The Mondi Merebank MultiFuel Boiler
Presented by Mark Miller
Date February 2010
The MultiFuel Boiler
South African Division
South Africa Division
Forests Richards Bay Merebank
• The leading brand in South Africa
• Key market South Africa
• Long standing reputation for all-round performance
• The paper for everyday use with high whiteness
• Ideally suited for use in inkjet and laser printers, copiers and fax machines
• Pioneer in FSC (Forest Stewardship Council) certification
Merebank: Core products -Domestic
Merebank: Core products -Export
• No. 1
• Copyrex
• Niveus Top
• Dolphin
• Southern Star
All available in 80gsm, A4 & A3
The Paper Making ProcessWood to chipsDebarkingSawingChippingScreening
Chips to PulpCookingWashingBleachingRefining
Pulp to paperFormingPressingDryingWinding
FinishingCuttingSizingWrappingPackaging
The Paper Making Process – Energy Intensive
To make paper is energy intensive.
The Merebank Mill typically consumes 125 MW of electricity.
This makes Mondi the biggest consumer in the Ethekwini Municipality
The second largest consumer in the boundaries of the Municipality is Toyota which consumes only a third of that of the Merebank Mill
This energy intensiveness demands an effective approach to Energy Efficiencies and Cost Effectiveness
The Paper Making Process
Significant quantities of low pressure steam are re quired
Mainly used to dry the wet paper sheet through the transfer of latent heat from the saturated steam through the drying cylinders
Up to 210 tons/hour of 5 bar saturated steam required with all machines operational
The steam were produced in the past with 2 x 85 tph coal fired stoker boilers
Make-up steam were generated with a 23 tph and 55 tph gas boiler
Two 25 tph oil boilers were used as standby boilers
It was possible to generate up to 9 MW of power using existing back pressure turbines
Waste Products
Waste Products produced (pre project)
Waste water from the water treatment plant. Recovered in belt pressesSignificant CV value due to fibre
Bark and saw dust from the debarking and chipping process.
Coarse ash from the coal boilersCarbon remnants for possible reburning
Waste as Fuel
An analysis of the typical waste products potentially available as fuel
Coal Bark Sawdust Sludge Coarse
Ash
Fine Ash
Cal. Value
(MJ/kg)
26.12 7.13 7.4 3.12 8.18 11.81
Mass Flow
(tons/day)
156 95 35 231 86 64
Input
Energy (%)
57 9 4 10 10 10
Steam flow
(tons/h)
51 8 3 9 9 9
The Multi-fuel Boiler - Strategic Significance
Mondi decided in the early 2000’s to install a new multi-fuel boiler. It made strategic sense:
Reduce landfill costs
Reduce environmental impacts
Secure sufficient and reliable steam resources for present and foreseeable future needs
Allow Mondi the opportunity to generate more electricity to counter rising electricity costs
Allow the mothballing of the aging oil fired boilers, thereby reducing the site SO2
emission levels
Transfer the gas boilers to standby, significantly reducing the cost of steam production
The Multi-fuel Boiler
Mondi decided to purchase a 90 tph bubbling bed fluidised boiler (BFB). Advantages over conventional boilers:
Compact size
Fuel Flexibility
High Combustion efficiency
Reduced emissions of noxious pollutants such as SOx
Relatively wide operating ranges
Low Combustion temperatures(800 – 900 oC)
Limestone is injected directly into the furnace to control SOx emissions
Fuels: natural gas (as a start-up fuel), coal, bark, sawdust, fine ash, coarse ash and sludge.
Coal is required as the base fuel
New Turbine
A 21.7 MW (27 MVA) back-pressure turbine-generator set was purchased with the boiler to generate electricity.
The Multi-fuel Boiler
The main features of the BFB are:
A low and evenly distributed bed temperature, high turbulence, and long residence time, all optimise the combustion process
In bed tubes to increase heat transfer
Grits re-firing to increase boiler efficiency
Lime stone injection directly into the furnace
Bag filters to reduce particle emissions
Metso DCS System
The Multi-fuel Boiler
The Multi-fuel Boiler – Material Handling
The challenges of material handling systems:
Site space constraints
Limited availability of certain fuels, inducing the need for buffer bins and silos
Special mixing requirements
Accurate control of fuel feed to ensure proper combustion
Removal, screening and recirculation of sand from the bed
Complicated over and under bed gas firing system
Fluidising ducting, piping and nozzles
Fuel re-feed systems
The Multi-fuel Boiler – Material Handling
The Multi-fuel Boiler - Integration
The MFB was designed to produce 90 tons/hour @ 65 Bar, with a superheated steam temperature of 475 oC
The MFB was designed to be the base load boiler, with the two coal boilers making up the difference
The new turbine was sized to produce 21.7 MW @ 65 Bar with an input steam requirement of 154 tons
The remaining steam obtained from the two coal boilers
LP steam leaves the turbine outlet 5 Bar
The LP steam (with backpressure control from the turbine) is sent to the Mill to satisfy the Mill steam requirements (mainly paper drying)
Original Steam System
C urren t S team S ys tem
O il30 t/h
C oa l85 t/h
O il30 t/h
C oa l85 t/h
G as30 t/h
1
3
5
6
4
4 .5 MW
4.5 MW
5 B ar, 163 oC
Sta
nd-b
yP
rodu
ctio
n
T o M ill62 Ba r, 485 oC
15 Bar, 220 oC
18 Bar, 212 oC
G as50 t/h
15 Bar, 220 oC
P R D S
P R D S
P R D S
P R D S8
Sta
nd-b
y
75T P H
75T P H
23T P H
39T P HS/W ater
5T PH
205T PH
T o M ill
30T PHT o M ill
N o te : If the M FB w ere no t insta lled , bo ile r 8 w ou ld be sw itched from standby to run fu ll tim e to cover the s team requ irem ents
N ote : P R D S = P ressure R educ ing S ta tion
New Steam System with MFB and Turbine
C oal85 t/h
M ulti-fue l90 t/h
G as55 t/h
8
5
6
7
4
4.5 MW
4.5 MW
20.8MW
Boiler N o.
Sta
nd-b
y G as30 t/h
Pro
duct
ion
5 Bar, 163 oCTo M ill
Steam S ystemW ith M u lti-Fuel Bo iler
NE W
To M ill5 Bar, 163oC
18 Bar, 212 oC
79tph
82 tph
C oal85 t/h
79tph
235tph
5tph
N ote . Spray water contribution w ill be sm all and has been ignored for calculation purposes
PRD S
PR D S
PR D S
PRD S
N ote : PR D S = Pressure Reducing S ta tion
Understanding fluidised bed combustion
Fluidised bed combustion differs from conventional pulverised coal combustion.
Continuous stream of air to create turbulence in a mixed bed of fuel, inert material and coarse fuel ash particles.
Combustion occurs at lower temperatures typically between 800°C and 900°C.
Fluidised bed combustion (FBC) technology is particularly suitable for coals that are difficult to mill and fire in pulverised coal combustion boilers.
FBC technology is commercially available for modules up to 300 MWe, with a number of projects being planned or presently implemented in the 250–350 MWe size range.
FBC represents about 2 per cent of the total coal-fired capacity worldwide but it is expected to be further adopted for a other applications.
Understanding fluidised bed combustion
Fuel is fed onto a packed bed of inert particles (sand).
An upward air flow fluidizes the bed via a distributor to provide uniform flow of air across the whole base area of the fluidized bed
If the upward air flow is turned off the particles can become de-fluidized or slumped
Fuel is burned in the bed and at times in the freeboard above the bed. (Combustion of around 800–900°C - considerably lower than in pulver ised coal combustion)
The freeboard can be designed with a much larger diameter than the bed. The reduced gas velocity in the section ensures that coarse solid particles entrained in the gas flow fall back to the bed by gravity. Fine particles (which encourage abrasion) are carried forward with the flue gas
Secondary air, is introduced into the freeboard region to ensure complete combustion of the fuel.
Fluidized bed combustion
After exiting the combustor, the flue gases pass into a convective section where heat is
further recovered and the gases are cooled to below 200°C.
Then through a particulate control unit that can be a cyclone, a bag filter or an electrostatic precipitator. The cleaned gases are finally discharged into the atmosphere through a stack.
Combustion leaves behind the mineral matter in the coal and the spent sorbent (if added when sulphur removal is required) in the bed.
Coarse particles can accumulate in the bed. Excessive material is removed by an off-take from the bottom of the bed or via an overflow weir. This ensures the bed is maintained at the
designed depth
Good Bed Management is required
Fluidized bed combustion
When the gas velocity exceeds the minimum fluidising velocity, the excess gas passes
through the bed as bubbles. Practically gas velocities are several times higher than the
minimum fluidizing velocity.
Bubbles passing through the bed typically occupy 20–50% of the bed volume. The passage
of the bubbles, in upwards and sideways coalescing movements, gives intensive agitation
and mixing of the bed particles.
Fluidising velocity is perhaps the most important parameter of fluidisation. Its choice affects
most of the other process parameters. It typically ranges from 1 to 3 m/s for bubbling fluidised beds
The bubbling bed
Air Systems
In Bed Tubes
In Bed Tubes
Nozzles
Bark, Sludge & Sawdust Delivery
Bark, Sawdust & Sludge to Boiler
The Multi-fuel Boiler – Benefits Review
The MFB allowed Mondi to retire the two oil boilers – reduced environmental impact
Significant reduction of waste products = cleaner environment
The two gas boilers now standby
Steam supply is now reliable and secure – less unplanned shuts
The new turbine improved volt dip immunity and reduced electrical import
The improved Boiler standby capacity allows improved maintenance on the boilers and hence reliability of the steam system
Lessons Learned
Research and design is a worthwhile investment
Involve the local community
Use well proven technology
Good planning is essential
Have a good contract in place
Automation is key in leading technologies
Good troubleshooting and information gathering tools
FORWARD - LOOKING STATEMENTS
It should be noted that certain statements herein which are not historical facts, including, without limitation those regarding expectations of market growth and developments; expectations of growth and profitability; and statements preceded by “believes”, “expects”, “anticipates”, “foresees”, “may” or similar expressions, are forward-looking statements. Since these statements are based on current knowledge, plans, estimates and projections, they involve risks and uncertainties which may cause actual results to materially differ from those expressed in such forward-looking statements. Various factors could cause actual future results, performance or events to differ materially from those described in these statements. Such factors include in particular but without any limitation: (1) operating factors such as continued success of manufacturing activities and the achievement of efficiencies therein, continued success of product development plans and targets, changes in the degree of protection created by Group’s patents and other intellectual property rights, the availability of capital on acceptable terms; (2) industry conditions, such as strength of product demand, intensity of competition, prevailing and future global market prices for the Group’s products and raw materials and the pricing pressures thereto, financial condition of the customers, suppliers and the competitors of the Group, potential introduction of competing products and technologies by competitors; and (3) general economic conditions, such as rates of economic growth in the Group’s principal geographical markets or fluctuations of exchange rates and interest rates.
Mondi does not
a) assume any warranty or liability as to accuracy or completeness of the information provided hereinb) undertake to review or confirm analysts’ expectations or estimates or to update any forward-looking statements to reflect events that occur or circumstances that arise after the date of making any forward-looking statements.
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