PYROLYSIS AND GASIFICATION OF SOLID WASTE

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Pyrolysis and Gasification of solid waste

In the pyrolysis technologies, the most efficient is the PPV process, which means Pyrolysis by Plasma with Vitrification. Pyrolysis is a technology dedicated to waste destruction, which works at high temperatures, more than the typical temperatures in incineration chambers, with low oxygen, in order to avoid the combustion phenomena (Camacho, 2005). To guarantee the absence of oxygen, the wastes are decomposed in an inert gaseous atmosphere, through the utilization of Nitrogen (N2) (Puna, 2002).

The process of pyrolysis can be defined, generally, as the chemical decomposition of organic matter by heat, in the absence of air, unlike the incineration methods. The pyrolysis processes are endothermic, on the contrary of dedicated incineration or co-incineration, because it needs to supply heat to the pyrolysis reactor in order to occur the pyrolysis reactions. If any gas is heated at higher temperatures, there are significant changes in their properties. In the range of temperatures between 2000ºC and 3000ºC, the gas molecules decompose in ionized atoms by losing electrons. This ionized gas is called plasma (Lapa & Oliveira, 2002).

Normally, the wastes are injected directly in the plasma, producing pyrolysis gas (essentially H2, CO, N2, CO2, CH4), and this gas can be burned in a combustion process, by incineration, in order to make profitable the entire process and to valorize it as gas fuel, since CO and CH4 are organic gases with high calorific power. Nevertheless, it’s necessary a higher and significant annual flow admittance solid waste to maintain the optimum operating conditions of PPV reactor and, also, to profit the all PPV system, since the production of plasma is a great consumer of thermal energy (Camacho, 2005). 

The co-products of this process, especially ashes and heavy metals, are encapsulated in a vitrified matrix, to avoid the production of leachates. This vitrified matrix transforms the PPV co-products in inert remaining wastes, without any chance of occur lixiviation. This is a great advantage in the environment and public health perspectives. This vitrified matric is called “obsidian” and, results from the cooling of glass file-dust, which is introduced in the pyrolysis reactor, on the temperature range of 2000ºC-3000ºC (Oliveira, 2000).

The glass at these temperatures is liquid and, in the cooling step, is submitted to a solidification process, covering the remaining wastes, heavy metals and other dangerous gaseous/solid substances produced in the pyrolysis reactor. These vitrified ashes have large applicability in the road flooring, landfills covering, and, as an additive to the cement in civil construction.

In this process, the application range of dangerous solid wastes is almost total and much more all-inclusive that the admittance wastes in incineration methods. In all thermal processes, this is the one that is considered, from an environmental point of view, the most sustainable, although the higher energy and economic costs (Puna, 2002). The general equation of a pyrolysis process can be traduced in the following way:

Organic matter + Heat → Gases + Refractory metals

The plasma is a special form of gaseous material, capable to conduct electricity and, it’s knower as the “fourth state of matter” (solid, liquid, gas, and plasma). In the state of plasma, the gas can achieve temperatures extremely high, which can change from 5000 to 50000 ºC, depending on its production conditions (Oliveira, 2000).

This plasma is generated by the formation of an electric arch, through the cross of electric current between the cathode and the anode.  Between them, a gas is injected and ionized.

This ionized gas is, subsequently injected over the solid wastes. The plasma jet is produced and controlled in a torch capable to convert electric energy in heat, at higher temperature through the gas flow. In the torch, any gas rapidly reaches the plasma state. Basically, there are two kinds of solid waste treatment by plasma: the direct heating system and heating system with gasification chamber.

Direct heating system

Through the plasma torch, it occurs the production of an electric field of radiant energy with higher intensity, capable to dissociate the existing intramolecular bindings of solid, liquid and gaseous wastes, dangerous or inerts, organics or inorganics. So, when the wastes are submitted to the plasma jet, they lose their original chemical composition to convert in more simple compounds. Figure 9 shows a directly heated system diagram used in the PPV system to treat municipal and hazardous solid wastes.

Heating system with gasification chamber

This system consists in two different stages of treatment. The solid wastes are injected in a first conventional gasification chamber, in order to gasify the organic compounds in a gas partially oxidized and, also, to melt the inorganic compounds. In this chamber, it’s produced a gas and a liquid, which they are, subsequently decomposed in a second chamber, with a PPV reactor.

After the dissociation of all molecules, the matter is recovered in the following forms (Puna, 2002):

• Plasma synthesized gas, which is conducted to a combustion chamber, in order to valorise its calorific power and, to reuse the release heated, supplying into the PPV reactor;

• Inorganic materials and vitrified silicates, which will swim on the surface of liquid phase. These inorganic compounds, in the case of directed heating technology, were submitted to temperatures substantially higher than in the gasification chamber method.

• Obsidian, which is a solid structure of higher hardness and, generally, with black color, similar to a mineral of the volcanic source. This solid contains the PPV ashes, the heavy metals, and other dangerous inorganic atoms, all vitrified, without any chance of occur lixiviation. Figure 10 shows the typical aspect of Obsidian.

Like other treatment techniques of industrial waste treatment, the use of pyrolysis with plasma presents advantages and disadvantages or inconvenient, as follows:

Advantages

• PPV is a process more environmentally friendly and safety, with “zero” pollutants emission or, with magnitudes lowers than those established in the environmental legal Framework related with air quality;

• Higher temperatures cause rapid and complete pyrolysis of organic wastes, melting and vitrifying certain inorganic compounds, in a high hardness structure, without lixiviation, called obsidian;

• The plasma synthesized gas, with high calorific power, can be used in other process or, it can be submitted to combustion in order to valorise it;

• In PPV reactor, there isn’t combustion of solid wastes, so, it doesn’t occur the production of toxic compounds, like dioxins, furan’s and PCB’s;

• The gas volume obtained is substantially less than the gas volume achieved in other treatment process, like incineration, so, it’s easier to be treated. The reduction rates volume from waste to gas, can be higher than 99%;

• The high temperature of PPV reactor to the molecule’s dissociation is produced from electricity, which is a clean energetic source;

• Enables the co-generation of energy, with production of electricity, steam and/or cold (freeze water/air conditioning).

Disadvantages

• PPV it’s a dedicated technology, requiring a high investment, due to the fact that, it can only be profitable when coupled with a thermoelectric powerplant, to supply the sufficient electricity for plasma production. It’s also necessary, a significant higher and stable flow of solid wastes, which compromises any waste reduction/reutilisation/recycling policy strategy in medium/long time;

• The PPV system can’t dispense a sophisticated washing gases system, as in any incineration process, especially for the retention of VOC’s and acid gases, after the combustion of plasma synthesized gas;

• For different waste treatment, in particular, those containing organic matter in significant amounts, the pyrolysis techniques can’t achieve great industrial development. The wastes are decomposed by pyrolysis but, after that, they are eliminated by combustion, through the incineration of plasma gas;

• The production of dioxins/furan’s/PCB’s in the incineration chamber, after PPV reactor, are strongly dependent of thermal recovery technologies used down the stream. It’s not clearly that it can assure a significant advantage over more advanced incineration Technologies or over gasification simple techniques.

Synthesizing, the main process characteristics of PPV, are

• It’s necessary a thermal source with high enthalpy and reduced mass, which is the plasma (boiled gas at high temperatures);

• The pyrolysis temperature, the applied heating rate and the waste composition will determine the gas pyrolysis composition;

• The plasma corresponds to the fourth state of matter, the ionized gas, under temperatures between 2000ºC and 3000ºC and, it’s produced by an electric discharge between the cathode and the anode, where flows a inert gas, which is injected over the wastes;

• Any organic compound, including wastes, is convertible in gas pyrolysis and in a mixture of retractable glass with PPV ashes and heavy metals, under a hardness solid structure without any material percolation;

• In this process, there isn’t a final liquid phase and the higher temperatures leads to the elimination of macromolecules traditionally produced in the combustion process (dioxins, furan’s, PCB’s);

• The plasma is controlled under a torch, converting electric energy into heat, through the supplying of a higher amount of electricity, proceeding from a own powerplant electricity production;

• In this process, occurs the following elementary reactions in the PPV reactor, with the important auxiliary of heat generated by the plasma:

C + H2O   → CO + H2    CO + H2O → CO2 + H2

C + CO2 → 2CO    C + 2H2 → CH4

The average volumetric composition of plasma gas is, normally, 41% of H2, 30% of CO, 17% of N2, 8% of CO2, 3% of CH4 and, O2, C2H2, C2H4 with contents lowers than 0,5%. The calorific recovery in PPV process around 10.51 MJ/Nm3 and the energetic yield is near from 612 kWh/ton of treated waste (Oliveira, 2000). Table 8 performs a comparison of the main characteristics between PPV and incineration processes.

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