σ - SMOX from SRF to HEAT

OVERVIEW

Direct flame oxidation is fast, and due to the high temperature of the process, can manage a large quantity of different materials. Nevertheless the direct flame oxidation, due its fast reaction speed, produce a lot of partially burned materials, known as smoke, polluting the environment.

To avoid pollution, the solution, is the SMOLDERING OXIDATION (SMOX). 

The SMOLDERING OXIDATION is a multi phase oxidation method having the purpose of fully oxidate a solid fuel, transforming most of the oxygen, hydrogen and carbon content of the input in inert gases, leaving at the end of the process a very low carbon content ash.

The low temperatures used during the first phases of the process and the slowness with which the process is deliberately carried out, avoid the formation of pollutants allowing an excellent energy recovery.

ADVANTAGES

The σ - SMOX advantages:

• Remove the volatile elements (Carbon, Hydrogen, Nitrogen, Oxygen, Sulphur, …)

• Recover the energy content into high quality heat (1000°C)

• Avoids the formation of NOx (due to the reducing environment of the initial phase of the process)

• Avoid the formation of dust (due to the slowness of the process)

• Avoid the formation of dioxins and furans

• Avoid the formation of carbon monoxide and volatile compounds

• Generates an inert, white ash with a very low carbon content

DESCRIPTION

The σ - SMOX OXIDATION DEVICE, specifically designed to avoid the formation of pollutants, is used to fully oxidize the SRF through the combination of different oxidation methods.

Smoldering combustion can be defined as a semi-self-sustaining, slowly propagating, low-temperature flameless combustion process in which solid fuel undergoes thermal decomposition, producing volatiles and carbonaceous char.

Subsequently to the smoldering phase, volatiles and char are fully oxidised in a gas burner to produce high quality thermal energy.

WORKING PRINCIPLES

The σ - SMOX OXIDATION is a multi-phase oxidation method having the purpose of fully oxidate a solid fuel, transforming most of the oxygen, hydrogen, and carbon content of the input in CO2 and H2O, leaving at the end of the process a very low carbon content ash, recovering heat.

The SMOLDERING phase, through a slow oxidation process in oxygen deficiency, produce a clean and burnable syngas. The syngas and its latent heat is then fully oxidate in an oxygen excess gas burner to recover the energy content of the SRF.

The low temperatures used during the first phases of the process and the slowness with which the process is deliberately carried out allows the creation of a very low environmental impact allowing instead an excellent energy recovery.

The exhaust gases resulting from the oxidation process after their energy recovery are sent to the filtration system. The non-combustible materials, contained in the loaded SRF, are rendered inert through their complete oxidation, separated from their metals content, and collected as inert ash.

Air feeding

Air is added in various places in the smoldering and in oxidation chamber according to the oxidation needs.

The oxidation air is sucked from the waste bunker to maintain a slight negative pressure in the bunker hall and eliminates most odour and dust emissions from the bunker area.

Before its arrival to the Smoldering Oxidation module, the air is preheated and used to dry the feedstock. This operation significantly increases the air humidity level.

The steam contained into the air is used, during the smoldering phase, to favour the “water shift reaction” that can extract the fixed carbon from the SRF according to the following: C + H2O = CO + H2 + H2O = CO2 + 2H2. The reaction is conducted with the help of a natural catalyser, extensively present inside the ash, Fe2O3.

The air is blown by fans into the smoldering areas, where its distribution is closely controlled using multiple distribution valves.

In the oxidation chamber air is blown at high speeds via inclined injectors, for increasing the combustion turbulence level, intensively mixing the flue-gases, and ensuring a better and complete oxidation.

Oxidation conditions

The smoldering oxidation process is designed and operate to achieve a good burnout of the combustion gases by ensuring that the combustion gases are maintained at a minimum temperature for a minimum residence time at a minimum oxygen level.

The design values of the system are:

• A temperature between 850°C to 1100°C (preferred: 960°C)

• A resident time after the last injected combustion air for at least 2 seconds

• An oxygen level of at least 6% into the exhaust gas

The carbon monoxide content of the flue-gas is continuously monitored and used as an indicator of the quality of combustion.

Auxiliary burners

The smoldering oxidation system is equipped with 2 auxiliary burners, one in the smoldering area, one in the oxidation area.

At start-up, auxiliary burners are used to heat up the oxidation chamber to 850°C and the smoldering chamber to 350°C is added before any SRF.

During operation, the burners are switched on automatically if the temperature falls below 850°C. During shutdown, the burners are used until no more volatile components are released by the smoldering area.

SRF feeder

The SRF is discharged from the SRF buffer to the smoldering filling hopper.

The hopper, through a double auger with crossed turns, send the SRF to a flap valve designed to avoid flashbacks and to prevent uncontrolled air infiltration. The flap valve discharges the SRF to a single auger that push the fuel inside the smoldering area.

The feeding system is designed to send, to the smoldering area, a uniform and constant amount of SRF, nevertheless the quantity is automatically adjusted according to the process needs.

The maximum amount of SRF that can be managed by every single SMOLDERING module depends on fuel’s energy content. 

Smoldering phase

The purpose of the SMOLDERING process is to fully separate the water and the volatile part of the SRF from its solid content. To avoid any dragging of solid components inside volatiles, the process is conducted in a very slow way and by phases.

• Evaporation phase, evaporate the residual water content (H2O)

• Pyrolysis phase, extract the light volatile compounds (CO; H2)

• Gasification phase, extract the heavy volatile compounds (tar)

To extract the remaining carbon (char), from the solid residues, solids are continuously mixed for about 120 minutes at 400°C in a high humidity environment.

Among the advantages of this kind of process:

• The homogeneity of the material sent to the heat treatment allows easier control of the oxidation process.

• In the pyrolysis stage, the gases and vapours are slowly released, and this prevents the formation of turbulent currents which could cause entrainments of solid particulate.

• The product entering the gasification phase (char) is just over 40% by weight compared to the initial product, it is homogeneous and porous. These characteristics make this material extremely reactive in contact with oxygen.

• The reduced quantities of material to be gasified (about 50% of the initial product) and its high reactivity allow to limit the quantity of air necessary for the almost total removal of the carbon from the inorganic fraction. The low air speeds through the gasification material also allow a strong reduction of the solid particulate entrained by the synthesis gas.

• The temperatures present in the pyrolysis and gasification phase are sufficiently low to limit the vaporisation of the metals.

Oxidation process

The purpose of the oxidation process is to completely oxidate the volatile compounds produced by the smoldering process, releasing a hot exhaust gas.

As soon as the volatile compounds are produced, they reach the syngas oxidation chamber, without losing the sensible heat. In the oxidation chamber, the volatile compound, meet the air to be completely oxidised.

The combustion chamber volume is designed to maintain the combustion gasses at a temperature between 850°C and 1000°C for at least 2 seconds. The oxidation chamber is equipped with an external burner not to allow the operating temperature to go down below the 850°C during the oxidation process.

Moreover, the oxidation chamber in equipped with a SNCR deNOx fed with Urea.

Among the advantages of this kind of process

• The conditions at the oxidation chamber (temperatures, residence times and excess air) are such as to allow the complete combustion of the gases generated in the previous processes and to ensure that there is no carbon monoxide in the gases at the chimney. Under the conditions adopted, the formation of thermal NOx is also attenuated.

• The low content of entrained dust and metal salts that enter the oxidation chamber, facilitate the oxidation process and the subsequent energy recovery and cleaning of the exhausted gases.

• The oxidation stage takes place in a homogeneous phase between air and synthesis / pyrolysis gases. This makes the process much more efficient by increasing the total efficiency of the system and considerably reducing the load of pollutants entering the exhaust gas cleaning system.

Process Temperatures

The graph below shows the trend of process temperatures, during the different phases of the smoldering oxidation process and the consequent solid mass reduction.

TEMPERATURE LIMITS

SRF surface (top zone):         500°C

SRF core (bottom zone):        300°C

Gasification phase:                      500°C (450°C set point)

Oxidation phase limits:                850°C – 1’000°C

SNCR reactor limits:                    850°C – 1’100°C

Selective non-catalytic reduction (SNCR) process

In the selective non-catalytic reduction (SNCR) process, nitrogen oxides (NO + NO2) are removed by injecting a reducing agent (urea) into the oxidation chamber.

The involved chemical reactions are:

• Conversion of urea into ammonia        NH2CONH2 + H2O → 2NH3 + CO2

• Reduction of NO to N2 with ammonia   4NO + 4NH3 + O2 → 4N2 + 6H2O

The selected reagent agent is UREA because its wider effective temperature range (850–1000°C) makes temperature control less critical and because of less storage and handling hazards being not flammable and not toxic.

The relationship between NOx reduction, ammonia slip and the reaction temperature is given in below.

At the oxidation chamber selected temperature range (950-1000°C), the reduction of NOx would be about 75 %, with an ammonia slip of less than 3%. 

Bottom Ash

The bottom ash is the solid residue of the smoldering chamber after the volatile part of the SRF has been removed. 

The bottom ash generated by a smoldering process, due to the long oxidation resident time and the low temperature adopted, are completely oxidised (inert), not melted, and with a very low carbon content (< 1%).

Metals can be easily removed from bottom ash; ferrous metals are removed using magnetic separation and non-ferrous metals are removed using eddy current separation.

Mechanical treatment operations, like size reduction by crushing or screening, intended to prepare materials for subsequent use, e.g. construction as a foundation material or road construction as a fill material, are not required because the ashes derived from SRF are already available in a granulometry < 3mm.

At the end of the process the ash, the magnetic and the not-magnetic metals are collected in 3 different containers, ready for recycling. 

MEDIA GALLERY