RASCHKA
Compact -Fluidized Bed Incinerator
Lonza Engineering Ltd
Muenchensteinerstrasse 38,CH-4002 Basel, Switzerland
phone: +41 61 316 8606 fax:+41 61 316 9606
e-mail: [email protected] website: www.lonza.com/engineering
1 Plant conception
The RASCHKA-Compact Fluidized Bed Incinerator (compact FBI) is the core
component of a fluidized bed incineration plant for the thermal disposal and
utilization directly at site-of sewage sludge and screenings that are produced in
smaller wastewater treatment plants.
The main components of the incineration plant are the reception-, storing-and
conveying sytems for sludge and screenings, the fluidized bed incinerator, the
warmwater boiler and the flue gas cleaning (dry system). The realized plant as
described below is located in a housing but the plant can be realized as outdoor
installation, too.
The heat set free by the process is recovered and it is utilized for the process
itself and for heating purposes on the wastewater treatment plant. By this heat
recovery and utilization fossile fuel is saved and accordingly the production of
CO2 is avoided.
2 Plant and process description of the compact FBI
2.1 Plant description
The compact FBI consists of a welded cylindrical steel casing that has an inner
refractory lining and an outer thermal insulation. The FBI consists of the following
main sections:
•Bottom construction with bottom plate
•Windbox
•Fluidized bed area
•Freeboard (postcombustion zone)
•Incinerator head
•Heating up combustion chamber
The bottom construction with bottom plate serves as basis for the incinerator and
for fastening it on the foundation. The bottom construction with bottom plate
bears the complete brick lined incinerator and the recuperator (combustion air
preaheater) that is located on the flue gas outlet in the incinerator head. All loads
and moments resulting from the incinerator are led into the foundation by the
bottom construction.
The cylindrical windbox is mounted upon the bottom plate. The windbox serves
for the distribution of the fluidizing air (main part of the total combustion air)
below the nozzle bottom. The heating up combustion chamber is mounted
horizontally to the windbox. The heated up fluidizing air flows from the heating up
combustion chamber into the windbox.
The conical fluidized bed area is mounted upon the windbox. The nozzle bottom
that is integrated in the refractory lining separates the fluidized bed area from the
wind-box. The nozzle bottom is a ceramic sandwich disc made of a special
refractory concrete. The nozzle bottom is equipped with the RASCHKA-air
nozzles made from heat resistant cast steel. The fluidizing air is blown through
these nozzles. The special shape of the nozzles and their sophisticated
arrangement in the nozzle bottom serve for an even distribution of the fluidizing
air over the total surface of the nozzle bottom.
Three RASCHKA-fuel injection lances are mounted in the fluidized bed area. By
means of these lances biogas can be injected directly into the fluidized bed. The
biogas is injected during the heating up of the incinerator and for maintaining the
incineration process.
The RASCHKA-bed material discharge device is located in the fluidized bed area,
too. It serves for the discharge of surplus bed material during the operation.
The cylindrical freeboard area is mounted upon the fluidized bed area. The flue
gases and ash particles coming from the fluidized bed burn out completely in this
post combustion zone. The freeboard is mainly equipped with the
RASCHA-spreader and the four secondary air injection pipes. The spreader
serves for the feeding, the opening up and the even distribution of the
combustible. The secondary air injection serves for a perfect temperature profile
and air distribution inside the furnace. The special arrangement of the secondary
air pipes leads to a rotation and thorough mixing of the flue gas and the
secondary air.
The conical incinerator head is mounted upon the freeboard. It is equipped with
the flue gas outlet socket.
The heating up combustion chamber is mounted to the windbox. It is equipped
with the combustion chamber burner. The heating up combustion chamber
consists of a cylindrical steel casing with the fluidizing air inlet socket, an inner
refractory lining and an outer thermal insulation. The fluidizing air is heated up by
means of the burner during the heating up process of the incinerator. The
windbox, the nozzle bottom, the sand layer upon the nozzle bottom and the
whole furnace are heated up by the hot fluidizing air. During the normal operation
of the incinerator the fluidizing air is preheated by a gas-gas heat exchanger. The
gas-gas heat exchanger (recuperator) is mounted upon the flue gas outlet of the
incinerator. The recuperator is heated by the flue gas leaving the incinerator. The
temperature of the preheated fluidizing air can be increased on demand by
means of the combustion chamber burner.
Technical data
Fluidized bed incinerator, with ceramic nozzle bottom, stationary atmospheric
fluidized bed.
•Height FBI (from upper end of foundation): ~ 12 m
•Outer diameter (steel casing freeboard): ~ 3.5 m
•Space required (floor area): ~ 8 x 10 m
•Total height incl. Recuperator and air lines: ~ 20 m
•Firing capacity: 1 MW
•Max. furnace temperature: 1,000 °C
2.2 Process description
The sewage sludge and the screenings are dewatered mechanically to dry solid
contents of 23-30% (sludge) resp. min. 21% (screenings). The screenings are
shredded additionally. The mixture of sludge and screenings (combustible)
amounts to 1,000 – 1,300 kg/h. The combustible has a high water content and
thus a low heating value. Due to the low heating value the combustion of the
mixture is not possible without additional measures: to make it possible that the
mixture combusts at 870 °C under equilibrated heat balance conditions it is
necessary to heat up the combustion air up to 950 °C and to feed biogas into the
fluidized bed additionally.
The main part of the combustion air serves as fluidizing air. The fluidizing air is
heated up in two stages. Firstly the fluidizing air is blown by a fan into the
recuperator which is heated by the hot flue gas leaving the incinerator. By means
of the recuperator the fluidizing air combustion air is preheated up to 600 – 650
°C. Then the preheated fluidizing air flows into the combustion chamber and is
heated further by means of the combustion chamber burner. Then the fluidizing
air flows into the wind-box below the nozzle bottom. Then the fluidizing air flows
through the nozzles into the fluidizing bed area and fluidizes the sand layer to the
fluidized bed.
The furnace with refractory lining and sand layer must be heated up to operation
temperature before starting the combustible feeding and the incineration process.
The heating up combustion chamber with combustion chamber burner (natural
gas burner) serves for heating up the furnace. Additionally biogas is injected
directly into the fluidzed bed by means of the three fuel injection lances that are
mounted in the fluidized bed area. The injection of biogas serves as well for
maintaining the combustion process and the minimum furnace temperature of
850 °C.
As soon as the furnace is heated up to operation temperature the combustible is
conveyed evenly and exactly dosed to the spreader. The spreader serves for
feeding the combustible into the furnace. The spreader opens up the combustible
and distributes it evenly over the surface of the fluidized bed. The incineration
process takes place in the evenly fluidized, hot fluidized bed at a temperature of
~ 870 °C. The combustible water is evaporated and superheated, the
combustible organic substance combusts and the combustible anorganic
substance glows out.
The controllable secondary air injection in the freeboard area of the incinerator
serves for a perfect temperature profile and air distribution inside the furnace. By
the special arrangement of the secondary air pipes (downwards and tangentially
to the incinerator casing) a rotation and thorough mixing of the flue gas and the
secondary air is achieved. The secondary air is a (small) part of the total
combustion air. The secondary air is cold air and not heated up – opposite to the
fluidizing air.
The retention time of the flue in the post combustion zone amounts to more than
two seconds from the last feeding of combustion air.
The fine grain ashes resulting from the combustion leave the incinerator together
with the flue gas through the flue gas outlet in the incinerator head.
During operation the quantity of the bed material can increase or decrease
depending on the characteristics of the combustible. Lack of bed material can be
compensated by feeding of silica sand and surplus bed material can be
discharged by means of the RASCHKA-bed material discharge device during
operation.
3 Combustible and process parameters
Mechanically dewatered combustible
Dry solids (ds)-content
Organic dry solids (ods) – content-of
ds
Lower heating value (Ihv) of ods
%
%
sludge
23-30
51
screenings
21
80
MJ/kg
25
19
Throughput/lay-out, based on the annual throughput of sludge and
screenings and a continuous operation of 7,500 hours/year.
Throughput
t/y
sludge
7,000 – 9,130
screenings
401
total
7,401–9,531
Throughput
kg/h
933 – 1,217
54
987 – 1,271
Throughput ds
t/y
2,100
84
2,184
Throughput ds
kg/h
280
11
291
Throughput water
t/y
4,900 – 7,027
317
5,217 –7,344
Throughput water
kg/h
653 - 937
43
696 - 980
Process parameters based on the incineration at 870 °C with feeding of
auxiliary fuel (natural gas, biogas), firing capacity ~ 1 MW.
Combustion air
Flue gas 870°C
-Flow (wet)
--N2
--O2
--CO2
--H2O
-Ashes
mn /h
lay -out
3,000-3,300
mn3/h
Vol.-%
Vol.-%
Vol.-%
Vol.-%
kg/h
4,000-4,700
56-60
8-9
5-6
25-30
139
3
4 pictures
General arrangement
Front view of housing
Rear view of housing
Fluidized bed incinerator with heating up combustion chamber
Incinerator lower part with heating up combustion chamber
Heating up combustion chamber with fluidizing air line and burner air line
Freeboard
RASCHKA spreader for combustible feeding
RASCHKA fuel injection lances for the injection of biogas