process
12,555,000
Power for process
28,553,000
Revenue from MARPOL treatment
9,585,000
Total
50,693,000
(iii) Emission Reduction
An MSW and ash-handling system with a series of air pollutant treatment devices are
designed for ths plant in order to meet the current disposal and emission
requirements. The airborne emissions from the Co-Combustion Process are
controlled, and must not exceed the concentration limits set by the Hong Kong
Environmental Protection Department.
In our simulation study, the concentration of the pollutant is dependent on the
removal efficiency of the equipment and the conversion in the combustion process.
Currently, 98% removal efficiency by the calcium carbonate/lime treatment is
assumed for HC1, and the overall conversion of CO to C 0 2 is 99.98% in the
combustion chamber. The removal efficiency of NO2 and SO2 in the Co-Co Process
is greater than 90%. The particulates are removed by a cyclone and filter bag, with a
total removal efficiency of 99%. The emission concentrations of these pollutants are
lower than the concentration limits set by the Hong Kong Environmental Protection
Department as shown in Table 4.
200
Use of Municipal Solid Wastefor Integrated Cement Production
Table 4. Concentration limits f o r emission from incineration processes.
Daily average
Air pollutant
concentration limit
Removal
efficiency
(mg/m3j
Simulation
results
(mg/m3)
Particulates
30
99%
9
Hydrogen chloride (HCr)
50
98%
17
Hydrogen fluoride (HF)
2
Sulphur dioxide (SO,!
200
90%
163
Nitrogen oxides {NO,!
400
90%
1
Carbon monoxide (CO)
100
99.98%
50
(iv) Economic Benefts of the Co-Co Process
Based on the simulated pilot plant study reported in the previous sections, the savings
for the 2 ton MSW/day pilot plant can be scaled up to the full-scale 250 ton cement
clinker per day plant. The values obtained are likely to be minimum value scenarios,
however more optimistic values cannot be confirmed until pilot-scale tests are
completed. The data in Table 3 reflect the direct difference between the MSW cement
integrated process and conventional incineration. In terms of assessing the rate of
return, a conventional cement plant will incur additional capital and operating costs
for the MRRF, the MSW incineration system and the emission plant, and treatment
chemicals.
Conclusions
The use of MSW as a source of both raw materials and energy in the production of
cement has been studied. A novel integrated co-combustion design of a cement
production facility has been developed, and a pilot-scale plant is under construction.
Compared to conventional MSW incinerators, waste minimisation will enable
significant economic savings to be made in the following categories: (i) landfill costs;
(ii) energy for process; (iii) power for process; (iv) power for export; (v) limestone
feed; (vi) silica sand feed; (vii) revenue from MARPOL treatment; (viii) AC
injection; and (ix) lime injection.
Acknowledgements
The authors are grateful to the Innovation Technology Fund, Hong Kong SAR, Green
Island Cement Co. Ltd. and Hong Kong University of Science and Technology for the
provision of financial support during t h s research programme. Thanks also to Barrie
201
W.H. Cheung, K.K.H. Choy, D.C.W.Hui, J.F.Porter and G. McKay
Cook, Gary Yu, Lambert Leung, Don Johnston, Henry Law, Peter Leung, Aung
Khine, Raymond Cheung, Thomas Tao, Michael Wong, Sunny Kwong and Vivian
Kwok for their assistance and advice throughout this research project.
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202
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