Final Design Report Plasma Arc Waste Disposal Group 11

8/7/2019 Final Design Report Plasma Arc Waste Disposal Group 11

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CHEMICAL PROCESS DESIGN 2

TO: PROFESSOR RONALD WILLEY

FROM: EDGAR HUERTA, MOHAMMED METTEDENANDADAM SOBELSUBJECT: PROPOSAL FOR BOSTON PLASMA ARC WASTE DISPOSAL FACILITY

DATE: 7/20/2011

Municipal solid waste management is of growing concern within the Boston

Metropolitan area. The Environmental Protection Agency estimates that 4.5 lbs of solid waste is

produced per capita every day within the United States[1], a value that has nearly doubled over

the past 50 years. This trend is expected to continue. The 2008 census estimated the population

of Boston as 608,352 residents[2]. This number is expected to increase by 9,000 people by

2015[3]. This equals a 40,500 lb increase in waste generated per day, yielding a total 1,389 tons

of solid waste per day. Currently there are two options available to the state of Massachusettsregarding this waste; send it to a dwindling number of domestic landfills or export it to another

state for incineration, as in-state incineration is unlawful. Tipping fees for landfills range from

$25-$75 per ton for non-hazardous waste and several hundreds of dollars per ton for hazardous

waste. Exportation of waste requires payment to the receiving state while incurring

transportation costs of shipping by non-government agents. The fact that the receiving state

likely allows incineration negates the environmental benefit of the domestic ban. Both of these

options cost the state a phenomenal amount of money.

Plasma arc waste disposal differs from incineration by thermally disintegrating waste in

an oxygen-free environment, as opposed to combusting waste in an oxygen-rich environment.


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This has a major effect on the chemical compounds produced by processing. Incineration tends

to produce volatile hydrocarbons, NO x molecules and compounds of significant health concern,

such as dioxins and furans. The plasma arc processes solid waste by breaking down compounds

into their respective atomic forms, then thermally controlling how these atoms recombine.

Organic matter, such as plastic, wood and paper produces inert gases and synthesis gas, which is

composed of CO, CO 2 and H 2. This synthesis gas is burned as fuel to generate power. Inorganic

solid waste, such as glass and metal, forms various types of slag depending on the cooling

method; air-cooled slag forms large crystalline structures resembling obsidian, compressed-air

cooled slag forms rock wool and water-cooled slag forms small crystalline structures resemblingsand.

The ability to handle metal, hazardous and radioactive waste makes plasma arc waste

disposal much more versatile than incineration, which is reserved for organic waste. This is very

important as electronic waste becomes more prominent. Currently all discarded cathode ray

televisions and computer monitors must be carefully disposed of to prevent heavy metals from

leeching into ground water.

Plasma arc waste disposal is an effective, environmentally friendly approach to handling

Bostons increasing solid waste production. It will greatly reduce the phenomenal cost of

exporting waste to other states as active landfills in Massachusetts are becoming scarce. This is

the answer to Bostons growing waste problem.


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Proposal References:

[1] U.S. Environmental Protection Agency, Region 7: SolidWaste Basic Facts .http://www.epa.gov. Web. Last visited Feb. 24, 2010

[2] Mayors Office. Boston Wins 4 th Census Challenge; Citys Population Continues toGrow. Released December, 2009.Web. http://www.cityofboston.gov. Last Visited Feb. 24, 2010.

[3] The Boston Indicators Project. Key Trends. Web.www.tbf.org. Last visited Feb. 24, 2010.


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NORTHEASTERN UNIVERSITYDEPARTMENT OF CHEMICAL ENGINEERINGSPRING 2010

Waste Disposal via Plasma

Adam SobelEdgar Huerta

Mohammed Metteden4/20/2010


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Table of Contents:

1. ABSTRACT...62. INTRODUCTION.7

3. PROCESS DESCRIPTION..84. ENERGY BALANCE AND UTILITY REQUIREMENTS155. ECONOMIC SUMMARY..226. SAFETY, HEALTH AND ENVIRONMENTAL CONSIDERATIONS257. CONCLUSIONS AND RECOMMENDATIONS288. ACKNOWLEDGMENTS..309. BIBLIOGRAPHY3110. APPENDIX..32

Table of Figures:

FIGURE 1. PROCESS FLOW DIAGRAM OF PLASMA ARC GASIFICATION PLANT..9FIGURE 2. CUSTOM ROCK WOOL SPINNER DESIGN...20

Table of Tables:TABLE 1. TYPICAL SLAG CHEMISTRY..12TABLE 2. HEAVY METALS DISTRIBUTION BETWEEN PYROLYSIS AND PLASMA ARC13TABLE 3. WASTECARE SHREDDER SPECIFICATIONS...17TABLE 4.SUPERMAX HEAT EXCHANGER SPECIFICATIONS.......18TABLE 5.BRIGDE BREAKER OPEN HOPPER PUMP SPECIFICATIONS19TABLE 6.SIEMENS STEAM TURBINE SPECIFICATIONS19TABLE 7.SIEMENS GAS TURBINE SPECIFICATIONS..20TABLE 8. CUSTOM GAS SYNTHESIS COMPRESSOR SPECIFICATIONS..20TABLE 9. CUSTOM ROCK WOOL SPINNER SPECIFICATIONS...21TABLE 10. PROCESS ECONOMICS SUMMARY..22TABLE 11.EQUIPMENT COSTS.23

TABLE 12. FIXED CAPITAL INVESTMENT SUMMARY23TABLE 13. TOTAL CAPITAL INVESTMENT SUMMARY..24TABLE 14. HAZOP ANALYSIS26


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1. Abstract

Plasma arc gasification is a method of processing large volumes of municipal solid waste inan efficient manner that has less environmental impact and health hazards than incineration andlandfill dumping respectively. Solid waste enters a gasification chamber and travels through anelectrically generated arc. The arc vitrifies the organic component of the waste to synthesis gas,forcing it out of the gasification chamber, while the inorganic component becomes molten andflows out the bottom of the chamber. The slag is pulled onto a porous rotary drum and cooledusing compressed air, forming useful rock wool. The synthesis gas is cooled using a heatrecovery system, filtered to a clean stream and combusted in a gas turbine engine. The exitstream from the turbine consists of CO 2, with trace amounts of uncombusted CO and NO x fromthe working air of the turbine, which are scrubbed. The overall energy recovery system producesenough energy to meet consumption needs of the process with enough left over to sell to the grid

for profit. This combined with the revenue from the rock wool and tipping fees gained fromaccepting solid waste yields a profitable system. While the CO 2 amount emitted by the process isabout half the amount emitted by a 500MW coal plant (when processing 4000 tons of solid waste

per day), the process does not emit toxic ash, SO x, dioxins, furans, heavy metal particulates or any other hazardous material associated with incineration.


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2. Introduction

In recent years, the quantity of municipal solid waste (MSW) generated has increasedsignificantly in the Boston metropolitan area. The proper management of waste is a crucial issuedue to the increasing amount of waste generated from residential and municipal buildings. It isestimated that in 2009 the City of Boston collected 259,680 tons of municipal solid waste [6],which was predominantly exported to other states or dumped in landfills. The Department of Public Works has little control over waste disposal technology, but has the belief that most of itis incinerated. The current method of waste management employed by the City of Boston failsto address factors such as population growth, environmental implications and increasing costsassociated with the exportation of waste.

Clearly this current system is not a sustainable or environmentally friendly method of waste management. The number of landfills available for waste disposal is graduallydiminishing; by 2015 it is estimated that there will be only 8 active landfill sites in the entirestate of Massachusetts [7]. Furthermore, the incineration of MSW generates fly and bottom ashesand releases leachable toxic heavy metals, dioxin, furans and volatile organic compounds that areharmful to the environment [1]. It is therefore imperative that an alternative method of MSWmanagement is found that is reliable, cost effective, sustainable and environmentally friendly.

Waste management systems include waste collection and sorting followed by one or more of the following options: recovery of waste materials (i.e., recycling), biological treatmentof organic waste (i.e., production of marketable compost), thermal treatment (i.e., incineration torecover energy in the form of heat and electricity) and finally land filling [1]. In more recenttimes, the incineration of combustible waste to generate energy has become the most commonmethod of dealing with MSW effectively as it decreases its volume and mass significantly. Asmentioned earlier, incineration has some environmentally harmful byproducts. One suchalternative that has shown great potential is the gasification of MSW. There are several studiesthat focus on an innovative technology called plasma gasification. This technology has beendemonstrated as one of the most effective and environmentally friendly methods for solid wastetreatment and energy recovery [3].

Plasma arc gasification is a method of waste management that has been employed in ahost of cities around the world. The process uses electrical energy to generate high temperaturescreated by an arc. This arc breaks down waste primarily into carbon gas and inorganic moltenwaste (slag), in a device called a plasma converter. The gases produced, called synthesis gases,are then used as fuel for a gas turbine engine to generate electricity. The slag is used as rawmaterial for construction and insulation. The process is a net generator of electricity, and reducesthe volume of waste sent to landfill sites.


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3. Process Description

3.1 Plasma Arc Gasification

In a plasma arc gasification plant, waste is first pretreated with a shredder ( f igure 1 ). The

shredded waste is then loaded in the solid feeder, where it is fed into the gasification chamber or plasma furnace. Commonly, graphite electrodes with male-female threads have a high currentdirected through them in order to generate a plasma arc. The arcing generates a large amount of heat (up to 3000 oC) which is used to heat the waste and subsequently vitrify the inorganic part of the waste and convert the organic part into useful syngas.

The syngas generated from the organic portion of the waste is compressed in anexplosion proof gas compressor and then transported to heat exchangers, where the temperatureis reduced. The gas compressor keeps the pressure within the gasifier low, while keeping the

pressure within the heat exchanger high, dramatically improving heat transfer. The gas is cooled

to 250oC, below the temperature required for toxic reformation, by water. The heat exchanger

causes the water to phase shift and generates steam at 500 oC. The steam flows to a steam turbine,generating power.

At this point the syngas is still dirty and needs to be filtered through an activated carbon filter.Any heavy particulates and hydrogen sulfide will leach out onto the carbon filter leaving cleansyngas on the other side.

From here the clean syngas is sent to a gas turbine engine. Syngas burns exothermicallyand is used as a fuel to expand the working gas of the turbine, generating power. If air is used asthe working gas then hazardous compounds such as NOx will form during the expansion

process. Scrubbers will filter out any NOx or trace un-combusted syngas from the turbine outletstream, leaving CO 2 and water. Water is then condensed out of the process stream.


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Figure 1. Process Flow Diagram of Plasma Arc Gasification Plant

3.2 Advantages of Plasma Arc Gasification

Plasma arc gasification of waste has many advantages when compared to other methodsof waste management. First, it is an environmentally friendly process, since all the waste is either converted into syngas or solid inert rock. There is no risk of releasing all of the harmful toxinsassociated with incineration.

Furthermore, this process significantly reduces the volume and mass of solid waste after processing. This is of particular importance due to the fact that the city of Boston has limitedlandfill availability. As the number of active landfills in the area gradually diminishes, a plasmaarc gasification plant would greatly facilitate more efficient disposal of waste.

Another important attribute of this process is the ability to process and handle hazardous

wastes. Private waste management companies charge exorbitant tipping fees for the handling of hazardous waste due to the high costs associated with the methods they employ.

Finally, the plant operates as a net generator of electricity; the plant generates moreenergy than it uses and therefore can sell this electricity back to the grid. Several studies have

been conducted that show that solid waste could be a viable alternative source of fuel [2][4] .


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3.3 Detailed Design

The following section will discuss in detail the: i) pre-processing stage of waste, ii)gasification process, iii) energy recovery and iv) utilization of solid by products (slag).

3.3.1 Waste Pre-treatment

The waste feedstock is delivered and discharged by truck or other means to the tippingfloor. The treated waste should have a maximum particle size of 2.5cm and a maximummoisture content of 50% [2]. A pre-crusher compacts the waste into a specially designedcompaction container. The waste is compressed to about one fifth of its initial volume. The highdegree of compaction improves heat conductivity by reducing the residual air content [1]. Oncefilled, the container is fitted with a metallic door that will be closed, thus preventing problemswith rodents and foul odors.

A conveyor system then moves the filled containers into the gasifier area. This will allow

efficient control of the process and will ensure that there is no chance that a filled container can be forgotten (a major cause of rodent and odor problems in MSW facilities). It is important tonote that due to nature of MSW, it is not realistic to have an exact composition of the waste

being fed into the system. For the purpose of this investigation, the waste will be assumed to becomposed of 80% organic waste and 20% inorganic (e.g. soil, metals-bearing wastes, and fly-ash, metals, etc.).This is a reasonable assumption as most of the inorganic waste is salvaged andrecycled before arriving at the plant.

Once the container reaches the gasifier, a small crane will load the contents into thegasifier-feeding platform. The empty container is placed in a second conveyor that will return it

to the container area. The feeding platform is an articulated tilting table where the container door is opened. Once the door is opened, the articulated table is inclined approximately 60degrees directly over the compactor/extruder, which then feeds the MSW into the gasifier. Thecompactor/extruder, in conjunction with the storage container, provides a unique advantage thatmaximizes the benefits of plasma gasification of MSW. The system feeds the waste feedstock into the gasifier after having extruded a significant portion of the entrained air in the wastefeedstock (the most important aspect to ensure the production of the highest quality synthesisgas). Finally, the feed rate can be calibrated to equal the rate of dissociation and gasificationwithin the gasifier chamber.

The feed rate of waste going into the gasifier could be increased by; feeding the wastefrom the top instead of the side, and placing a refractory cone to spread the waste evenly over thesurface of the molten bath [2].Keeping a layer of untreated waste on top of the molten pool servedas insulation to keep the chamber temperature stable [2].

Feeding the waste from the top ensures that the entire surface of the whole bath is used tomelt the waste, instead of a localized area which was found to occur when the waste was fed


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from the side. Keeping a layer of untreated waste improves the stability of the arc and increasesthe energy efficiency of the process, because the waste provided insulation for the molten slag.

The figure below shows the range of feed rates obtained from a pilot unit before and after the change in feeding patterns. It can be seen from the figure that feed rates above 25 kg/h were

difficult to attain due to the waste not being able to melt fast enough [2]. Whereas when fedevenly from the top, feed rates of up to 92 kg/h were obtained.

To satisfy the waste disposal needs for the City of Boston, the plant is designed to process 4000 tons a day of waste. The plant will consist of 9 gasifiers in total which will bediscussed in further detail in the following section. 8 gasifiers will operate simultaneously inorder to process the large amount of waste. Therefore, each gasifier will have a feed rate of 20tons per hour.

3.3.2 Gasification Process

The gasification of the organic component of MSW is dictated by 5 equilibriumreactions:

22)( H COO H sC (Heterogeneous water gas shift reaction-endothermic)

COCO sC 2)( 2 (Boudouard equilibrium-endothermic)

422)( C H H sC (Hydrogenating gasification-exothermic)

224 3 H COO H C H (Methane decomposition-endothermic)

222 H COO H CO (Waster gas shift reaction-exothermic)

These reactions require some explanation. The formation of products on the right side of the equation relates to whether that reaction is categorized as endothermic or exothermic. Thehigh temperature of the plasma arc furnace, where recombination of molecules from atoms takes

place, favors endothermic reactions. This yields an effluent stream of mostly carbon monoxide,some carbon dioxide and hydrogen gas, and trace amounts of methane.

The residence time of the syngas is on the order of seconds within the reactor; the generalrange of time associated with most reactors is 3-5 seconds. The inorganic slag requires an hour to

become sufficiently molten, depending on distance from the electrical arc. Slag collects withinthe gasifier, causing the slag at the top of the collection to heat quicker than the slag below it.The molten slag overflows the breech in the reactor and is transported to the rock wool spinner.The remaining slag within the gasifier acts as insulation and a heat source for the next batch of solid waste.


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3.3.3 Energy Recovery

The high temperature of the syngas exiting from the gasifier stresses the importance of thermal recovery. 33 Kg/s of syngas exiting at 1600C is compressed to 200 bar by a gascompressor. Increasing the pressure of the syngas improves the heat transfer coefficient of thegas by a factor of 1000. This is very important as the temperature of the syngas must decreasefrom 500C to 250C very quickly to eliminate the possibility of toxic reformation of the carbongases. This heat transfer requires 13 Kg/s of water on the cold side of the exchanger. The water

phase changes to steam at 500C. The steam flows to a steam turbine that generates 9.3 MW of power.

The cooled syngas passes through an activated carbon filter; the activated carbon absorbsheavy metal particulates and sulfide compounds that may have been gasified and carried out thegas exit. This leaves a stream of clean syngas consisting of approximately 17 Kg/s CO, 16 Kg/sCO2 and 14 g/s H 2. The clean syngas is transported to and used as fuel in the gas turbine engine.The primary energy generating reaction is the oxidation of CO to CO 2, generating 282,984KJ/Kg-mol. This corresponds to a power generation of 65.8 MW with a thermal efficiency of

38%.

3.3.4 Solid By-products

Upon completion of the vitrification process, a molten inorganic pool of slag is tappedout of the gasification chamber. Slag is a mixture of various inorganic compounds that are foundin MSW such as soil, metals-bearing wastes, and fly-ash, metals, etc. These attribute toapproximately 20% of the waste treated with respect to mass.

The composition of slag, as with the waste coming in to the plant is very difficult toestablish due to the varying composition of the waste being processed. From literature [8] [10] , anapproximate composition was ascertained which will be used as a basis for this investigation.The typical slag composition is shown below

Table 1. Typical Slag Composition

Elements Composition (% byMass)

Silica 37.2

Alumina 19.5

CaO 19.5

Fe2O3 6.21

Na2O 3.87

MgO 2.31

MnO 1.7


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K2O 1.31

NiO 0.32

Cr2O3 0.26

CuO 0.26

ZnO 0.24

PbO 0.11

CdO


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The most promising of these products as a commercially sellable commodity is rockwool.Rockwool serves as a replacement to asbestos without any of the harmful side effects. Rockwoolis fabricated by passing the molten slag through a perforated drum that has air shooting out of it.The slag is spun in this drum whilst simultaneously having jets of air steam past it. The end

product is an insulator with excellent properties. Its fibers are non-combustible and have amelting point of well over 900 oC [11] . Rockwool is also used as ceiling tile and sprayed fire

proofing, as well as sound control and attenuation.

In addition, the use of rockwool as a horticultural growing medium has increased inrecent years [12] . By slightly changing the mineral composition, rockwool has proven to be asuccessful substrate for the cultivation of produce using hydroponics. This method of horticulture has been implemented successfully in the Netherlands where of 3550 hectares of vegetables, 2350 are on substrate [12].


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3. Energy Balance and Utility Requirements

Energy generation is a large component of the overall process and is one reason why this process is successful. The method below shows how this process is capable of generating

sufficient power to be self sustaining with enough extra to sell to the electrical grid, even when processing 4000 tons per day of solid waste.

The calculations were based on processing 4000 tons per day, with a synthesis gastemperature exiting the plasma arc furnace at 1600C. The gas must be cooled to a temperatureof 250C to prevent the reformation of toxins. These flow rate and temperature requirements arethe basis of the heat recovery system. The energy needed to be removed by a heat exchanger toachieve this was calculated using:

T C mQ (yy! Where;Q = energy m = mass flow rate C = heat capacity

T = temperature change

The overall heat capacity of the synthesis gas was calculated by taking the weighted average of all the synthesis gas components.

The mass flow rate of the water entering the heat exchanger was determined using thevalues from this equation and steam tables. The steam produced had to be of sufficient pressure

and temperature to drive the steam turbine, around 500C. The size of the heat exchanger neededfor such a transfer of energy was calculated using the equation:

Ly(y!

T U Q

A

Where;A = heat transfer area Q = energy

T = temperature change = efficiency

U = heat transfer coefficient

The heat transfer coefficient was estimated using a value typical of compressed gases.The energy generated by the steam turbine was calculated using enthalpies found in the

steam tables for the inlet and outlet conditions of the steam turbine. The equation used assumesisentropic expansion factored by a standard turbine efficiency:


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)( H o H imWs y!

Where;Ws = shaft work m = mass flow rate Hi = enthalpy of inletHo = enthalpy of outlet

= efficiency

Once cooled by the heat exchanger and cleaned by the activated carbon filter, thesynthesis gas was combusted in a gas turbine engine to produce more energy. The energyreleased by combustion was calculated using the difference of the heats of formation of thecomponents of the synthesis gas before combustion and the products post combustion:

22 22 COOCO

O H O H 222 22

This difference was factored by the mass flowrate of the respective reactants. The energygenerated by the gas turbine engine was calculated using a thermal efficiency typical of most gasturbine engines.


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4. Equipment List and Unit Descriptions

4.1 Plasma Arc Reactor

The plasma arc furnaces are from Dutemp, and can process about 500 tons of MSW per day. This tank includes the electrodes, the gas exhaust pipe and the conveyor belt for the bottomslag. The plant design is to have 9 reactors installed with 8 working at all times and one as aspare.

4.2 WasteCare S72-725 all purpose shredder

In order to have small uniform pieces of MSW the process requires an industrial grinder.The grinder can process up to 45 tons per hour of MSW. One WasteCare S72-725 all-purposeshredder is enough to supply 2 reactors with the waste to process. A total of 4 are required.

Table 3.WasteCare shredder specifications [15]

Standard Drive Motor - (HP ) 200 - 250 HP Standard

Highest Available HP 300 HP (Other Models up to 1,000 HP)

Lowest Available HP 60

MSW Production Rate (from reported facilities) 45 Tons / Hour (Other Models up to 65 TPH)

Tires Production Rate (from reported facilities) 1,000 / Hour (Other Models up to 2,000 / Hr)

Bulk or Metered Feeding from Reported Facilities Bulk

Length of Smallest Infeed Opening 72"

Width of Smallest Infeed Opening 72"

Maximum Design Infeed Opening 140"

Shaft Center Design Distance - Center to Center of Shafts

24"

Shaft Size for Hex Shaft (Flat to Flat Distance) 12.5"

Cutter Diameter 32"

Cutter Thickness Up to 3" Stacked 1 x 1, 2 x 2, 3 x 3


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4.3 Tranter Inc. SuperMax Heat and Plate Heat Exchange

In order to create steam for the steam turbine, a heat exchanger is needed. Two of theseare enough to provide the flow rate needed for the syngas to heat the water to produce steam.

Table 4. SuperMax heat exchanger specifications

Technology/Type

Technology Plate

Type Welded

Performance Specification

Working Pressure 600 psi

Temperature Range -20 to 600 F (Cold)

Liquid Flow Rate 825 GPM

Construction

Primary Material Steel

Electric Drive Yes

RPM of Shafts on Standard Machine 4

Single or Dual Drive Dual

Shaft & Cutter Material 4140 Alloy Steel - Heat treated for OptimumStrength

Spacers & Cleaning Fingers Material 4140 Alloy Steel - Heat treated for OptimumStrength

Overall Length (Main Machine - Rounded) 23'

Overall Width (Main Machine - Rounded) 13'

Overall Height (Main Machine - Rounded) 17'

Main Machine Weight / Approx: - (lbs) 55,000+ lbs


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Overall Dimensions 12.32" diameter

4.4 Bridge Breaker Open Hopper Pump

A pump is needed to produce a flow to the heat exchanger in order to produce the steam.In order to reach a flow of 220 GPM we will be using two pumps; one for each of the heatexchangers. Each pump will be running around half capacity, since each heat exchanger onlyneeds water flow of 110 GPM.

Table 5. Bridge Breaker Open Hopper Pump specifications

Flow/Pressure Specifications

Liquid Flow 200 GPM

Discharge Pressure 700 psi

Discharge Size 10 inch

Hp 250 HP

Max RPM 150 RPM

4.5Siemens Steam Turbine SST-200

The steam turbine is used to convert the steam from the heat exchanger to energy.

Table 6. Siemens Steam Turbine specifications [16]

Specifications

Power Output up to 10 MW

Inlet Steam Pressure up to 110 bar

Inlet Steam Temperature up to 520 C

Bleed up to 60 Bar

4.6 Siemens Gas Turbine SGT-200


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Another piece of energy generating equipment that we need is a gas turbine. Once the syngas passes through the heat exchanger, it will be combusted in the turbine to produce the majority of the energy.

Table 7. Siemens Gas Turbine specifications [16]

Specifications

Electrical efficiency 37.50%

Heat rate 9597 kJ/kWh

Turbine speed 6608 rpm

Compressor pressure ratio 19:1

Exhaust gas flow/temp 131.5 kg/s, 544 C

NOx emission capability less than 15 ppmV

4.7 Custom Synthesis Gas Compressor

The compressor needed for this project has specific requirements not readily availablefrom a standard vendor. The compressor must be able to withstand a working temperature up to1600C, the temperature of the syngas leaving the gasifier. Compressors able to do this exist onthe market, and are available from Haskel and Fluitron, however the available equipment cannotkeep up with the volumetric flowrate of the gasifiers.

The compressor, or compressor system, must be able to compress carbon monoxide up toa pressure of 200 bar at a flowrate 11,000 SCFM. Hydraulic piston, centrifugal and screwcompressors are able to achieve pressures of 50 bar at the required flowrate, and are availablefrom Camerons Compression Systems and Aerzen USA Corp. The large volumetric compressor can act as a step-up to the high pressure compressor.

Table 8. Custom Synthesis Gas Compressor specifications

Specifications

Temperature 1600C

Pressure 200 bar

Flowrate 11,000 SCFM


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4.8 Custom rock wool spinner

The rockwool spinner is a machine that converts a steady flow of molten slag intorockwool. The stream of molten slag flows down a channel onto a spinning, perforated rotarydrum. The drum itself must operate at a working temperature of 1600C, which is the

temperature of the slag exiting the plasma furnace. Blasting molten slag with compressed air isthe principle method of producing rockwool from furnace byproducts. High pressure air iscontinuously pumped into the center of the rotary drum. The air then escapes through the

perforations, cooling the slag into the fibrous structure of rockwool. The rockwool is scraped off the rotary drum using a wedge at the end of the rotation.

Table 9. Custom rock wool spinner specifications

Specifications

Working temperature 1600C

Internal working pressure 100 psig

Explosion proof Required

Figure 2. Custom rock wool spinner design

CompressedAir

Conveyer System

M o l t e n S l a g

Rock Wool

Scraper


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5. Economic Summary

The economics of the project are based on the assumption that 1,500,000 tons of municipalsolid waste per year will be processed.It wasthen determined how much electrical power, tons of slag and tons of CO 2 are produced from our process from literature available and processcalculations. A tipping fee is associated with accepting and processing MSW, which adds to the

process revenue. CO 2 is the major component for gases recovered from this process. Carbondioxide can be recovered and purified to be sold. The other byproduct of this process is slag,which can be converted to the versatile material rockwool. A market has already beenestablished for this material. Using the average price for products we are able to determine therevenue of this process.

Table 10. Process Economics Summary

Unit Price

Electric Rates per MW $50.00Tipping Fee Per ton of MSW $30.00Sale Price of Rockwool per ton $175.00Sale Price of CO 2 per ton $90.00ProductsTons of MSW processed 1,500,000Tons of rock wool produced 300,000Tons of CO2 produced 1,460,000

Net MW for sale per year 511,000RevenueMSW $45,000,000Rock wool $52,500,000CO2 $131,400,000Electricity (MW) $25,550,000Total Annual $254,450,000


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In order to calculate the total capital investment, the equipment cost was first determinedfrom quotations from various vendors. These values are an important factor needed for theeconomic analysis. Estimations for the variables in the total fixed cost were calculated using thetotal equipment cost as a basis andwere used as estimations for asolid-liquid process [17]. The

prices for our major equipment are shown in the table below.

Table 11. Equipment costs

EquipmentGrinder (MSW Processing) $9,120,0009 Reactors $70,000,000Scrubber System $23,600,000Energy Recover System $92,000,000

In order to calculate the fixed capital investment (FCI), an estimate for the

instrumentation, piping, electrical and other variables was obtained by multiplying the total purchased equipment by certain factors. These factors can be found in the Plant Design andEconomics for Chemical Engineers, for a solid-liquid process. A summary of these figures isshown below.

Table 12. Fixed Capital Investment Summary

Total Purchased Equipment $194,720,000Purchased equipment installation $75,940,800Instrumentation $50,627,200Piping $60,363,200Electrical $19,472,000Building ( incl. services) $29,208,000Yard Improvement $23,366,400Services facilities $107,096,000Total Direct Cost $560,793,600Engineering and supervision $62,310,400Construction expenses $66,204,800Legal expenses $7,788,800Contractor's fee $36,996,800Contingency $72,046,400

Total Indirect Costs $245,347,200Fixed Capital Investment $806,140,800Working Capital $146,040,000

The FCI is the sum of our total indirect investment and the total direct investment. Fromthe total equipment cost it was then possible to determine our working capital. Upon calculatingthe working capital and fixed capital investment, it was then possible to determine the total


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capital investment (TCI). The TCI for the process was calculated to be $952 million, this is alarge value compared to our net cash flow of $150 million a year. This large difference can beattributed to the significant costs associated with the necessary equipment, in particular, thegasification chambers and energy recovery system. A summary of the TCI is shown below.

Table 13. Total Capital Investment Summary

Working Capital (W) $146,000,000Total Fixed Cost $806,000,000Total Capital Investment $952,000,000Cost of Operation $28,000,000Income (sales) $254,000,000Depreciation $10,000,000

Gross Profit $226,000,000Gross Profit(depreciation) $216,000,000Tax rate 0.35Income Tax $76,000,000

Net Profit $140,000,000 Net cash flow $150,000,000

In order to determine the economical feasibility of this project a payback period, the net present value and the internal rate of returnneeded to be determined. These three factorswill form

the basis of the economic feasibility of this process. The process is shown to have a payback period of 6.8 years. Next, the net present value (NPV) of this process is assessed over the next 20years, the lifespan of the project. The NPV, with a discounted factor of 10%,was found to be$326 million. Another good indicator to look at is the internal rate of return; for this project itwas found to be 6%. The payback period and NPVboth indicatethat the process is economicallyfeasible.

The rate of return is a low value at 6%, a good value would be around 15%. This should not be a reason to stop this project. The city of Boston will not be losing money in this endeavor.Much land can be reclaimed in the process of solving Boston waste management problem. There

other options that we have, about start up. Starting a plant to handle less waste will be able tolower the total capital investment.


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6. Safety, Health, and Environmental Consideration

6.1 Safety Considerations

As with any manufacturing process, safety precautions and considerations need to betaken into account in order to conform to industry standards. It is imperative that the necessarysafety precautions are implemented in order to ensure the efficient running of the plant and thegeneral well being of all persons in the vicinity. In order to effectively identify all the possiblesafety hazards it is necessary to carry out a HAZOP analysis and go through all the componentsof the system.

Besides conducting a HAZOP analysis there are several other areas that have to beconsidered with regards to safety, these include; the general layout of the plant, piping, venting,equipment, instrument and electrical, safety equipment and raw materials. The plant is designed

to address each of these areas; the proceeding section will go into greater detail on each area.The general layout of the plant plays a significant role in general safety. The plant floor

has to be fitted with adequate drainage, in order to avoid physical injury to employees. The planthandles a significant amount of electricity, and due to the incompatible nature of electricity andliquids adequate drainage is essential to prevent electrical shocks. Furthermore, proper drainagewould prevent accidents arising from a loss of balance. Sufficient aisle ways, guardrails and

platforms have to be in place to ensure for the safe operation of the plant by its employees. It isalso necessary for there to be adequate headroom for the employees from all the piping and

power lines. Finally, all emergency exits have to be fully visible, accessible and permit access for

emergency vehicles.Safety considerations need to be taken when dealing with piping in a facility. These pipes

carry a host of fluids, the most significant for this process being high temperature fluids andthose that are under a high pressure. The syngas coming out of the gasification chamber is at avery high temperature (1600 oC) before going through a heat exchanger, and therefore thefollowing safety measures have to be in place:

I. Ensure all the steam lines and syngas pipes are well insulated and labeled.

II. Relief valves in place to prevent plugging and subsequently rupturing the pipe

line.

III. Drains to relieve pressure on suction and discharge on all process pumps

IV. All overflow lines directed to safe areas.

All the instruments in the plant require fail safe controls. They should be properly labeledand grounded. The equipment should also be designed to permit lockout protection in the case of


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an emergency. Finally, all process equipment should be connected to a standby power source,and emergency lighting put in place in the advent of a power outage.

General safety equipment required at the plant include fire extinguishers throughout the plant, respiratory equipment, flammable vapor detection apparatus, safety alarms and a detailed

emergency evacuation procedure for all employees. There should also be a designated safe areaoutside the plant where all employees assemble in advent of an emergency.

Any raw materials coming into the plant should be properly labeled and handled in theappropriate manner. Any hazardous or toxic material should be treated with the correct personal

protective equipment, such as gloves, glasses and masks.

Finally, a detailed HAZOP analysis has been carried out to identify all the possible safetyhazards or risks that could arise from the process. The results of this analysis are shown below.

Table 14. HAZOP analysis

Equipment Hazardous State Cause Consequence Resolution/fix

Gasifier Ov erpressurization

Blockage of line Rupture of Gasifier Isolation of o v er pressurizedgasifier, PRV to HeatSink/secondary chamber topre v ent o v er pressurization

Blockage of line Inadequate Heat Transfer,formation of dioxins,furans, other toxins

PRV to v ent toxic gasses toHolding tank

Ov er Accumulationof Slag

Lots of Inorganic Waste Increased Residence timeof slag

~

Loss of Power to

Gasifier

Broken/Disconnected

Power Line

Gasifier shuts down,

possible underpressurization

Close Gas exit line, monitored

by pressure sensors

Crusher UnexpectedShutdown

Loss of Power, switch trip Too much waste water Close gasifier inlet, stop wastewater pump

HeatExchanger

Rupture of lines ~ Syngas enters steam/waterline

Div ert gas to second heatexchanger, decrease trashprocessing rate

Loss of Water Flow Pump Break Inadequate Heat Transfer,formation of dioxins,furans, other toxins

Div ert gas to second heatexchanger, decrease trashprocessing rate

Loss of Syngas flow Not enough O rganics,blockage

Water in steam turbine Flow and temp sensors onsteam line, shut down steamturbine

Filter Clogged Filter Ov er accumulation of particulates

Ov er pressurization of housing, line

PRV to Syngas holding tank

Ruptured Filter ~ Particulates continue on togas turbine

Flow meter will register toohigh, di v ert flow to secondaryfilter

GasTurbine

Loss of Flame Temp falls below autoignition of C O

Not generating power, C O to v ent

Flame sensor, restart turbinewith fuel

Slag Solid Slag Temp falls below melting blockage of slag o v erflow Secondary line


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Ov erflow point of slag

6.2 Health Considerations

The main health considerations that have to be taken into account for this process are theformation of dioxins and furans when vitrifying MSW. Dioxins and furans can cause anumber of health risks, such as cancer and changes in hormone levels. Animal studies haveshown that animals exposed to dioxins and furans experienced changes in their hormonesystems, changes in the development of the fetus, decreased ability to reproduce and asuppressed immune system [14].This is a very serious health risk if exposed to humans, for that reason the process has been specifically designed to minimize the possibility of theformation of these toxins.

Dioxin formation typically occurs if the temperatures produced by the combustion

process do not exceed 250o

C throughout the entire combustion chamber. However, when thechamber temperatures exceed the 250 oC threshold, as will occur in the plasma gasifier, thechlorinated materials will dissociate themselves of the chlorine atoms and the chlorine will

preferentially combine with hydrogen to form HCl (which is then removed in the gastreatment system and removed in the scrubber with NaOH to form a benign salt).Additionally, the rapid cooling of the syngas from 1600 oC to below 250 oC in the heatexchanger will prevent the reformation of dioxins and furans from elementary molecules inthe syngas due to the de novo synthesis back reactions [1].

Finally, any residual particulates or mercury in the syngas stream will be removed using a

carbon filter to leave a clean syngas that is the fed to the gas turbine. The exhaust streamfrom the turbine will have scrubber in place to take care of any trace amounts of NOx, SOxand CO that may be left over.

6.3 Environmental Considerations

The main environmental concerns that could potentially arise from this process are tracemetals and halides escaping into the air or soil. Carbon filters and scrubber are in place totrap any of these potentially hazardous elements, and thus should not pose a significant risk to the environment. Heavy metals could potentially leach into the soil through the slag that is

produced as a byproduct of the process. However, it has been found that the slag createdfrom the process contains trace amounts of these metals which are below regulatory limits,on the order of less than 0.05 mg/L for most of the heavy metals [8].


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7. Conclusions and Recommendations

7.1 Conclusion

The proposed design of a plasma gasification plant to manage the MSW generated by the

city of Boston has been reported in this paper. From calculations and literature it has beendeduced that this process has satisfied the design goals specified. The following conclusionswere arrived at:

1) The plasma gasification plant was found to manage and treat the MSW generated by thecity of Boston. The proposed plant design has the capacity to handle the estimated 1400tons a day of MSW generated by the city, with a net weight reduction of approximately80%.

2) Ability to process MSW using an environmentally sound method. Waste treated with plasma does not produce hazardous bottom or fly ash. The by-products (slag) do notleach into the soil.

3) A positive net energy balance was obtained. From calculations conducted it was foundthat the plant utilized approximately a third of the energy generated, and sold the excess

back to the grid.

4) Ability to handle a wide range of waste, such as hazardous and biomedical waste. This isdue to the robust nature of the plasma arc system which requires little or no modificationsto handle the various waste feeds.

5) Eliminates the need for landfills. The byproducts of the process are energy, slag andCO2, all of which can be sold for a profit. The system could use landfills as a source of fuel and thereby reclaim landfills that are no longer active whilst simultaneouslygenerating energy.

This process is economically feasible according to the real word data collected at other plasma arc facilities. Our goals of being environmentally friendly, with a positive net energy gainand being safe have a real potential of being achieved. The plasma arc technology put forth bythis report shows that it is the solution for Bostons solid waste problem.

7.2Recommendations

Based on the findings of the report, the following recommendations are proposed:

1) Branch out of handling only MSW, and look into processing hazardous and biomedical waste. The process does not need significant modifications and wouldgenerate more income in the form of tipping fees.


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2) Design an isolated gasifier to handle biomedical waste that is generated from thevarious medical facilities found in the Boston area.

3) Scale back the size of the facility, this is only due to the capital intensive nature of the plant. Securing the funds required for this project might prove to be difficult whether

it be from private or government sources. At present the facility has been designed tohandle the waste generated by the City of Boston and also reclaim used up landfills.


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8. Acknowledgements

We would like to acknowledge the following for giving us the support we needed:

Dr. Ronald Willey, thank you for all the help and guidance you have given us throughout our

stay at Northeastern University. You were a large part of our experience here and we areappreciative.

Dr. Jillian Goldfarb, thank you for all your help you have given us over the course of our projectand keeping us properly motivated during the tough parts. We couldnt have done it without you.

Dutemp Corporation, thank you for all real world data you have provided us. It was invaluable toour project.


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9. Bibliography

1) Kwak, T.H., Maken, S., Lee, S. E nvironmental aspects o f gasi f ication o f Koreanmunicipal solid waste in a pilot plant . Fuel 18 April 2006.

2) Moustakas, K., Xydis, G., Malamis, S., Haralambou, K-J Analysis o f results f romthe operation o f a pilot plasma gasi f ication/ vitri f ication unit f or optimizing its per f ormance Journal of Hazardous Materials 8 June 2007

3) Minutillo, M., Perna, A., Di Bona, D. M odelling and per f ormance analysis o f anintegrated plasma gasi f ication combined cycle (IPGCC)power plant EnergyConversion and Management 20 July 2009

4) Mountouris, A., Voutsas, E., Tassios, D. Solid waste plasma gasi f ication: Equilibrium model development and exergy analysis Energy Conversion andManagement 7 December 2005

5) Gomez, E., Amutha Rani, D., Cheeseman, C.R., Wise, M. Thermal plasma

technology f or the treatment o f wastes: A critical review Journal of HazardousMaterials 11 April 2008

6) City of Boston The M ayors Per f ormance Report 2009http://www.cityofboston.gov/bar/PDFs/2009/PWD-BTD_BAR%20FY09%20Q4.pdf

7) Bureau of Waste Prevention Active Land f ills web.http://www.mass.gov/dep/recycle/actlf.pdf May 2009

8) Dutemp Corporation web. www.dutemp.com9) Morgenthaler (Patent)10) Saffarzadeh, A., Shimoka, T., Watanabe, K. Characterization study o f heavy-

metal bearing phases in M SW slag Journal of Hazardous Materials 4 September 2008

11) N.A.I.M.A North American Insulation Manufacturers Association web.www.naima.org

12) Grodan web. www.hydroponics101.com13) Specific Heat of Gasses at Various Pressures. Engineeringtoolbox.com. web14) EPA web.

http://www.epa.gov/wastes/hazard/wastemin/minimize/factshts/dioxfura.pdf

15) Wastecare web. www.wastecare.com16) Siemens web www.siemens.com17) Peters, M.S, Timmerhaus, K.D Plant Design and Economics for Chemical

Engineers Fifth Edition, McGraw-Hill, New York (2003)


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10. Appendix

10.1 MSDS

The following MSDSs were obtained from Sigma Aldrich and include all the hazardous

chemicals that are expected to be found in the process. Please refer to www.sigma-aldrich for greater detail.

AluminaCalcium OxideCadmium OxideChromium (III) OxideCopper (II) OxideIron (III) OxideLead (II) OxideManganese (II) Oxide

Magnesium Oxide Nickel (II) OxideSilicaSodium OxideZinc Oxide


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10.2 Energy Generation Calculations


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Equ ation: Ws=m(H1-H2)* Eff iciencyms H1 500 C and 8600 K pa H2 500 C and 10 Kpa Eff iciency Ws


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10.3 NPV calculations

Final Design Report Plasma Arc Waste Disposal Group 11

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