The purpose of the combustion system is to turn the carbon, hydrogen and possibly sulphur in the fuel completely into CO2, H2O and SO2. So that combustion is as clean as possible, the correct mixture ratio of fuel and combustion air must be present at the right time and place in the combustion chamber.
Pressurised combustion is almost exclusively used for this in shell boilers. This means that the combustion air fan must provide the positive pressure necessary to overcome the 5 – ≤ 50 mbar resistance caused by the boiler and, if installed, the downstream heat exchanger. A slight positive pressure therefore always exists in the combustion chamber.
Technical information: requirements for an on-site burner system
Natural gas and fuel oil are still the most commonly used fuels. Depending on the mode of operation of the steam boiler, the required output or emission level requirements, each of these standard fuels offer different benefits and are suitable for different applications.
Info on Criteria for selection between fuel oil and natural gas
The correct combination of fuel, combustion equipment and combustion chamber is particularly decisive for the cleanest possible combustion.
In addition to the standard fuels, different gaseous and liquid fuels can also be used in shell boilers. Depending on the fuel, this is easy to implement or may require a great deal of effort. Whatever the case, use of these fuels should be carefully considered in the project phase as, in addition to higher investment, stricter monitoring and maintenance during operation is sometimes required.
Info on Further fuels
Examples of liquid special fuels:
Examples of gaseous special fuels:
Combustion of these fuels can take the form of individual auxiliary fuel combustion, e.g. when using a dual-fuel burner with natural gas and a liquid special fuel, or as proportioning combustion, e.g. natural gas with biogas.
Sectional representation of monoblock burner (Weishaupt)
Combustion systems in which the combustion air fan is directly integrated into the casing of the burner are referred to as monoblock or pressure-jet burners. This burner system is suitable for fuel oil, gaseous fuels and also as a combination system in which a straightforward changeover between gas and oil combustion is possible. The most obvious advantage of monoblock burners is their compactness and therefore favourable design, and also that all systems in the combustion system can be mounted directly at the boiler in a space-saving manner. Monoblock burners can be used up to a combustion output of roughly 10MW. They are however not suitable for use with an air preheating system.
The term duoblock burner is used to describe burners whose combustion air fan (shown mounted on the boiler) and combustion unit are installed separately. The combustion air fan and burner are connected by a combustion air duct. Duoblock burners are used specially for large combustion outputs and when using air preheating.
Duoblock burner with fan on the boiler crown and combustion air ducts (Saacke)
The most important terms and distinguishing features for combustion systems and the necessary equipment are described below.
Image and sectional representation of oil burner (Weishaupt)
With pressurised atomisation, the oil is guided through a nozzle and discharged as a fine spray into the combustion chamber. The pre-charge pressures in the oil infeed must be roughly 6 – 30 bar. When the oil jet exits from the nozzle, fine oil droplets with a large reaction cross-section form. The prerequisite for this is that the viscosity of the fuel must be within the range of 5 – 8mm²/s. If this is not the case at ambient temperature, the oil must be preheated.
The burner can be controlled in different ways. With step burners, several nozzles are installed in the burner head. Depending on the required output, nozzles are switched on or off by activating solenoid valves. Burners with up to three nozzles are available.
Return flow atomising burners are used so that the output can be controlled steplessly. To do this, a valve in the fuel oil return flow regulates the quantity of fuel admitted to the combustion chamber. This valve is activated in combination with the position of the combustion air damper.
The liquid fuel is admitted at low pressure to a conical cup atomiser via a hollow shaft which rotates at high speed. The oil film that forms on this migrates to the edge of the cup which opens out towards the combustion chamber. Due to the centrifugal force, the oil film at the edge of the cup breaks off and forms fine oil droplets which are flung into the combustion chamber in a swirling motion.
Some of the combustion air is guided into the cup and the rest flows into an annular gap around the cup which normally has an opposing swirling motion. The admission and distribution of combustion air affects the flame appearance. This results in an intensive mixing of the oil with the combustion air.
A significant advantage of the rotary atomiser compared to the pressure atomiser is that it depends less on the viscosity characteristics of the fuel. This means that fuels with varying quality can also be reliably burnt. The rotation of the cup can also be monitored to ensure clean combustion without CO and soot formation.
Sectional view of a rotary atomising burner (Saacke)
Example of a light fuel oil combustion system with pressure-jet oil burner
Flow rate indicator
Pressure indicator (pressure gauge)
Air deficiency safety device
Maximum pressure limiter
Oil supply module OSM
Oil supply module
The oil supply module pumps the fuel from the oil storage tank which is set up outside the boiler house via the oil ring line to the individual oil circulation modules that supply each oil burner individually.
It is pre-assembled as a single or double station with 100% reserve to ensure security of supply, also in the event of an oil filter change, with all valves in an oil sump for ease of installation in the ring line.
Oil pressure regulation module ORM
The purpose of the oil pressure regulation module is to set a constant oil pressure in the flow of the oil ring line. It consists of an oil pressure controller, upstream and downstream shut-off valves that allow the oil pressure controller to be removed, a pressure indicator (PI) and a bypass valve. It is always integrated downstream of the last branch line that supplies the burner.
Oil circulation module OCM
Oil circulation module
The oil circulation module prepares liquid fuels by means of filtering and air separation and measures the oil flow rate. It is designed for light and heavy fuel oil pressure-jet burners equipped with return flow atomiser system and is installed as ready-to-use unit including casing for every burner in the ring lines with a pre-charge pressure of ≥ 1.5 bar.
The module contains an oil reservoir with two chambers which supplies oil directly to the burner and receives the return flow quantity from the burner. The pipework can be directly connected to the oil hoses of the burner.
It includes a filter valve (FI), oil volumeter, shut-off valves, overpressure safety valve, ventilation shut-off valve and drain plug. In the case of heavy fuel oil operation, insulation is also fitted under the metal cladding.
Oil preheating module OPM
Oil preheating module
Temperature control valve
When using medium and heavy oils as fuel, they must be preheated because at ambient temperature they do not have the flow characteristics required for atomisation. The oil must be preheated in order to reduce the viscosity. Depending on the burner make and fuel, heating up to a temperature of 100 – 180°C is required to ensure reliable combustion.
The fuel is heated by a heat exchanger with extendable tube bundle which can be operated with steam or a combination of steam heating and electric heater. It must be ensured that trace heating is also fitted to all pipework and valves. In the start-up condition, the oil is initially heated by electricity then during continuous operation it is heated with steam to a constant temperature via the temperature control valve (TC). The module is pre-assembled ready to use, including the heating control, thermal insulation and all valves.
Nowadays, natural gas is available at most locations and normally costs less than oil. The market share of gas combustion systems has been steadily increasing in recent years.
In addition to the economic advantage, using gas as fuel has other benefits:
Burner with monoblock design (Dreizler)
Sectional representation of gas burner (Weishaupt)
Example showing schematic representation of gas combustion (high-pressure supply)
Minimum gas pressure limiter
Maximum gas pressure limiter
Safety shut-off valve: shuts off the gas supply
in the event of faults and an emergency stop
(located outside the boiler installation room)
Safety shut-off valve: locks out gas supply in the
event of unacceptable increase in gas pressure
Thermal shut-off valve: shuts off the gas supply
if fire breaks out in the boiler house
Gas pressure governor: ensures uniform gas
pressure for combustion
Gas regulation module
Safety relief valve: cuts in if an unacceptable
positive pressure occurs downstream of the gas
Shut-off valve: for shutting off manually
Pressure indicator (PI)
Gas filter: protects sensitive parts from
Expansion joint: compensates for pipework expansion
Flame monitoring (BA): switches the combustion
system off if combustion in the combustion chamber
is not stable after a starting time interval
Minimum gas pressure limiter (PRZA-):
switches off the combustion system if the gas
pressure is too low
Fan: combustion air supply
Maximum gas pressure limiter (PRZA+):
switches off the combustion system if the gas
pressure is too high
Air deficiency safety device (PZA-): shuts down the
combustion system if the delivery pressure of the
combustion air fan is low
Double solenoid valve: double shut-off of gas
supply when the burner is not in operation
Air damper: controls fuel/air ratio
Tightness test (PRZA+): tests the tightness
of the solenoid valves
Burner control unit/safety chain
Gas control damper: controls the gas volume
Gas regulation module GRM
The gas regulation module contains all the control and safety equipment required for safe and fault-free combustion. The gas and air supply in particular are set via an electronic or pneumatic integrated control so that the correct fuel/air ratio for complete, safe and efficient combustion in the combustion chamber exists at all load points.
The gas pressure regulator is installed to ensure a uniform gas pressure upstream of the burner irrespective of the changing pre-charge pressures. If the gas pressure was changed, the gas/air ratio at the burner would change and consequently either an unstable flame or combustion leading to a heavy build-up of soot and CO formation would occur. If there is a possibility of the safeguarded upstream gas pressure exceeding the permissible operating pressure of the components of the gas train, then a safety shut-off valve and safety relief valve must be installed upstream of the control unit.
The pressure switches monitor the minimum and maximum permissible gas pressure if the gas pressure governor indicates a defect. During downtimes or pre-ventilation, gas must not enter the combustion chamber as otherwise a hard ignition may occur. The solenoid valves in the gas train must therefore close reliably. For safety reasons, the gas solenoid valves are redundantly configured and the burner sequence program checks before every burner start whether the valves are tight (gas tightness check).
The heat of flue gases from upstream processes, e.g. combined heat and power from CHP modules or gas turbines, industrial manufacturing and production processes in the metal industry or the utilisation of thermal energy from waste material is suitable for generating steam in shell boilers.
Info on Combined heat and power
The possible steam output from the flue gases essentially depends on three criteria:
Additional framework parameters for selection of a suitable boiler are the corresponding sulphur content, solids content or other corrosive substances, e.g. chlorine content in flue gas.
In this case, due to the large number of possible flue gas variations, detailed engineering is always recommended in preparation for a waste heat steam boiler system in order to use the available thermal energy as efficiently as possible.
Combined heat and power unit with 4-pass waste heat boiler