Operation results of power
station with petroleum cokefiring boiler
YASUNORI HIRAYAMA* 1

SHUJI MAKIURA* 1

MASASHI HISHIDA* 1

YOSHIHISA ARAKAWA* 1

YOSHIHISA YAMAMOTO* 1

AKIYASU OKAMOTO* 2

Petroleum cokes (PC) is a by-product from the oil refining process. In recent years, deep drawing oil technology has beendeveloped and surplus PC with low volatility and low quality is increasing in the market. If the surplus PC could be usedas the main fuel for thermal power stations, it will become most economical fuel in the next 5 to 20 years. Mitsubishi
Heavy Industries, Ltd. constructed the thermal power station with a 100% PC firing boiler for Frontier Energy Niigata Co.,
Ltd. Commercial operation started in July 2005. This power station is highly praised both in Japan and foreign countries
as a large boiler with the stable combustion of 100% PC, while most of the boilers with PC firing needs heavy fuel or otherfuels for stable combustion. This report states the distinctive technology for a thermal power station with a 100% PC firingboiler, including the results of continuous operation and first maintenance in April 2006.
heavy metal and less volatile matter content in comparison with general fuels, reduction corrosion by hydrogensulfate and high-temperature oxidation corrosion tend tooccur on the heat transfer surface, causing difficulties instable combustion and operation. Because of these, PC isclassified as a poor quality fuel, whose cost is generally
lower than bituminous coal. In view of the power generation cost, therefore, the PC firing power plant is sufficiently
competitive with bituminous coal-firing power plants.
Focusing on this point MHI built a power plant which
fires PC which was lower in cost than bituminous coalfired power plants and which contributes to the effective
use of resources.

1. Introduction of petroleum coke
Petroleum coke is a by-product produced in the proFig. 1
cess of refining petroleum (Fig.
1). Vacuum residue (VR)obtained after refining major fuels such as gasoline andheavy oil is processed in a coking machine, where kerosene and diesel oil are extracted from gas. The residue
from this process is referred to as PC.
Table 1 is a comparison of the general properties ofPC, typical bituminous coal, and VR. PC has a heating
value about 1.24 times higher than bituminous coal.
According to a provisional estimate based on the heating value, it can be assumed that the amount of PC required
for electric power generation will be 80% (1/1.24) of that ofbituminous coal. However, due to PC's higher sulfur and2. Toward realization of commercial operation
for PC firing power plant
In view of the stable supply of this cost-effective fuel,the use of PC with higher sulfur/higher vanadium was assumed as the premise of this project. Therefore, stablecombustion technology and countermeasures against reduction corrosion by hydrogen sulfate and high slagging
caused by vanadium are both necessary meaning that thefollowing technological problems had to be solved. In thissection, each technological issue for designing a PC firing
power plant is stated below.

Crude oilLP gasAtmosphericdistillation planto30 - 100 Co
100 - 150 Co
150 - 200 Co
250 - 300 Co
350 C <-

NaphthaGasoline
Kerosene
Diesel oil
Heavy oil

Vacuumdistillation plant

Residue oil

Table 1 Comparison of general properties between planned PC andother fuels

VR
PC
Higher heating value (air-dried)

Coking machine
Asphalt
PC

Fig. 1 Outline of PC generation process

Bituminous coalVR

(kJ/kg) 34 893

28 18041 850Volatile matter content (air-dried) (wt %) 9.9~13

26.2

-

Fixed carbon (air-dried)(wt %)

87~90

56.3

20~30

Sulfur content (dry ash-free)(wt %)<6.5

0.40

4.0~6.0

Vanadium(ppm)<1 500-<300

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)*1 Nagasaki Shipyard & Machinery Works*2 Nakasaki Research & Development Center, Technical Headquaters12.1 PC combustion technology with Direct Mill
SystemOne method of stably burning solid fuel is to pulverize
the fuel into fine particles, blow them into a furnace together with air for combustion, and make them self-ignite
by the radiant heat of the furnace. This technology has
already been put to wide practical use in coal-firing boilers, and when decreasing the ratio of the air for combustion
and transportation to the fine pulverized fuel, which is
known as A/C (air by coal), - the fuel density becomes
higher, and ignition more stable. Therefore, the A/C is set
to a low level for lower ignitability solid fuels.
The bin system which collects and delivers pulverizedfuel into a bin is well known as a system which can easily
Fig. 2
reduce A/C (Fig.
2). The PC firing boiler which was first
introduced in the 1980s also used this bin system.
On the other hand, because there is no need for devices
such as a cyclone and a pulverized fuel storage bin, theFig. 3
direct mill system (Fig.
3) is advantageous because of its
smaller space requirements and less operation and repair costs. However, the direct mill system has a higher
A/C lower limit than the bin system and has some remaining technological problems for stable ignition.
From the point of view of reducing the construction
and power generation costs, MHI conducted a technological study of PC firing technology with the direct millsystem. As a result, we decided that it was possible tosolve the problems and the PC firing technology using thedirect mill system was adopted.

In order to compensate for the difficulties in reducing
A/C, the following techniques were adopted to improve
the stable ignition of PC.
2.1.1 Optimization of burner locations
Figure 4 is a comparison of burner and flame locations of the CCF (circular corner firing) system which isadopted for bituminous coal-fired boilers and the CUF(circular ultra firing) system.
In the CUF combustion system, the burners are located near the center of each furnace wall with a highradiation intensity so that circular firing is formed by
the burner flame from each furnace wall and lower
ignitability PC can be ignited stably at low load. MHI
adopted this CUF combustion system.
2.1.2 Development of a special burner for PC
firing and PC pulverization technology
Fig. 5A special burner for firing PC was adopted (Fig.
5).
Its separating performance for concentrated and weakened fuel flows was enhanced as well as its flamestabilization performance. This burner was developedby modifying the latest model burner which has a reputation as a low NOx, coal-fired combustion burner.
Also for the mill (fuel pulverizer), in order to achieve
the high fineness of "#200 pass 95% or above" for improvement combustion performance, we adopted the latest
model of vertical mill with a built-in rotary separator.

From coal feeder

Burnerposition

Exhaust fan
Pulverization
systemAH

Mitsubishi
vertical mill
FDF

PAF

SAH

CycloneDeliverysystemPulverizedfuel bin(a) CCFTo
burnerBurnerposition

Coal feed controller

Fig. 2 Bin system (indirect combustion system)(b) CUFTo burner

Fig. 4 Comparison of CCF and CUF
Unnecessary devices:
cyclone, exhaust fan,pulverized fuel bin,coal feed controller

AH

FDF

Built-in type fuel separator
Conc. flame

PC burner

PAF
Mitsubishi
vertical mill

Weak flame
Conc. flame

Installation areareduction
Maintenance cost reduction

Oil burner
for A-heavy oil
Fig. 5 PC burner

Fig. 3 Direct mill system (direct combustion system)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)2

2.2 Technologies for molten ash caused by highvanadium content
As shown in Table 2
2, vanadium oxide may form low
melting point ash depending on its combined oxygen
weight and other fuel properties, causing problems in
the continuous operation of the boiler due to slagging
and fouling.

2.3.4 Rotary air preheater (AH)
To prevent corrosion of the low-temperature element
caused by sulfated ash, a steam heating type air preheater(SAH) is mounted on the air inlet of the AH, which maintains the exhaust gas temperature at the AH outlet above
the acid dew point. In addition, by using ceramic for the
low-temperature elements, we reduced the corrosion of theelements and ash clogging.

Table 2 Vanadium oxide compoundsoCompound Melting point ( C)

3. Outline of system and operation resultsoCompoundMelting point ( C)V 2O 41970

Na2O . 3V2O5560

V 2O 5690

10Na2O . 7V2O5573

3.1 Specifications of major equipment and schematic
diagram of system(1) Boiler
Type: single drum natural circulation type
Boiler Maximum Continious Rate: 428 tons/hr
Steam condition: 12.95 MPa x 541oC(This system has no reheater.)
Fuel: PC firing (normal operation)/A-heavy oil(Note)(during unit start-up, and combustion stabilizer)

..
.
.
.
.
.
.
...
..
.

In order to solve this problem, we built a test device
which is simulates the actual boiler, and investigated
the ash deposition and stripping characteristics. Finally,
we optimized the tube pitch as well as the sootblower
location. Further we succeeded in reducing the generation of molten ash on the heat transfer tube surface byfeeding an MgO fuel additive to increase the ash melting point.
2.3 Technologies against corrosion caused by highsulfur and high vanadium content
Anti-corrosion technologies differ by location, from thefurnace to the rotary air preheater (AH), which can besummarized as follows:
2.3.1 Furnace waterwall/burner zone
High Cr sprayed film, which has good anti-corrosion
performance in VR firing boilers, is coated on the watercooling wall to protect it.
2.3.2 Superheater
High Cr stainless steel pipe is used for the high-temperature parts to prevent thickness reduction caused bycorrosion.
2.3.3 Dinitrification system
A catalyst with a large pitch is used to prevent ash deposition. Also the sootblower does not use a steam as theblowing medium but a sonic wave to remove deposited ash.

Note: Kinetic viscosity (cSt, mm 2/s): 20 or less
See JIS (Japanese Industrial Standards) K2205.

Number of burner stages: PC 3 stages (direct ignition)/A-heavy oil (2 stages)
Main steam temperature control: feed water spraytype (1 stage only)
Draft system: balanced draft system (furnace
draft control: -0.2kPa)(2) Turbine
Type: single cylinder, single flow, impulse type, condensing turbine, axial exhaust, indoor type
Steam condition: 12.45 MPa x 538oC
Rated speed: 3,000 min-1
Exhaust condition: -93.3 kPa
Number of turbine stages: HP 13 stages/LP 3 stages
Number of bleed stages: 5 stages(3) Generator
Output (generator terminal): 122,223 kVA/110,000
kW
Figure 6 is a schematic diagram of this power plant.

WES
Cooling
tower

NH3

HP-HTR

CWP

PC
bunkerDeaerator

Generator
LP-HTR

BoilerDinitrification
systemVacuum blower
Desulfurization
system
EP

BFP

Lock
hopper

TurbineAsh silo
FDF
Pulverizer

Transformer
CP

AH

IDF

PAF
Fig. 6 Schematic of system

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)3.

3.2 Operation results
It was confirmed that PC firing is stabilized from 35 to100% of the boiler load the same as the boiler design.
This means that this PC firing boiler has a performanceas good as the ordinary bituminous coal-fired boilers.
Figure 7 shows the PC burner flame of the actual
equipment. Combustion was stable both for 100% and
35% boiler loads, showing the intended good results ofstable combustion technology for lower ignitability PC.

FA handling system
As the temperature inside the ash conveying pipe
decreases, sulfuric acid or sulfated compounds in theash absorbs water from the air and eventually become
viscous. To deal with this, SAH is installed at the air
intake of the ash conveying pipe. In addition, to prevent temperature drops during winter, a steam trace ismounted on the ash conveying pipe to keep the temperatures inside the ash conveying pipe above thesulfuric acid dew point.

4. System inspection results of the first periodic inspection

Burnernozzle(a) 100% load operation(b) 35% load operation

Radiation intensity iscomparable with that ofa pulverized coal firingboiler and the furnace
is filled with flame.

Flames are formed besideof the burner nozzle tothe injected fuel outercircumferences.
Combustion is also stable.

The first periodic inspection was conducted in April
2006, lasting about 30 days. Neither the turbine nor thegenerator were overhauled and the inspection mainly concerned the boiler and its ancillary facilities.
The inspection results of each part are summarized
below, based on which the technology adopted in thisboiler is evaluated:
4.1 Technologies against molten ash
As a result of the optimum layout of the tube pitch andsootblower based on basic research, it was confirmed thatno persistent ash deposits bringing about bridging between tubes were seen and the ash deposits weremaintained at an appropriate level for continuous operation. Figure 8 shows ash deposits on the pressure parts,
where both the superheater and the economizer are free ofbridges and the conditions are favorable.
4.2 Technologies against corrosion
4.2.1 Pressure parts (furnace, superheater, andeconomizer)
High-temperature oxidation corrosion was evaluated,
for which the residual film thickness of the sprayed film
was measured at the furnace bottom and the tube thickness was measured for each pressure part. As a result, it
was confirmed that both the sprayed film and the tube
thickness were satisfactory and the anti-corrosion technology of the sprayed film and fuel additive werefunctioning as designed.

Fig. 7 PC burner flames during PC firing

3.3 Ash handling
The furnace bottom ash is collected by a water sealed
type furnace bottom conveyor and the fly ash (referred below to as FA) is collected by vacuuming. Although the ash
handling system is basically the same as that of a coalfired power plant, the furnace bottom conveyor and the FA
handling system are each fitted with the following special
technologies to deal with the high sulfur content ash:
Water sealed bottom conveyor
Because of using circulated seal water, the seal water tends to become acidic depending on the sulfur
content of the bottom ash. Therefore, by injecting sodium hydroxide, the water quality is automaticallycontrolled to neutral.

.

(a) Penetrated position of platen
superheater to furnace front wall(Gas flow: from the bottom towards the
top in the photo)(b) Tertiary superheater and suspended tubes(Gas flow: from the bottom towards the top
in the photo)(c) Middle-temperature economizer:
Spiral fins
(Gas flow: from the front towards theback in the photo)Fig. 8 Ash deposition inside furnace and flue gas duct equipment

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)4

4.2.2 Dinitrification system
Of the three catalyst layers, only a slight ash deposit
and clogging were observed on the upstream side of theuppermost upstream layer (first layer), which remained
Fig. 9within the expected result (Fig.
9). These results wereattained because the technology to prevent ash deposition caused by the NOx removal catalyst functioned
satisfactorily as designed through the combination of thepitch expanding catalyst and the sonic type sootblower.
4.2.3 AH (rotary air preheater)
All the elements (high, medium, and low temperatures)of the AH were also free of clogging and corrosion. The
same was observed for the basket and seal sections. As an
example, Fig. 10 shows photos of the high-temperatureelement and the low-temperature element. The AH
sootblowers were located on both the high-temperature andlow-temperature sides. They were operated as follows:
High-temperature side: 1 time/week
Low-temperature side: 3 times/day
As a result, no significant increase in differential pressure was observed even during continuous operation. Thus,
the application of the AH sootblower was judged as appropriate.

1st layer(uppermost upstream layer)
Some solidified ash and slight
clogging are observed.

4.3 Water washing
There was a worry that corrosion caused by moistureabsorption and sulfurization of the high sulfur content
ash might occur especially on the equipment in the boiler
flue gas line during shutdown of the unit. As a preventive
measure, washing the electrostatic precipitator (EP) withwater is required during a long-term shutdown (three days
Fig. 1
1 ).
or longer, as a rough guide) (Fig.
11

5. Future tasks
As mentioned above, although positive results wereobtained with regard to stable operation of the PC firingboiler, tasks to be worked on have appeared in connection
with continuous operation and workability during periodic inspection in the FA handling system.
In this section, we would like to take this case as an
example and summarize the improvement measures for
the FA handling system of the PC firing boiler.
5.1 Blockage of ash handling system (case example
during continuous operation)
5.1.1 Outline of FA system
This is a vacuuming FA handling system which uses
a vacuum blower (100% capacity machine x 2; 1 machineas a spare), ash is separated from the air through the bag
filter and is stored in an ash silo via a lock hopper. It isremoved from the silo by trucks at regular intervals.
A lock hopper system has been adopted for the mechanism in which the ash conveying pipe, a vacuum region, isseparated from the open-to-air ash silo, and FA, which isseparated by the bag filter, is steadily dropped into theFig. 12ash silo (Fig.
12).

2nd and 3rd layers(photo shows the 2nd layer.)
Neither solidified ash nor
clogging are observed.

Washing AH with waterand draining
Sonic sootblower(sonic horn)

Washing EP with waterand ash discharge

Fig. 11 Washing flue gas duct equipment with water

Ammonia nozzle
Only ash deposits.
No blockage of nozzle.

Lock hopper

Fig. 9 Internal inspection of Dinitrification system

Bag filter

EP hoppers
Flue gas
duct

Boiler hoppers

Air intake with SAH

VacuumblowerAsh siloHigh-temperature side
Low-temperature side
(DU/mild steel)(Square/ceramic)
Fig. 10 Clogging of AH elements

Fig. 12 Outline of FA treatment system

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)55.1.2 Ash trapping on lock hopper flap valve
and line blockage
In the lock hopper system, two tanks are located on
top of the ash silo, where the vacuum region and the opento-air region of each tank are alternately partitioned byvalves to convey the FA downward (See (a) to (c) in
Fig.13
Fig.13). Although various kinds of valve were available
for partitioning each hopper, flap valves were adopted inthis case.
Compared with a slide gate or a rotary valve, as a flap
valve does not have many parts which slide and catchash, it has the advantage of being less likely to be
abraded by ash. On the other hand, if a foreign substance is caught in the flap valve seat, the vacuum region
and the open-to-air region cannot be partitioned, and ash
clogging may take place. In Fig. 13, (d) to (f) explain thisprocess.
When a clearance is created on the flap valve seat asshown in (d), a flow in the direction of the green arrow in
the figure occurs when the lower flap valve is opened. At
this time, as FA turbulence occurs in the direction of thegray arrows inside the lower hopper, a portion of the FAremains inside the lower hopper when the lower flap valve
is fully closed (e). When the FA is again fed from theupper hopper as shown in (f) in this state, a more thanplanned amount of FA accumulates in the lower hopper.
As this is repeated, the lower hopper is gradually filledwith FA up to the level of the upper flap valve. Thus,
blockage of the ash handling line takes place, causing
problems in the continuous operation of the unit.
5.1.3 Plate-like FA
When a flap valve is examined, a plate of FA is often

found inserted in the seat. This phenomenon occurs frequently, particularly during unit startup after EP water
washing. This plate-like FA is presumed to be generated
when residual FA absorbs moisture after the EP water
washing, solidifies, and breaks away in the form of a plate.
The number of EP water washings for the PC firing
power plant during its service life is larger than the caseof EP for ash from a coal firing power plant. Therefore,
additional improvements of the FA handling system arenecessary in consideration of the formation of the platelike ash.
5.1.4 Idea for improvement of FA system
In order to prevent the above problem, we have beenFig. 14studying the following new system (Fig.
14).
In the FA handling system, ash is handled according
to the following cycle: ash handling at boiler outlet, cleaning the ash conveying pipe, ash handling at EP, cleaning
the ash conveying pipe, ash handling at boiler outlet.
In the existing system, as ash is transferred by valves(1) and (2) being alternately opened and closed on asteady basis, the valves are opened and closed manytimes. The risk of trapping plate-like FA accordingly
becomes high.
Meanwhile in the new system being studied, an intermediate hopper, referred to as the "temporary ash-storage
hopper," is given enough capacity to store an amount ofash equal to that from one cycle of EP ash handling andthe ash in the temporary ash-storage hopper is transferred
into the ash silo while the ash conveying pipe is cleaned.
This helps reduce the number of times each valve is opened
and closed, and is expected to reduce problems in ash treatment caused by trapping the plate-like ash.

Pressureequalizing
valve

Pressureequalizing
valve

Upper
hopperBag filter

Bag filter

Lower
hopperM
Temporaryash-storage
hopperM
Valve 1

Normal
(a)

(b)

(c)

M

Valve 2M
Ash silo
Ash silo

ClearanceAshtrapping(d)(e): Vacuum region(f )M
Existing system: Open-to-air region

Valve 3M

New system being studied

Fig. 14 Suggestions for improvement of FA handling system

Fig. 13 Lock hopper ash conveying system

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)6

5.2 Cleaning of flue gas duct of EP Inlet (caseexample during periodic inspection work)
In this power plant, a WES (Waste Elimination System) has been adopted, in which waste liquid fromdesulfurization is sprayed into the EP inlet flue gas duct
so that its water content evaporates and its solid content
is collected by the EP. Therefore, as ash is deposited in
the EP inlet flue gas duct during continuous operation
Fig(Fig
Fig.. 1515), this needs to be removed during periodic inspection.
To remove ash from the flue gas duct, we have fitted astraight tube chute underneath the flue gas duct. However, if ash clogging occurs in the middle of the chute during
ash removal work, there is no measure for removing this.
Therefore, it is not used during periodic inspections andash is removed from the duct manholes.
With regard to similar plants in the future, in consideration of this point, we need improved measures suchas optimizing the manhole size and securing a work spaceon the exterior platform of the duct to increase the efficiency of the ash removal work.

Flue gasstraighter for EP

Fig. 15 Ash depositionat the EP inlet flue gas duct

Although this power plant has almost reached the levelof completion from the technological point of view, there is
still possibility for improvement of the system to support
boiler operations such as the ash handling system. We
would like to work on these improvements, aiming to build
a plant which is trusted by our clients as a highly stable
power supply facility.
Last but not least, we would like to extend our special
thanks to Frontier Energy Niigata Co., Ltd. and the clientsconcerned for their kind guidance and cooperation.

6. Conclusion
After overcoming various challenges such as the stable
ignition of lower ignitability fuel, anti-corrosion technologies for pressure parts, and clarification of the ash
deposition mechanism, we have succeeded in utilizing
PC, which is a poor quality fuel for power generation,and operating a PC power generating unit which is comparable with a coal firing type unit. This technology, we
think, is significant not only from the viewpoint of its
high competitiveness in power generating costs but also
from the viewpoint of the effective use of energy.

Yasunori Hirayama

Shuji Makiura

Reference(1) Arakaka, Y. et al., Development and Operation Result ofPetroleum Coke Firing Boiler, Mitsubishi Juko Giho Vol.
42 No. 3 (2005)

Masashi Hishida

Yoshihisa Arakawa Yoshihisa Yamamoto,

Akiyasu Okamoto

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)

7