Institute of Energy Process Engineering and Fuel Technology > Equipment > Specially developed methods for fuel characterization

Specially developed methods for fuel characterization

Contact person:Dipl.-Ing. Yunus Poyraz

Investigation of fuel ignition behavior in ignition furnaces

ressurized gas (air or nitrogen) charges are blown into a heated muffle furnace over a suitable apparatus with a defined amount of pulverized fuel of a specific quality. The ignition of the pulveniged fuel sample depends on its ignitability as well as the temperature and composition of the gases in the chamber. The time that elapses until ignition occurs (the so called ignition time) is measured and then evaluated. The following parameters can be varied during implementation of the experiments: particle size of the investigated dust, reaction gas composition (air, O2, CO2, Nx), and reaction temperature (400°C - 1100°C).

Figure 3.2.1: Ignition Furnace: Construction and Photo of the Device

Technical Data

Assay mass300mg
Bore diamater300mm
Calorific output6kW
Duration of ignation20ms
Inlet velocity20m/s

Determining the grinding behaviour with CMT (Clausthal Grindibilty Test)

With this method it is possible to study the grinding behavior of dust fuels in a roller mill sized for laboratory use (see Figure 3.2.2). This method's procedure simply representes the start-up procedure of a mill until it reaches a stationary operating point (called the mill balance). In the first cycle (starting of the mill operation) the raw coal is crushed. Then the crushed coal is ground, classified, and the product separated into dust (mass fraction with acceptable fineness) and recycled (mass fraction is coarser than the acceptable fineness) streams. The fines are removed from the circuit. The mill feed for the following cycles is composed of the recycle and of raw coal of an amount equal to the newly created dust removed from the system. The experimental results provide information on:

  • Cycle factor,
  • Energy availability for grinding,
  • Milling load,
  • Grain size of the generated dust.

 

Figure 3.2.2: Clausthal Grindability Test

Thermogravimetric investigations of solid fuels

The combustion of solid and lumpy fuels proceeds over the following process steps: drying, devolatilisation/pyrolysis, gasification and burnout of solids. The thermogravimetric investigations can be used to examine, in particular, the processes of drying and devolatilization. Here, the conversion of solid fuel depends on the main variables, temperature, oxygen partial pressure, total pressure and residence time. The thermogravimetric investigations of the conversion of solid, are carried out in a technical thermal balance, which is shown in simplified form in Figure 3.2.3. The centerpiece of the system is the electrically heated reaction chamber, which can be operated with variable heating rates (0.5 - 20 K / min) and different isothermal holding phases (1000°C) using program controls. To produce an inert atmosphere, nitrogen is pumped into the reaction chamber from the bottom through a packed bed layer, which serves to equalize the flow. In the middle of the reaction chamber is a cylindrical sample container, proportional to scale, to which the sample is added. The experiments for drying and pyrolysis take place under inert gas. The reaction chamber is operated under atmospheric pressure. The main parameters of the experiments, including the heating rate, final temperature, duration and level of isothermal holding phase, the total test duration, moisture content, and sample weight are varied. Typical test results provide:

  • the change in mass as a function of time,
  • the change in mass as a function of temperature,
  • possible emerging threshold temperatures or temperature intervals from reactions that are experiencing a mass change. 

Figure 3.2.3: Simplied design of the thermal balance

Technical Data:

Swashplate array (disaggregation: 0,1 g)0-3kg
Max. attitude50mm
Max. diameter80mm
Max. temperature1200°C
Max. warming rate heater20K/min
Reaction gas (pyrolyse-bustle)Nitrogen
Reaction gas (oxidation-bustle)Air or other gases

Studies with a patch process

There exists a wide range of solid fuels that differ in their combustion relevant properties (gasification, ignition, etc.) considerably.  In terms of reliable plant operation, it is of crucial importance that one has precise knowledge of the selected fuel. The theoretical ascertainment of the listed fuel properties is, in most cases, not possible, and must therefore be empirically determined during the experiment.\\The IEVB has developed a reactor for comprehensive studies regarding kinetics of solid fuel combustion and gasification. The exhaust gas composition (CO2, O2, CO, SO2, HF, HCl, H2O, NO2, NH3, H2, CH4, CxHy) within the batch reactor is measured.A schematic representation of this batch-grate reactor is given in Figure 3.2.4. The material is fed onto the grate (No. 2) in Segment 1. The volume above the grate, the feedstock, is approximately 30 litre. The feedstock can be heated by means of radiation from the upper segments (segment 2 to 4), using preheated air (max. 500~°C) from the secondary air (No. 13), or over a gas burner (No. 17 heated up). Next to the four thermocouples that record the bed temperature at various heights along the bed (No. 10) are devices to record continuoously the measurements of process gas components CH\textsubscript{4}, CO, CO2, H2, O2, and NOx. Furthermore, the wall and exhaust gas temperatures in the furnace (over the bed) and the pressure profile in the reactor are identified over the time (No. 11) (three measuring points over the height).

 

Figure 3.2.4: Schematic representation of the batch grate (batch reactor)

Figure 3.2.5: Experimental reactor at IEVB

Technical Data:

Bed height400mm
Bed weight10kg
Combustion chamber temperature1200°C
Primary air preheating300°C
Secundary air preheating500°C

Investigation of the sintering behavior of ash in a sintering furnace

Experiments concerning the sintering behavior are conducted with special laboratory equipment (a tube furnace). The furnace is shown schematically in Figure 3.2.6. The sintering behavior of the ash/slag samples is based on measuring the compressive strength of ash pellets.  These pellets are held below a certain temperature for a relatively long time (16 hours) in air or in a simulated flue gas atmosphere. Using this method,influence of thermal treatment on the change in pellet mass, density, and mechanical compression strength is assessed in relation to the original pellets. One can, using this method, investigate the sintering behavior, assess the tendency of the system to create hard, sintered build-up at varying operating conditions, such as temperature and gas composition.

The investigations are carried out as follows:

  • 1. Pellets with a diameter of about 8 mm and height of about 6 - 8 mm are pressed from the previously crushed ash/slag pellets (pressing force about 2 kN / cm2),
  • The height, diameter and mass of the pellets are measured before the thermal treatment in the oven,
  • The pellets are then treated in the tube furnace for 16 h (the furnace temperature is in the range of about 600 °C to 1200 °C),
  • The thermal treatment can take place both in air and in a simulated ue gas atmosphere (for example, the following gas composition: CO2 = 16 vol%, O2 = 6 vol%, SO2 = 0.5 vol%, N2 = 77.5 vol%),
  • After the thermal treatment, the pellets are cooled down,
  • 6. Next, the height, diameter, and mass of the pellets are measured again,
  • 7. Finally, the compressive strength of the ash pellets is measured with a special facility (the pellets are destroyed during this process).

 

 

Figure 3.2.6: design of a sintering furnace

Technical Data:

Thermal capacity3kW
Pellet bulk6-8mm
Heating duration16h
Temperature600-1200°C

Investigation of pulverized fuel combustion, emission, and ash deposition behavior in a 50 kW vertical combustion chamber (50 kW Down Fired Combustion Chamber - DFCC)

To investigate the combustion, emission characteristics and slagging behaviors, it is helpful to examine samples of coal, coal blends, biomass-derived fuels, and alternative fuels under defined conditions. For this purpose, a special 50 kW combustion chamber equipped with a swirl burner is used (see Figure 3.2.7).\\

The essential components of the system are: The fuel injection system, the burner, the vertical reactor and the ash removal equipment. The fuel supply rate (between 1-7kg/h)(1) can be kept constant with a maximum deviation of 4\%. The fuel is placed into a hopper with a capacity of 11 kg. A screw conveyor then transports the bulk material to the burner. It uses a twin-screw discharge which can transport especially difficult substances such as sewage sludge. Powdered fuel can be transported pneumatically using an air stream leading to the burner.\\

The experimental burner (2), an IFRF-burner, was developed for use in a wide range of operating conditions. The nominal burner output is 50 kW utilizing gaseous, liquid, powdered fuels, or mixtures thereof. One also has the option of using a swirling secondary air stream preheated to about 400~°C through use of a movable swirl generator. A notable feature of the burner is that its aerodynamics very well known. The burner outlet velocities, the velocity profiles, and the turbulence were measured with a laser anemometer for various swirl numbers. This information is of fundamental importance for scientific measurements. 

The reactor (3) consists of a vertical combustion chamber. The burner is found on the top, situated axially to the combustion chamber. The reactor is divided into several parts. The upper part consists of a refractory-lined high-radiation zone, measuring 2.2 m high and 0.3 meters in diameter. The lower part consists of a 1.8 m long low-temperature convection zone of stainless steel with a diameter of 0.25 m. The high-temperature radiation zone has four heating elements to compensate for heat loss. These heaters can be configured to a obtain a desired temperature profile in the radiation zone. The thermal performance of the heating elements is 8.8 kW, and they are operated up to temperatures reaching 1600°C. The low-temperature convection zone is water cooled. 

The primary task of the convection zone is to cool the exhaust gases to temperatures below 1000°C. Both zones have openings for sampling probes, with which the gas temperature and composition can be determined. There exists also the possibility to extract solid matter samples with a special probe to analyze the burnout.

The exhaust gas can be optionally cooled to about 180°C using a compressed air quenching system (5). This cooling may be necessary in order to avoid high temperatures in subsequent parts of the plant.

There are three outlets within the exhaust system for the removal of fly ash. An ash container (4) is located directly behind the exit of the convection zone. Ash is not separated in this stage, but it is separated further downstream in a cyclone (6) and a filter (7). Using this arrangement, even very fine particles can be separated. Because all downstream units lead to a pressure loss, a suction device must also be used downstream to force the exhaust gases through the lines. 

The temperature in the radiation zone is measured by a suction pyrometer, whereby measurement errors caused by the radiation exchange between the thermocouple and the environment are minimized. The thermocouple is surrounded by two concentric ceramic shielts that minimize the influence of radiation from the surrounding environment. The gas flows at very high speeds (> 100 m / s) along the ceramic tubes, where the dominant heat transfer path of the thermal element is convection. This method reduces the measurement error over a simple thermocouple by more than 95\%. The temperatures in the convection zone, however, can be measured with simple NiCr-Ni thermocouples. 

To measure the gas composition, an online gas analyzer is used. Hereby, the concentration of CO, CO2} NO and SO2 in the flue gas is measured with NDIR (Non-dispersive Infra Red) sensors. Oxygen is measured with an electrochemical sensor.

 

Figure 3.2.7: Design of the 50kW vertical combustion chamber (DFCC)

Technical Data:

Max. fuel input50kW
Fuel ow1-7kg/h
Max. wall temperature1400°C
Max. combustion chamber temperature1600°C
Combustion air ow rate10-60Nm3/h
Caloric output air preheater2,8kW
Air temperature500°C
Paricle retention timemax. 4s
lengh of radiation zone2,2m
Minor diameter of radiation zone330mm
Caloric output of radiation zone - 4 heating elements4 Heating elements 32kW
Lengh of convection zone2m
Minor diameter of convection zone200mm

Investigation of devolatilization and char combustion rates in a drop tube furnace ("Isothermal Plug Flow Reactor" - IPFR)

The characterization of fuel in a downpipe reactor (see Figure 3.2.8) is being carried out with both an inert atmosphere consisting of (N2) and with certain oxygen concentrations (5\%, 10\%, 15\% O2). Under these conditions, the individual process steps of the particle combustion are simulated: particle heating, release of the volatile components, and combustion of residual char. The advantage of this method is the high heating rate (up to 105  K/s) of the fuel sample, which corresponds to the actual firing conditions. A special, water-cooled probe is used to allow particulate sampling along the reaction pipe. The kinetic parameters for the devolatilization and char combustion models are derived from the measured data.

Figure 3.2.8: Downpipe reactor - Isothermal Plug Flow Reactor - IPFR

The characterization of fuel in a downpipe reactor (see Figure 3.2.8) is being carried out with both an inert atmosphere consisting of (N2) and with certain oxygen concentrations (5\%, 10\%, 15\% O2). Under these conditions, the individual process steps of the particle combustion are simulated: particle heating, release of the volatile components, and combustion of residual char. The advantage of this method is the high heating rate (up to 105  K/s) of the fuel sample, which corresponds to the actual firing conditions. A special, water-cooled probe is used to allow particulate sampling along the reaction pipe. The kinetic parameters for the devolatilization and char combustion models are derived from the measured data.

 Technical Data

Temperatureup to 1400°C
Detention time for outgassing10-250ms (with 5 ms Hub)
Detention time for burnout100-2500ms (with 150-200ms Hub)
Reactor atmosphereN2, CO2, O2, H2O (steam) and mixtures
Fuel ow25-500g/h
Exhaust gas analysisCO, CO2, O2, NO, SO2, H2, CH4

Pyrolysis and devolatilization experiments on a rotary kiln

Pyrolysis is a process that has certainly gained importance in the area of thermal waste treatment. This comes about as it is considered to be one of the primary process steps occurring before the combustion process.

The pyrolysis rotary drum allows us to investigate the relationship between the pyrolysis behavior of residues and the process parameters such as temperature and residence time.

The rotary kiln has been designed as shown in Figure 3.2.9. The feeding of the material is accomplished through an airtight storage container with an auger feed system in the ceramic rotary drum. The achievable temperatures are in the range between 500°C - 1000°C. The residence time of feedstock can be controlled by altering the tilt and rotation (rpm), as well as the mass flow of the feedstock. The latter amounts to 1-4 kg/h. At the outlet of the rotary tube, the pyrolysis coke is collected. Pyrolysis oil is produced by condensing the resulting pyrolysis gas. This gas is then burned in a flare.

Figure 3.2.9: Modell of rotary kiln for pyrolysis

Figure 3.2.10: Rotary kiln at IEVB

Technical data rotary klin:

Flow-rate1kg/h
Caloric output27kWel
Max. temperature of heater elements1150°C
Tube lenght (heated)1300mm
Tube lenght (total)1700mm
Tube diameter120mm
Rotary speed1-10U/min
Inert gas (pyrolyse-duty)Nitrogen
 

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