Biomass Fired Superheater for more Efficient
Electricity Generation from
Waste Incineration Plants
1. Aim of the Project
The aim of the project is to demonstrate in full scale how to increase steam data for a steam
cycle based waste to energy plant, avoiding the normal negative side effect of high
temperature corrosion/erosion. Corrosion at high steam temperature is reduced by reduction
in the concentration of alkali-metals, chlorides, and fly ash in the flue gas. The waste can be
municipal solid waste or agricultural waste, however, in this project it is straw.
Photo 1: General view of the carbonisation unit
2. Introduction
COWI has developed a new concept based on mild pyrolysis of straw in a carbonisation unit.
During, the carbonisation process the straw is converted into a gas fraction and a char
fraction. Due to a relatively low process temperature in the carbonisation reactor, the major
part of alkali chlorides remains in the char fraction. Therefore, the less corrosive product gas
can be used in a separate superheater to boost steam data to for example 80-110 bar and above
500 °C depending on the fuel type and used alloys in the superheater. Thus the power to heat
ratio can be improved considerably for new plants. For existing plants, the technical
limitations and necessary modifications must be evaluated in each individual case.
Char particles are removed from the product gas using a gas/char separator followed by a hot
cyclone. The biomass char is used as top-up fuel the steam boiler, with a possibility of load
levelling in order to stabilise the steam flow, in case of fluctuations. The metal temperature in
the steam boiler is kept below the limits of accelerating high temperature corrosion.
The plant was commissioned in May 1996. The demonstration plant is located at Haslev, 65
km south-west of Copenhagen, at the Haslev Combined Heat and Power (CHP) plant which
hosted this demonstration project. Haslev CHP plant is the world's first 100% straw fired
CHP plant. It was put into operation in 1989 and produces about 5 MWe electricity and 3.6
MWh district heating. The plant uses 5.3 t/h straw delivered in large bales and supplies
district heating to about 2000 domestic consumers in the town of Haslev. District heating is
supplied at a temperature of 90 °C and returned with a temperature range of 50-60 °C. Main
steam data are: 25 ton/h, 65 bar and 420 °C under typical working conditions. The turbine
runs at 10,000 rpm and is connected to a generator running 1500 rpm. The electricity is
transformed to 50 kV and connected to the main grid. The yearly electricity production is
about 17 GWh, corresponding to the need of approximately 3000 households. The overall
efficiency is 17.6% electricity and 70% heat on an annual basis. Under optimum conditions,
an efficiency of 23% electricity can be reached with 85% overall efficiency. The low
electricity gain is due to the low steam data and due to the on-off operation. New plants are
showing slightly better performances.
3. Technical Description
Based on very positive results from testing of a pilot plant for more than 1000 operation
hours, a 15 times larger full-scale plant has been constructed. On the steam line from the
boiler to turbine a bypass has been established for connection and testing of the demonstration
plant.
Figure 1: Simplified diagram of the concept to use product gas to boost steam data
The demonstration plant consists of one process line with a design capacity of 675 kg straw
per hour. The carbonising unit is fed with loosely cut straw using a 2 step hydraulic piston
feeder creating a gas-tight seal. The carbonising unit is a special designed 2 level horizontal
double screw plug flow reactor, which is indirectly heated to approx. 600 °C by flue gas in a
heat jacket. After separation of char and fines in the hot cyclone, the produced 500-550 °C hot
gas fuels a special low NOx burner installed in an adiabatic combustion chamber, where
combustion takes place at approx. 1400 °C. Heat is initially used for additional superheating,
of the steam from the boiler to the maximum allowable temperature, then at 580-650 °C for
heating the carbonising unit, and finally for pre-heating of the combustion air to 360-400 °C,
before it is led to the chimney at 150-200 °C. Part of the flue gas can be re-circulated in order
to reduce NOx formation and increase efficiency.
The separated char is cooled with a slight water dosing and extracted using a hydraulic piston
feeder. From here, the char is conveyed to the combustion compartment of the boiler of the
basic plant using a pneumatic transport system. The char is used as topping fuel, and the
dosing can be controlled according to operational needs of the plant. The process is started by
using a start-up fuel i.e. gas or oil, which also can be used as back-up.
4. Performance of the Haslev CHP Plant
The operating conditions and the performance of the plant when operated on straw is given in
table 1.
Table 1: Energy data
Parameter Data
Straw for the carbonising unit: 0.675 t/h nom, gross (0.240 t/h net)
Steam flow from boiler: 11-26 t/h at 410-430 °C
Steam from superheater: 480°C (max. 505 °C)
Superheater surface: < 580 °C .
Combustion temperature: 1250-1500 °C
LHV gas: 7-8 MJ/kg
LHV char: 26.8 MJ/kg
Nominal gas capacity: 900 kW
Marginal power share: 42%
Marginal heat share: 58%
Gross overall efficiency: 78-82%
Gas production, nom.: 430 kg/h
Char production, nom.: 245 kg/h
Power output marginal: 360 - 680 kWe *)
Own consumption: 56 kWe, at full load
Straw for the basic boiler: 5.3 t/h
Power output total plant: Approx. 5-5.5 MWe
Heat output total plant: Approx. 13 MJ/s
*)Depending on the actual upgrading of steam data.
The operational experience from monitoring more than 1000 hours of operation is very
positive. The results of testing show an output well above the design value and a stable
operation after the commissioning period. The operation safety and availability figures under
gas operation have now reached a stage comparable to conventional biomass fired plants. The
monitoring period has given all the basic data for validation of the plant and design of the
single components for further commercial plants.
The plant is fed with up to 675 kg/h straw with a moisture content of 10-22%, producing
roughly 0.9-1 MW of gas and 1.7-1.8 MW char. Due to the low process temperature and
absence of air, the corrosive elements remain in the char fraction, which is used as topping
fuel in the steam boiler. The produced gas is utilised in a separate combustion chamber
producing a flue gas with very low content of alkali metals, chlorides and particles, making
the flue gas much less corrosive at high steam temperatures than flue gasses from direct
combustion of municipal solid waste or straw. Therefore, the produced gas can be used to
boost steam data in an external superheater without increasing the corrosion rate. The supply
of char to the steam boiler can be controlled in order to stabilise the steam production, which
also will lead to increased efficiency and output.
If the concept demonstrated in this project is used, when building new plants, the electricity
gain could be increased by typically 10-15% on a given heat demand. This is an improvement
of approximately 3% points in electricity efficiency which is considered significant. Potential
users for the dissemination of this technology are public and private utilities in charge of
waste management and energy production. The concept mainly addresses MSW (municipal
solid waste) fired CHP plants, either new plants or plants to be retrofitted in respect to
combustion line and steam turbine.
The environmental impact of the plant is given in table 2. In principle the utilisation of straw
is CO2 neutral. The noise level is not significant, however, there is a slight smell at the plant
due to the carbonisation process. Therefore future plants must be located in an industrial area.
Table 2: Measured emissions
Component Measurement
Particles: 13 - 15 mg/Nm3
SO2: 160 -173 mg/Nm3
CO: 41 - 44 mg/NM3
O2 reference: 10 %
5. Economic Performance
The total project cost including development and testing was 26 million DKK. The
carbonisation demonstration plant was installed at a host plant at conditions which can not be
considered optimum. The Haslev CHP plant has not been upgraded to take full advantages of
the straw carbonisation unit which would allow to increase the pressure by 5-10 bar at higher
steam temperatures, (however, a technical and economic evaluation of an eventual upgrading
is under investigation). Therefore the economic performance of the Haslev CHP plant is not
optimum.
For new plants the pay-back period is estimated to be 6-9 years, which normally is acceptable
for CHP plants.
6. Project Identifiers
Project : BM/422/91-DK
Owner : Haslev CHP Plant
Contractors : ANSALDO VOLUND, COWI & ELKRAFT
Contact: COWI Consulting Engineers & Planners AS
Parallelvej 15,
DK - 2800 Lyngby,
Denmark
Tel: [phone],
Fax: [phone]
Technology: ANSALDO VOLUND & COWI (patent holder of
the carbonisation technology)
Total Cost: 2,387,324 ECU
EC Support: 704 728 ECU