INCINERATION

The incineration consists of mass combustion techniques of solid wastes, which are admitted to an incineration furnace. Inside the furnace occur the combustion of organic wastes including the dangerous ones with air excess to promote mixing and turbulence, in order to ensure safety and completely burn of those substances. As co-products of this process occurs the formation of ashes and solid slags.

The first ones are completely inertized in cement matrices for succeeding compaction in landfills and, the second ones, are valorized, separating the metals from the inerts. The metals are recycled to the recycling industry and the inerts are normally used on civil construction, such as road flooring, landfills covering, etc. (Puna, 2005). There are two incineration processes, regarded with energetic valorization. As a great advantage of this process, it’s possible to produce a large quantity of electric energy, and therefore, it makes profitable this process, becoming autonomous and supplier of electricity.

COMPOSTING AND IT’S TYPE

It is important refer the control of main process variables of furnace incinerator, like, temperature, waste time residence on furnace, qualitative and quantitative analysis of solid wastes in admittance. Besides, the incineration and co-incineration processes are very restrictive in the admittance of several dangerous solid wastes, due to the legal limit values of gases that are produced in the incineration furnace and emitted to the atmosphere, which they are severally rigid, like, Dioxins, Furans, PCB’s and Heavy Metals (Brunner, 1994).

Basically, the incineration furnace is a combustion chamber where, the solid wastes chemical elements (carbon, hydrogen and, if exists also in the wastes, sulphur) are burned to produced combustion gases, especially CO, CO₂, H₂O, NO_x and, if it is the case, SO₂. With fewer proportions, it’s produced, also, acid gases like HCl and HF and, last but not least, heavy metals and macromolecules with high stability and higher molecular weight (Dioxins, Furan’s and PCB’s) (Freeman, 1988). The main elementary reactions of solid wastes in the combustion process at the incinerator are the follow ones:

C + O₂ → CO₂ 2H₂ + O₂ → 2H₂O S + O₂→ SO₂

The dedicated incineration works like an appropriate industrial infra-structure, which uses, to operate the incineration furnace, a secondary fuel, like natural gas, propane or fuel oil to improve and maintain the combustion of solid wastes. However, the main source of fuel is the solid wastes due to its higher specific heat. It’s estimated that, the solid wastes can substitute the use of secondary fuel until a percentage of utilisation about 40-50% (Formosinho et. al., 2000).

THE AEROBIC COMPOSTING PROCESS

The rate of organic molecules destruction depends of the high temperature inside in the furnace and the time residence of gases combustion in the incinerator. Normally, a temperature higher than 900 ºC and, a time residence between 2 and 5 seconds, with a Thermal Conversion Technologies for Solid Wastes: A New Way to Produce Sustainable Energy excess of air (oxygen) higher than 6% is sufficient to ensure the destruction of all organic molecules.

Valorsul is an integrated urban solid waste system, which include an incinerator, in order to burn urban solid wastes. This incinerator is located in Loures, near Lisbon, Portugal and, it operates since 1994 (Puna, 2002).

In the incineration of dangerous solid wastes with more than 1% of halogen organic compounds in their composition, expressed in chlorine, the temperature in the incinerator has to achieve 1100ºC and the residence time of combustion gases must be, at minimum, 2 seconds (Brunner, 1994; European Legislation, 1994). The main final products in the incineration process are, the combustion gases.

The appearance of CO in the combustion gases results from the inefficient burning of solid wastes with, most probably, a less air excess. To ensure the reduction of CO, it’s important to increase the air flow and, consequently, the excess of oxygen. With the combustion gases, flows particles, specially, those with a diameter smaller than 10 μm (Russo, 2005). As remaining solid wastes, the incineration process produces ashes and slags, which will be characterized more ahead.

Down the incineration chambers, the gases combustion has to be submitted to physical and chemical treatments, in order to ensure contents below to their respective legal limits. The main concerns are to achieve very low limits regarded with CO and volatile organic compounds (VOC’s), which are proceeding from incomplete combustion with low air excess), NO_x (produced due to high temperatures in the furnace interior), acid gases (HF, HCl and SO₂), produced due to the presence of halogen (F, Cl) and Sulphur atoms in the solid wastes composition, heavy metals (Cu, Cr, Cd, Be, Mn, Hg and As), Dioxins/Furan’s/PCB’s, and particles.

Today, the European Directive on waste incineration (76/2000/EC), overcome to the Portuguese legislation, by the DL n.º 85/2005) considers a limit value for Cr together with other eight heavy metals (Sb + As + Pb + Cr + Co + Cu + Mn + Ni + V) equal of 0.5 mg/Nm³ without taking into account the different toxicity of total Cr compared to Cr^VI (Cocarta, et al., 2007).

An incinerator powerplant must be gifted also, with sophisticated systems for treating the combustion gases; a turbine to convert thermal energy proceeded from super-heated steam water into mechanical energy and, also, an alternator to convert this mechanical energy into electric energy. This last equipment is extremely important to profit the heat release from the combustion (exothermic process) to produce super-heated steam water for subsequent production of electricity.

Figure 1; Monitorization of atmospheric pollutants at stack outlet, in the Valorsul’s incinerator (CTRSU), as well, their respective legal limits.

The co-products resulted from waste solid combustion in incineration chambers are easy to compact and to store, due to their reduced volume and, they are classified as ashes and slags. Both are considered as remaining solid wastes. The ashes are submitted to an inertization process before their deposition in controlled landfill, while the slags are submitted to a valorisation process, which consists in separate the metals from the inerts.

The metals can be conducted to the recycling industry and, the inerts can be used as covering of landfills, flooring roads, etc. The typical composition of slag’s is 40% of SiO2, 10% to 20% of Al2O3 and Fe2O3, 15% of H2O and oxides, phosphates and sulphates, with a content below to 6%. By another hand, the ashes typical composition is 15% of sulphates, 13% of chlorides, 7-8% of SiO2, 4-7% of Al and other alkaline and heavy metals with a proportion not higher than 5% (Puna, 2005).

There is still the production of volatile ashes, proceeding from the treatment of combustion gases in the depuration process. These ashes have to be also, submitted to an inertization procedure, before their deposition in landfill.

Figure 2; Processes data of Valorsul’s dedicated incineration (S. João da Talha, Loures,Portugal).

In summary, it’s possible to express the main processes characteristics which define the dedicated incineration of solid wastes:

• The temperature in the inside of furnaces has to be controlled between 900ºC and 1200ºC;

• The combustion gases must have a short time residence in the chamber incineration, at high temperature, to avoid the considerable production of Dioxins/Furan’s/PCB’s. This can be achieved with 2-5 seconds for a temperature of 900ºC and with an oxygen excess higher than 6% (Brunner, 1994);

• The admittance of solid wastes in the incineration chambers will only be possible with chlorine contents below 1% (w/w), to minimize and avoid, once again, the production of Dioxins/Furan’s, which occurs in the combustion process, at high temperatures (below 900ºC) and with high contents of Chlorine (Puna, 2002);

• An incineration powerplant has to be an installation to treat the combustion gases and, has to treat also, the co-products of incineration process (ashes, slag’s and volatile ashes resulted from combustion gases treatment process).

• The dedicated incineration will only be an energetic valorization process if the energy spontaneously released in the combustion chambers could be profited to produce superheated steam water. The high enthalpy of steam-water at high temperature has a significant economic value and, it can be used to produce electric energy, in order to supply electricity, not only for the own incineration powerplant, but, more important, supply to the national electric network;

• The main incomes of an incineration unit are the admittance of solid wastes and, the supply of electricity to the electric network. Besides that, it’s possible to avoid costs related to the purchase of electric energy coming from the operator’s supplier electricity.

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