new water saving fgd technology in south africa l 2018 new ... · new water saving fgd technology...

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New water saving FGD technology in South Africa VGB PowerTech 11 l 2018

New water saving FGD technology in South Africa

Authors

Kurzfassung

Einsatz einer neue wassersparenden REA-Technologie in Südafrika: Die CFB-FGD-DemonstrationsanlageWasserverfügbarkeit wird eine zunehmend wichti-gere Frage, vor allem in Ländern wie Südafrika, eines der trockensten Länder. Weltweit werden was-sersparende Technologien für die Entschwefelung von Rauchgasen eine interessante Alternative, da Wasser zu einer knappen und wertvollen Ressource zählt. Die meist genutzte Technologie zur Abtren-nung von Schwefeldioxid aus Rauchgasen großer Kraftwerke ist die nasse Entschwefelung mit Kalk-stein und anschließender Oxidation (WFGD). Die-ser Prozess nutzt eine große Menge an Wasser zur Sättigung des gereinigten Gases, gebunden im Re-aktionsprodukt Gips und im ausgeschleusten Ab-wasserstrom.

Derzeit ist eine Demonstrationsanlage am Eskom-Standort Kendal in der Evaluierungs- und Pla-nungsphase, um den Einsatz einer halb-trockenen Technologie, der sog. zirkulierenden Wirbelschicht (CFB FGD), mit südafrikanischen Bedingungen zu testen. Die Demonstrationsanlage wird einen Teil-strom von ca. 5 % des Volumenstroms einer Kessel-linie behandeln und ist nach dem bestehenden Elektro-Filter positioniert. Der optimale Betriebs-punkt und der Einfluss verschiedener Prozesspara-meter wie Flugaschegehalt und Qualität des Sor-bents werden dabei untersucht, um den Prozess mit südafrikanische Randbedingungen zu optimieren.

Der Bau einer Demonstrationsanlage hat verschie-dene Vorteile für Eskom. Während der Engineering Phase können die beteiligten Ingenieure vor Ort Wissen über das Design einer CFB FGD erlangen. Während der Beschaffung ergeben sich Erkenntnis-se über mögliche Lieferanten, Materialverfügbar-keiten, Lieferzeiten und Preise für die Anlagenteile. Während der Montage Phase können Herausforde-rungen beim Bau identifiziert und gelöst werden um als „lessons-learned“ bei einer möglichen, zu-künftigen Implementierung der Großanlage zu dienen. l

Annikie MoganelwaProcess Engineer Air Quality ControlPuseletso GodanaProcess Engineer Air Quality Control Eskom Holdings SOC Ltd Megawatt Park, Sandton, South AfricaSabrina SchäferProcess Engineer Flue Gas Cleaning Steinmüller Engineering GmbH IHI Group Company, Gummersbach, Germany

New water saving FGD technology in South Africa: CFB FGD demonstration plant Annikie Moganelwa, Puseletso Godana and Sabrina Schäfer

Introduction

Environmental considerations have in-creased over the years in South Africa with a focus on sustainable development. Sus-tainable development integrates social, economic and environmental factors in planning, implementation and evaluation of decisions to ensure a sustainable life for present and future generations. Core focus areas are pollution prevention or reduc-tion, prevention of ecological degradation and conservation of natural resources. The National Environmental Management Act (NEMA) No. 107 of 1998 governs regula-tions concerning the environment and is supported by other Acts. The legislative re-quirements provide a driver for existing and new plants to comply with the defined limits.Kendal Power Station utilises indirect dry-cooling which consumes approximately 95 % less water than the conventional method of wet cooling1. This is an advantage given the recent water shortages faced in the country. South Africa is ranked as one of the 30 driest countries in the world2. In or-der for the power station to continue to meet the more stringent future air quality requirements whilst considering the limit-ed water availability, options that would benefit both aspects were considered. The Circulating Fluidised Bed (CFB) flue gas desulphurisation was chosen as the most viable option based on preliminary desktop assessments and a demonstration trial has been planned for to confirm the viability.The selected power station consists of six 686 MW (total 4,116 MW) pulverised coal

boiler units, each split into two flue gas streams. There is no flue gas desulphurisa-tion in place however the station complies with existing SO2 limits which will de-crease from 2025. The plant has a remain-ing operating life of 35 years which is the longest remaining life of the power utility’s older fleet of coal fired power stations and has been identified as the largest emitter of SO2 emissions in the Highveld Priority Area (HPA). This was considered when se-lecting the existing power station as the site for the desulphurisation demonstra-tion plant.

Factors contributing to desulphurisation within the South African power generation sector

Air Quality compliancePower generation is a large contributor to the SO2 emissions (82 %) contributing 1,633,655 tons per annum. Sulphur oxide emissions are governed by the Minimum Source Emission Standards of the National Environmental Management Air Quality Act (NEMAQA) No. 39 of 2004 (F i g u r e 1 ). Each Eskom generating facility has its own Emission License which will detail the compliance requirement for the facility. This varies between facilities due to ambi-ent air quality priority area and age of the power generation facility.The current legislation (Ta b l e 1 ) re-quires that SO2 emissions comply with a limit of 500 mg/Nm3 (10 % O2) for new

Priority AreaAir Quality

ManagementPlan

AmbientAir QualityStandards

Dust FalloutManagementRegulations

MinimumSource

EmissionStandards

MunicipalBy-laws

EmissionLicense

NationalEnvironmentalManagement:Air Quality Act

(No 39 of 2004)

Fig. 1. NEMAQA Statutes.

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VGB PowerTech 11 l 2018 New water saving FGD technology in South Africa

plants, 3,500 mg/Nm3 (10 % O2) by 2015 for existing plants and a limit of 500 mg/Nm3 (10 % O2) by 20207 for all plants. On 24th February 2015 the Department of En-vironmental Affairs (DEA) resolved to postpone the effective date for implemen-tation of the 500 mg/Nm3 emissions limit for certain existing power plants. Kendal Power Station’s current emissions of SO2 are > 2,700 mg/Nm3.The selected power station is located in the Highveld Priority Area (HPA)6. The power station was identified using ambient air quality monitoring and dispersion model-ling as the largest emitter in the HPA5, as seen in F i g u r e 2 . SO2 emissions trended over a 9 year period in the HPA indicate that although the SO2 concentrations average below the South African National Ambient Air Quality Standards (SA NAAQS) these values are far above the World Health Organisation (WHO) figures, as seen in F i g u r e 2 . The associated daily averages are shown in F i g u r e 4 .

Water availabilityWater restrictions in South Africa have re-cently become a reality in certain parts of the country due to drought and other con-tributing factors. Water saving initiatives are therefore gaining prominence for mu-nicipal, agricultural and industrial applica-tions. A 2012 Water Research Commission study found that South Africa utilises 235 litres per capita per day (l/c/d) compared to the global average of approximately 175 l/c/d and a model developed by the Uni-versity of Denver forecasts that the water demand will increase till 203513. Most of

South Africa’s currently available water supplies have been allocated with only a small amount available to cater for addi-tional desulphurisation demands. The se-lected power station is the largest indirect dry-cooled power station in the world; hence the proposed desulphurisation tech-nology should consider this and mitigate the need for additional water resources to meet environmental limits.

Use of coalThe use of coal as a primary source for power generation is largely the case in South Africa. Approximately 95 % of the sulphur in coal is released as SO2 with the raw flue gas3. Alternate methods for power production have yet to be developed to a stage where the country can drastically re-duce its use of coal. On average 224 million

tonnes of marketable coal is produced an-nually and 53 % of this is utilised for power generation. The local coal reserves are esti-mated at 53 billion tonnes, providing an estimate of 200 years coal supply4.

Sorbent cost and availabilityThe availability and cost of sorbent is an important consideration. Wet flue gas des-ulphurisation utilises limestone (CaCO3) as a sorbent and circulating fluidised bed desulphurisation utilises hydrated lime (Ca(OH)2) or burnt lime (CaO) which is then slaked to form hydrated lime. Isolated high-grade deposits of limestone occur12 in South Africa; the Highveld region contains Limestone deposits which are available, however the quality and purity is lower than what is conventionally required for the chemical process.South Africa’s good limestone sources are located far from the Highveld area. The use of “dolomitic lime” as an alternative will be evaluated during the demonstration trials. Dolomitic lime is cheaper however con-tains a lower usable CaO content.

By-product disposalThe National Waste Management Strategy (NWMS) is a legislative requirement of the National Environmental Management Waste Act (the “Waste Act”) No. 59 of 2008. The purpose of the NWMS is to achieve the objectives of the Waste Act. Wet desulphurisation currently produces a waste stream which is further treated in a waste water treatment plant (WWTP) and is recirculated back to the process plant for further use. The WWTP mixed waste salt is disposed off-site. The WFGD gypsum by-product produced is saleable. In the Circulating Fluidised Bed option no waste water is produced so further treat-ment is not required. The CFB FGD by-product is composed of reaction products, unreacted sorbent and fly ash. This prod-uct is deposited in a landfill and based on its composition it can be reused in other industries.

Tab. 1. SO2 Emissions Limits (Point Source Emissions).

SO2 Emissions Limit Applicable to Compliance Date

500 mg/Nm3 at 10 % O2 New Plants 2010

3,500 mg/Nm3 at 10 % O2 Existing Plants 2015

500 mg/Nm3 at 10 % O2 Existing Plants 2025

Highveld Priority Area

Vaal Priority Area

Waterberg Bojanala Priority Area

SA Provinces

Fig. 2. Air Shared Priority Area of South Africa (Reference: South African Air Quality Information System – 2018).

SO2

Conc

entra

tion

in p

pb

Highveld Priority Area

SO2 – 9 Year Trend

30

25

20

15

10

5

0

19SANAAQS

WHOguideline

Ermelo Hendrina Middelburg Secunda Emalahleni

2008 2009 2010 2011 2012 2013 2014 2015 2016

Fig. 3. Highveld Priority Area SO2 emissions 9-year trend (Reference: South African Department of Environmental Affairs).

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Technology selection decision

Air quality control using desulphurisation technology in power plants is new within South Africa. The following criteria was used to evaluate which technology should be selected based on the South African con-ditions: SO2 removal efficiency, low capital expenditure (CAPEX), low operating ex-penditure (OPEX), reduced water con-sumption, retrofit simplicity and economi-cal with respect to remaining plant life.

Various technology alternatives were in-vestigated, the alternatives can be catego-rised in groups of pre-combustion and post-combustion desulphurisation tech-nology options as seen in F i g u r e 5 .

The technology evaluation was a phased approached; in the first step feasible com-mercial scale technologies were scanned for further evaluation. During evaluation two qualifying criteria were used to enable

to the arrival to a recommended FGD tech-nology. Firstly, a qualitative criteria was defined which served as a gate keeper for all technologies identified during the scan-ning study. Secondly, a quantitative crite-ria was tabled which was a set of technical requirements which were used to compare technologies against each other.

Qualitative CriteriaThe qualitative criteria used entailed the following

– Level to which requirements are met – Ability of the technology to meet the stakeholder‘s requirements

– Level to which design criteria are met – Ability of the technology to meet the design requirements.

– Level to which stakeholder values are met – Ability of the technology to meet the strategic objectives

– Maturity of Technology – Commercial availability of the technology.

– Constructability within normal GO period – Impact of the technology’s ret-rofit to the plant on the station’s general outage duration.

Quantitative CriteriaThe quantitative criteria were made up of the following:

– Maximum SO2 Removal Efficiency % – maximum SO2 removal efficiency pos-sible with the existing technology

– Pressure Drop Impact – level of impact to the system pressure, and requirement of additional fans to cater for the pres-sure drop

– Power consumption of plant utilities – maximum power consumption due to FGD technology installation (% MW im-pact on installed capacity)

– Environmental Impact (Effects on Waste Water Treatment System) – Number of waste streams required based on waste classification

– Water Usage – water consumption asso-ciated with the FGD technology (l/kWh gross).

– Estimated construction time frames – timelines required to construct [Target date 2025]

– Required Footprint outside stations boundary – relative footprint required in addition outside of the station bound-aries (Hectares).

– Site Arrangement – ease of retrofitting the FGD technology to site.

– Plant Operating Complexity – Addi-tional operation interventions required (linked to number of systems/unit oper-ations).

The technology selection exercise derived two technology option which would be suited in the South African context; the WFGD and the CFB FGD. A brief summary of the major comparisons is summarised in Ta b l e 2 and Ta b l e 3 ; these compari-sons were based on the large scale retrofit scenario.CFB FGD has the capability to achieve the required emissions limits while operating with reduced water consumption of up to >30 % less than conventional wet FGD technology. The development of CFB FGD technology could mitigate the need for ad-ditional water recourses to achieve envi-ronmental compliance.Best-fit FGD technology should have the following attributes:

– Meet emissions limits – Fuel flexibility – Low Capex – Low Opex – Reduced water consumption – Economical with remaining operating

lifePreliminary technology selection studies for the selected power station indicated that CFB FGD would be the most viable op-

SO2

Conc

entra

tion

in p

pb

SANAAQS

WHOguideline

Ermelo

Middelburg

Emalahleni

SO2 Daily Averages90

80

70

60

50

40

30

20

10

0

Hendrina

Secunda

NAAQS

20 A

pr

20 M

ay19

Jun

19 Ju

l

19 A

ug

17 Se

p

17 O

ct

16 N

ov

16 D

ec

15 Ja

n

14 Fe

b

16 M

ar

15 A

pr

15 M

ay

Fig. 4. SO2 Daily Averages (Reference: South African Department of Environmental Affairs – May 2017 Multi Stakeholder Reference Group Report).

EconomiserInjection SDA/CFB WFGD

Furnace Injection

Coal

Coal Switching

Coal Beneficiation

Fig. 5. FGD Technology Overview.

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tion provided that all criteria are met. A decision was made to evaluate this option further by initiating a demonstration plant project.Various advantages of the CFB FGD tech-nology were identified and need to be vali-dated with the demonstration plant before full scale implementation. If one or more of the criteria are not met, the initial envis-aged savings for this option will not be re-alised and prove more costly to operate compared to the WFGD.The sorbent cost is the largest contributor to the overall OPEX cost. An advantage of the CFB FGD is the possibility to use low quality sorbent, which could reduce the OPEX considerably. The use of “dolomitic lime” (calcination of dolomitic limestone) as a low quality sorbent has to be verified. If the lower quality lime does not prove fea-sible, the cost saving will not be realised and the WFGD would be the cheaper op-tion to operate.

CAPEXThe overall CAPEX cost is higher for WFGD than CFB FGD for application at the select-

ed power station. F i g u r e 6 shows the contributing costs for the two technolo-gies.

OPEXCFB FGD has a higher operating cost that WFGD for application at the selected pow-er station (F i g u r e 7 ). Sorbent utilisation is the largest contributing factor to the high operating costs for CFB FGD. The cost of burnt lime is 3 times that of the limestone cost in South Africa15.

CFB FGD overview

Raw flue gas containing fly ash from the up-stream boiler unit enters into the CFB FGD reactor where the main desulphurisation takes place. The flue gas enters the bottom of the reactor through venturi nozzle/s. The reactor relies on turbulent mixing of the flue gas and sorbent. The sorbent parti-cles are continuously abraded exposing the unreacted alkali surface areas of the parti-cles8. The CFB FGD process utilises hydrat-ed lime (Ca(OH)2) as a sorbent to bind the sulphur oxides and other gases in the flue

gas as shown in the below reactions:Hydrated lime reaction with SO2:

Ca(OH)2 + SO2 CaSO3 + H2O (Eq. 1)

Ca(OH)2 + SO3 CaSO4 + H2O (Eq. 2)

Hydrated lime reaction with other gases eg. HF and HCl:

Ca(OH)2 + 2HF CaF2 + 2H2O (Eq. 3)

Ca(OH)2 + 2HCl CaCl2 + 2H2O (Eq. 4)

The hydrated lime can be bought or pro-duced on site by the slaking process. The active pores on the hydrated lime particle influence the reactions and depend on the slaking process as well as the burnt lime used. The optimum temperature for the above reactions is 70-90 °C and this can be achieved by quenching the flue gas in the reactor with water. Water is added with high pressure nozzles in the form of fine droplets. The nozzles ensure are to be ar-ranged to ensure homogenous distribution across the reactor. The process must oper-ate above the dew point in order to prevent water from condensing out of the flue gas. The desired absorption efficiency of 95 % is achieved by choosing a stoichiometric ra-tio11. The stoichiometry ratio is the molar ratio between Ca(OH2) and flue gas SO2 and is chosen as a factor of the raw gas SO2 concentration. The consumption of hydrat-ed lime is dependent on the SO2 concentra-tion and stoichiometric factor hence the factor will influence the design of the slak-ing plant.The flue gas moves up through the reactor and the height is chosen to ensure an ade-quate retention time is reached. A fabric filter plant (FFP) is located downstream of the reactor in which the solids from the re-actor are separated. The process filter pro-vides additional time for desulphurisation as the solids adhere to the tubes to form a filter cake as the flue gas passes through. An additional reaction efficiency of ap-

Tab. 2. FGD Water Usage Comparison.

Water usage

Baseline station water usage WFGD CFB FGD

l/kWh 0.21 0.42* 0.35*

* as determined during the in-house process designs of the two technologies.

Tab. 3. FGD Waste Management Comparison.

Waste management applied

WFGD

Gypsum (Type 3) Re-use, recovery, disposal

Chemical Salts (Type 1) Disposal

Chemical Sludge (Type 1) Disposal

CFB FGD

Calcium Sulphate/sulphite mixture(requires classification, likely Type 3)

Re-use, disposal

Mio

. ZAR

BMH (Bulk MaterialHandling)

Sum Indirect CapitalCosts

Spare Parts first 2 years

Minor Equipment

C&l (General)

Electrical (General)

LPS

Civil

Chemical (ZLD)

FGD ProcessWFGD CFB FGD

Fig. 6. CAPEX comparison for total scope (Reference data is from an internal feasibility study).

Mio

. ZAR

/yea

r

WFGD CFB FGD

Rail Yard

Station Power Costs

LandfilI of by-product Gypsum

Waste water Disposal (ZLD)

Exis. ESP cost

Water costs

Reagent consumption

Personnel costs

Maintenance costs

Fig. 7. OPEX comparison for total scope (Reference data from an internal feasibility study).

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proximately 10 % is achieved. The reactor and process filter can replace the upstream de-dusting system if the fly ash is not re-quired for sale. A hopper for collection of the solids product is located at the bottom of the process filter.

CFB FGD demonstration plant implementation

JustificationFive main reasons were considered to mo-tivate for the implementation of the dem-onstration plant:

– Currently one power station in the coal fired fleet is in commercial with FGD op-eration utilising WFGD, treating flue gas from one of six boiler units. The util-ity has no experience in the applica-tion of CFB FGD technology for South African and existing power station conditions. The viability and risks associ-ated with CFB FGD technology un-der these conditions needs to be evalu-ated.

– The CFB FGD utilises burnt lime (CaO) as sorbent which is hydrated either on site to Ca(OH)2 or is directly purchased for use in the process. A limestone source can be used as a sorbent; the limestone will be first calcinated to burnt lime and secondly hydrated to hydrated lime. The quality of limestone in the Highveld re-gion is expected to be low (below 85 % CaCO3 content) therefore the usability of such quality needs to be investigated. In addition the possibility of using lime sources close to the power plant area could contribute to savings in transpor-tation costs.

– Due to the current water scarcity in South Africa and since the selected pow-er station already utilises indirect dry-cooling, which contributes to significant water savings compared to wet-cooling, the actual water usage should be con-firmed with further possibility for opti-misations.

– Despite using less water compared to wet technologies, the CFB FGD still requires water for its process requirements. This will increase the station’s existing water requirement. Provisions have to be made for the additional water allocation and necessary infrastructure from the catch-ment. The demonstration plant will test if lower quality water can be used for quenching versus the raw water. A waste water stream from a nearby power sta-tion will be evaluated as a potential re-placement.

– Another challenge is that South African coal typically contains high ash content. The CFB reactor has limitations for par-ticulate matter concentration and the particle size distribution. The need for fly ash pre-separation will be investigat-ed during the demonstration plant trials.

The solids recirculation will need to be well controlled however if the fly ash in the system exceeds the limits, it has to be bled out of the system. This directly in-creases the loss of unreacted lime.

ObjectivesThe test campaign objectives shown in Ta -b l e 4 have been outlined to prove the vi-ability of the CFB FGD technology in South Africa:

Feedstock limitations and optimisationSorbentIt is envisioned that the sorbent including the dolomitic limestones into the process will come from within a defined radius around the power station. The plan is to split the campaigning into two phases namely phase 1 which tests the operations and performance with sorbent of good quality and phase 2 with sorbents of sub-standard quality. This should enable the test team to separate the operational from the performance issues and be able to pro-ject the requirements for the major plant’s retrofit.

WaterThe quality of the waste water stream from an adjacent plant is known, however, the stream is a high chloride content stream. The impact of the chlorides on the mechan-ical design of the water distribution and on process operations need to be assessed. The stream also contains a moderate amount of abrasive solids which need to be

assessed for suitability for the high pres-sure nozzles.

Coal and load variationsThe power plant receives a consistent qual-ity of coal; it is therefore expected that the flue gas stream into the demonstration plant will be consistent. On the other hand, the plant reduces its load to the minimum during certain points during the day; this transient nature of the plant will be built-into the testing campaigns to test the dem-onstration plant’s performance in transient modes.

Capacity considerationsThe full scale proposed CFB FGD retrofit will treat 4,700,000 m3/h (wet, actual con-ditions). The demonstration plant is planned to be in operation for 15 to 24 months in order to obtain sufficient data for the different variables. Three flue gas vol-ume flow capacities were considered; 10,000, 127,000 Nm3/h and 180,000 Nm3/h. The 10,000 Nm3/h plant had the advantage of being built on a transportable framework for later use at the other power plants how-ever the scaling up factor was large and the process equipment were not standard. The next capacity of 127,000 Nm3/h (5 % of flue gas flow of one boiler) was considered as standard equipment could be used and this reduced the scaling up factor however it was found that the actual restriction on the demonstration plant sizing was the hydra-tion plant. A minimum CFB plant size had to be considered to ensure continuous op-

Tab. 4. Objectives of CFB FGD Demonstration Plant.

Test Description Objective

Lower Burnt Lime Qualities Calcination of Dolomitic Limestone The capability to calcinate lower quality limestone may result in the reduction of OPEX costs and the sourcing of sorbent in closer prox-imity to plant.

Optimisation of Sorbent Use Determine Sorbent Stoichiometry factor Variation of the recirculation flow rate has an impact on the utilisa-tion of the sorbent.

Fly Ash Concentration Limit at Reactor Inlet

Requirement for pre-separation Operation experience based on South African coal characteristics and high ash content.

Low quality water use Impact of low quality water on process design and operation

The use of low quality water sourc-es for humidification may relieve the adjacent power plant of the need for a waste water treatment plant and will reduce the need for raw water.

Operation optimisation • Part load variation• Ramping rate• By-product recirculation• Shutdown • Pressure drop improvements• Lime Hydration

Water consumption • Low quality water• Humidification requirement

Process efficiency • Lime usage• Ash at inlet to absorber• Influence of ash and low lime quality

Multipollutant • Removal efficiency of other acids • (SO2, CO2, HCl, HF, Hg)

Mechanical • Assessment of Materials of construction

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eration of the hydration plant for a consist-ent quality of hydrated lime. A demonstra-tion plant capacity of 180,000 Nm3/h was chosen which met these requirements.

Demonstration plant design conceptProcess designThe demonstration plant is designed to achieve 500 mg/Nm3 SO2 removal (10 % O2 reference) and a 50 mg/Nm3 particulate removal. A worst case coal condition was taken as the design reference with a sulphur and ash content of 1 % and 41.2 % respectively (moisture free). Burnt lime quality of 90 % was used as an input to the process design. To simulate real plant op-eration, the inlet fly ash content range of 0 to 51,000 mg/Nm3 (10 % O2 reference) is used. The test will demonstrate if fly ash pre-separation is required based on the fly ash loading and particle size distribu-tion.

Equipment designThe CFB FGD plant for one unit will consist of a reactor, a process filter, booster fan, a sorbent preparation plant (lime silo and hydrator), waste handling/recycle facili-ties and auxiliary equipment. The tie-in points will be downstream the existing ID fan and upstream the stack inlet. Figure 8 below illustrates the process flow of the demonstration plant.A single reactor is required. Water for quenching is added to the reactor via high pressure nozzles installed around the reac-tor cross section. A booster fan is installed downstream of the process filter to compensate for the pressure drop from the reactor, ductwork and FFP. Burnt lime is delivered onsite by trucks. The burnt lime is stored in a burnt lime silo. Hydrated lime will be produced onsite in a hydrator. The hydrated lime produced will be stored in a hydrated lime silo until use and will be pneumatically conveyed from the hydrated lime silo and injected directly into the reactor. By-product collected from the process filter hopper will be sent to the by-product silo for storage. The silo will store the by-prod-uct until disposal by truck to the dedicated disposal facility.

Arrangement designAs shown in F i g u r e 9 and F i g u r e 10 , the ideal location for the demonstration plant is downstream the induced draft fan as it provides sufficient space. A slip stream is taken from the “clean gas duct” after the existing ESPs and returned into the same duct after the CFB FGD plant for discharge via the stack. Two dampers at the inlet and outlet of the slip stream are provided to control the flue gas flow and to isolate the demonstration plant.No material modifications are envisioned for the existing ductwork since at 85 °C the

flue gas exiting the CFB FGD plant is above the saturation temperature. The demon-stration plant has been designed to avoid any clashes with existing plant.

By-product handling and disposalCurrently the selected power station for the CFB FGD retrofit sells 23,000 tons of fly ash per month to commercial users; this reduces the burden on the station’s ash dams lightly. However, if a CFB-FGD is in-stalled, it would likely replace the current ESPs and if the campaigns prove that the CFB-FGD can operate with higher ash load at inlet, this will require a business case to source saleable ash at a nearby power plant or elsewhere. This is because a typical CFB by-product consists of fly ash, unreacted sorbent, CaSO3, CaSO4, CaCO3, CaCl2 and CaF2 thereby modifying the quality of the ash being sold currently.

The demonstration plant will also address the classification of the by-product. The South African waste legislation recognises wastes of various classes according to the wastes’ hazardous nature which imposes where and how the wastes are disposed. The classification from the demonstra-tion plant waste will then be used as a ref-erence for the full scale retrofit if the dem-onstration plant proves successful. Waste generated during the trials will be tempo-rarily stored in a silo and transported to a hazardous waste disposal facility until clas-sified.

Demonstration implementation challenges and risks – Retrofit and arrangement The requirement for the demonstration plant by the host plant was that there should be least disruptions to the opera-tions. The host plant is a 6 unit plant with

Fig. 8. CFB FGD Demonstration Plant Main Equipment.

Fig. 9. CFB FGD Demonstration Plant Proposed Layout (Top View).

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substantial infrastructure around the plant stacks and identifying the host unit was subject to several iterations.The ideal slip stream would have been from air heater outlet but this was deemed disruptive to draught group balance since the station is a parallel twin gas flow draught system.

Sorbent sourcingThe calcination of dolomitic limestones for the demonstration plant is by far the larg-est risk for the project. The project will need to partner with local calcination plants to enable calcination and therefore the success of a large portion of the project.

Conclusion

Stricter air quality requirements in South Africa has prompted an investigation into possible SOx reduction measures at exist-ing coal fired power. The selected power station for the retrofit has been chosen due to the longest remaining operating life and has been identified as the largest SO2 emit-ter in the Highveld Priority Area.Two technologies were compared for im-plementation: wet and semi-dry circulat-ing fluidised bed flue gas desulphurisation. The CFB FGD was chosen due to its poten-tially lower lifecycle costs and retrofit ben-efits. A demonstration plant project is in progress to validate the successful imple-mentation of the technology with respect to achieving SO2 emission limits, multi-pollutant removal, process flexibility in re-lation to feedstock availability and charac-terisation of by-products.

A demonstration plant is of important in-terest to the FGD body of knowledge. The demonstration plant will verify and vali-date unconventional CFB FGD operation. There is high motivation in the direction of lower water FGD technologies in South Af-rica. The project benefits the country through local skills development and local resources utilisation potential.

Acknowledgements

The authors would like to thank Steinmül-ler Engineering GmbH and Graf EnviroPro for their knowledge sharing and contribu-tions. The authors would also like to thank following persons: Ms Shalini Govindsamy, Mr Ebrahim Patel, Mr Yokesh Singh and Mr Naushaad Haripersad. The demonstration plant project development is currently in progress, thus only information available at the time of publication has been made available.

Abbreviations

CFB – Circulating Fluidised BedFGD – Flue Gas Desulphurisation l/kWh – Litres Per Kilowatt HourMW – MegawattNAAQS – National Ambient Air Quality

StandardsNm3/h – Normal Cubic Meters Per HourNEMA – National Environmental Man-

agement ActNWMS – National Waste Management

StrategySA – South AfricaSO2 – Sulphur DioxideWFGD – Wet FGD

References1 Eskom, 2016. Media Room Dry Cooling Tech-

nology. [Online] Available at: http://www.eskom.co.za/news/Pages/Feb4X.aspx [Ac-cessed May 2018].

2 Communications, G., 2015. Government me-dia briefing on water scarcity and drought. [Online] Available at: http://www.gcis.gov.za/newsroom/media-releases/govern-ment-media-briefing-water-scarcity-and-drought [Accessed April 2018].

3 Franco, A. & Diaz, A. R., 2009. The future challenges for “clean coal technologies”: join-ing efficiency increase and pollutant emission control. Energy, 34(3), pp. 348-354.

4 Eskom, November 04, 2016. Coal Power. [Online] Available at: www.eskom.co.za: http://www.eskom.co.za/aboutelectricity/electricitytechnologies/pages/coal_power.aspx.

5 Affairs, D. o. E., 2013. National List of Activi-ties & Associated Minimum Emission Stand-ards. s.l.:s.n.

6 Affairs, D. o. E., n.d. Highveld priority area air quality management plan: Executive Sum-mary, s.l.: s.n.

7 Air Quality Act, 2004 (Act 39 of 2004), List-ed Activities and Associated Minimum Emis-sion Standards Identified in Terms of Section 21 of the National Environmental Manage-ment.

8 Srivastava, R.K. & Jozewicz, W., 2001. Flue Gas Desulphurization: The State of the Art. Journal of the Air & Waste Management As-sociation. 51(12): 1676-1688.

9 Sargent & Lundy, 2002. Circulating Fluid-ised Bed Flue Gas Desulphurization Tech-nology Evaluation. National Lime Associa-tion. Project number 11311-000. Chicago, USA.

10 Department of Minerals and Energy, 2003, A Review of the Dolomite and Limestone in-dustry in South Africa.

11 Donnenfeld, Z. et al, 2018, A delicate bal-ance: Water scarcity in South Africa.

12 Gomes G.M.F et al, 2015, Dolomite desul-phurisation behaviour in a bubbling fluidised bed pilot plant for high ash coal.

13 Eskom, 2018. Medupi Flue Gas Desulphuri-sation: Technology Selection Study Report, Document number 474-10175. l

Fig. 10. CFB FGD Demonstration Plant Proposed Layout.

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