Technology

The thermal treatment process annually covers approximately 20,000 tons of previously treated waste and 5,000 tons of sludge from the municipal waste treatment plant with an overall average calorific value of up to 16 MJ/kg.

The thermal power of the combustion plant is 15 MW with 2 MW of electricity produced. The electric power will be supplied to the distribution network, while the heat energy will be used in the district heating system for the city of Celje.

The thermal treatment process is conducted in the following stages: 

- transport and dosage of waste and sludge to the combustion plant in a ratio of 4:1,
- the gradual combustion of waste and the exhaust of ashes,
- cooling the flue gasses and the utilisation of the heat released during that process for the production of heat and electric energy,
- flue gas purification of harmful substances.

THE GRADUAL COMBUSTION OF WASTE AND THE EXHAUST OF ASHES

Based on the comparison of environmental, technological and economic criteria, modular incineration on a grate was chosen as the most appropriate thermal treatment process.

This technology is one of the alternatives to classical incineration on a grate and is appropriate for the incineration of smaller annual capacities – up to 50 thousand tons of light fraction waste and the incineration of previously treated waste and sludge from the waste treatment plant.
Combustion is conducted in two stages – in the primary and secondary chambers.

In the primary chamber, the combustion process is conducted with an air deficiency – approximately 70% of the theoretically required air, so pyrolytic gasification processes prevail.
Large quantities of flue gases develop, which travel to the secondary chamber for full combustion with the feeding of sufficient quantities of secondary and if needed tertiary air. The temperature of the gases leaving the primary chamber is usually between 650 and 850 ËšC, as a large part of the generated heat is used in endothermic pyrolytic processes.
The heterogeneous burning down of solid residue needs to be ensured towards the end of the revolving grate where the amount of air fed is sufficient for the complete oxidation of solid carbon.

In the second chamber, a temperature of up to 1,200 ËšC prevails which, together with intensive mixing with secondary air and a sufficient detention time (more than 2 seconds), ensures the full combustion of organic substances together with the polychlorinated biphenyls, polychlorinated dibenzodioxins, polychlorinated dibenzo-furans and polycyclic aromatic hydrocarbons eventually generated in the primary chamber.

COOLING THE FLUE GASES – The Production of Heat and Electrical Energy

The main components of the energy production system are the steam boiler with steam superheater and the steam turbine with the generator.

The feed water is vaporised in the boiler and superheated to the required temperature in the superheater. The superheated steam is then conducted through the steam turbine, driving the power generator.
The steam exiting the turbine is condensed in the condenser and from here it is led to the water preparation system and then, with the help of the boiler feed pump, back to the boiler. Air is normally used for smaller units of the condenser cooling. Part of the steam is used for the generation of hot water for the needs of district heating through the heat exchanger.

By using state-of-the-art technology comparable to the best plants for the thermal treatment of municipal waste in Europe, all requirements and stipulations of the Environment Protection Act and the implementing regulations, along with the EC 2000/76 DIRECTIVE on the incineration of waste, will be met.


FLUE GAS PURIFICATION

The selected combustion technology is distinguished by a highly controlled combustion process and the low emission of flue gases, which has positive effects in reducing the possibility of catalytic processes of harmful substances being generated during the cooling of the gases and less residue after flue gas purification.

The flue gas purification covers the elimination of particles, nitrogen oxides, acid gases and the eventually present organic substances and heavy metals.
The purification system is conducted in three stages:

-  semi-dry adsorption with lime milk for extracting acid gases,
-  fabric filter for the extraction of particles,
-  coke adsorbent for the extraction of organic substances and the eventually present heavy metals.

In order to reduce emission values for nitrogen oxide, recirculation of the flue gases and the dispersion of the ammonia in hot flue gases is foreseen.
The process of thermal treatment will ensure the organic carbon content in the ash and slag is below the limit values for inert waste – less than 3%.
Thermal treatment does not affect the other characteristics of ash and slag, which primarily depend on fuel composition. Ash and slag usually represent the inert or non-hazardous waste which can be deposited in the landfill for non-hazardous waste.

Products generated from flue gas purification are considered hazardous waste due to the increased content of metals and salts and are therefore taken by authorised collectors of hazardous waste which treat or deposit the waste at the landfill for hazardous waste in accordance with the legislative stipulations.

The saturated coke from the coke adsorbent is thermally regenerated in the combustion plant and fed back into the primary chamber, where it is incinerated together with the waste. 

The planned material and energy consumption for the planned 8,000 annual hours of operation is evident from the table below:

Description    unit    Norm    Unit/annually    Annual consumption
1. Fuel        3,125.0    t/year    25.000
* waste residue from mechanical and biological treatment 
* dehydrated sludge from the waste treatment plant    kg/h
kg/h    2,500.0
625.0    t/year
t/year    20.000
5.000
2. Supplementary fuel
* natural gas    m3/h    64.58    m3/year    516.000
3. Electric power    kWh/h    0.86    kWh/year    6.880
4. Process water    m3/h    2.00    m3/year    16.000
5. Compressed air     m3/h    15.00    m3/year    120.000
6. Additives (1)        125    t/year    1.000
* Ammonium solution (25%)
* Calcium hydroxide 
* Activated coke    kg/h
g/h
kg/h    10
100 
15    t/year
t/year
t/year    80 
800 
120

(1) The additives are for flue gas purification

THE WASTE AFTER THERMAL TREATMENT 

After thermal treatment, the following remain:

-  ash and slag up to 2,200 t/y,
-  filter residue up to 1,200 t/y,
-  saturated coke up to 120 t/y

Ash and slag are non-hazardous waste and, as such, can be deposited in the landfill or used in construction if interest arises.

The filter residue is the product of flue gas purification and contains increased metal and salt content and, as such, is regarded as hazardous waste.
It will be removed by collectors or waste processors qualified and authorised for performing services relating to hazardous waste.
Saturated coke from the coke adsorbent will be treated in the same manner as the filter dust.


CLOSURE OF THE PLANT

After the plant’s lifetime of 25 years has expired, the closure and dismantling of the devices for thermal waste treatment (IPPC devices) is not planned.
An expert evaluation will be prepared and a reconstruction of the plant carried out.