6. Using of combustible waste

Combustible wastes can be characterized by: ▪

• Higher and lower heating value (HHV, LHV).

• Short or proximate analysis, i.e. a determination of their moisture, ash and combustible content, further subdivided into volatile matter and fixed carbon.

• Ultimate analysis, i.e. element analysis of the combustible fraction and ash.

Other important properties are:

• Form and size, physical and bulk density, flammability and putrescence of solid waste.

• Vapor pressure, boiling point, density, viscosity, explosion limits, flash point, self-ignition temperature, corrosiveness, toxicity, and gas evolution or decomposition during preheating, possible auto-oxidation, polymerization or other incontrollable, exothermic or dangerous reactions of liquid waste.

• Density, explosion limits, toxicity and corrosiveness of off-gases.

Heating values determine whether waste is highly, moderately or almost not combustible. Heating values are required to establish the capacity and heat balance over the furnace, the combustion air requirements, and to decide whether heat recovery, combustion air preheating, or disposal in combination with other wastes are desirable.

The proximate analysis defines the amount of moisture to be evaporated prior to combustion and the required size of ash handling equipment. Moist waste, such as garbage, sewage sludge and aqueous solutions, burns only after at least superficial evaporation of the moisture contained. Hence, fast drying should be stimulated by adequate measures, such as exposure to radiant heat or contact with easily ignitable waste.

From the chemical analysis of combustibles the amount and composition of the flue gas can be estimated at a given excess of air. In a first approximation, the S-, Cl- and (much less so) N- content during combustion will be converted into SO₂, HCl and NO. Their concentration determines whether wet scrubbing of the flue gas is required. When the flue gas is cooled slowly and in the presence of catalytic fly ash, SO₂ can be further oxidized to SO₃, and HCl to Cl₂. Some part of SO₂/SO₃ and HCl is removed by adsorption on fly ash. NO formation is lowered by staged combustion, i.e. when the combustion is conducted in two steps: the first under reducing conditions, the second at moderately low temperatures.

Chemical analysts and dedicated testing of ash inform on its softening and melting behavior and hence about its tackiness and possible attack on refractory. Sodium and potassium compounds decrease the melting point, in particular when present as persulfates, vanadates and borates. The presence of volatile elements, such as mercury, thallium, cadmium, arsenic, antimony, etc. makes waste improper for incineration in conventional units, unless their air pollution controls are adapted.

Some properties are vital for selecting and specifying adequate storage, handling and feeding facilities and safety provisions. Information is also required on frequency and timing of deliveries, kind of containers and packaging.

There are several ways that are available for utilization of waste in the form of energy:

1. Incineration, a process of controlled combustion to reduce volume and getting the heat.

2. Combustion – the coefficient of excess air is above 1; comes to thermo-chemical conversion by releasing the chemical energy of fuel, heat, apply to the fuel with limited moisture comment and higher heating power.

3. Pyrolysis, a process of thermal decomposition in which the material is heated under the influence of an external heat source without presence of air, and as a result of that we get mixture of solid, liquid and gaseous fuel, a portion of this fuel can be used as a asource of thermal energy for pyrolysis.

4. Gasification, a process of the thermal decomposition that takes place similar to combustion, but with the coefficient of excess air is lower from 1; waste is converted into gas which mainly consists of CO₂, H₂, CH₄.

5. Plasma process – waste is heated to high temperature (3000⁰C to 10000⁰C) using the «plasma arc»; energy released by electric discharge in inert atmosphere. In this way organic waste is converted into gas that is rich with hydrogen and inorganic waste into inert glassy residue.

6. Anaerobic digestion is a process of microbiological decomposition without the presence of air; organic matter is processed with high moisture content; with the decomposition is obtained gas which consists of methane and CO₂.

7. Landfill gas – mainly from landfill gas is formed by bacterial decomposition, bacteria that are present in the waste and soil covering the landfill, unlike the previous one in this case microbiological decomposition is not completely comtrolled, and party takes place and anaerobic digestion.

Each of these technology requires different amounts of input materials, and emitted different amounts of CO₂, have different outputs and different efficiency.

There is another type of combustible waste – waste of different technological process that can be used as a fuel in another one. Examples are process gases of ferrous and nonferrous metallurgy, fuel, liquid and solid wastes from chemical and oil and gas industries, wood chips, sawdust, shavings, wood waste liquors and pulp paper industries.

Many combustible waste, for example steel, have a lower heating value (LHV) and chemically aggressive. This creates considerable difficulties in their disposal. It can takes place in case of combustion of high-energy, but at the same time flammable, explosive and toxic waste (hydrogen, dry exhaust gases, etc.).

For the disposal of combustible waste is often necessary to use special equipment, but the main way of using them - application in industrial technologies.

In the steel industry top gases are the most significant combustible waste. In particular, due to its combustion 35-45% of the heat demand of metallurgical enterprises can be covered. Its temperature is 175 – 250⁰C, it contents 23-40% of CO, 12-22% of CO₂, 1,5 – 6% of H₂, the rest is N₂; its heating value is about 4MJ/m³.

Dust-free top gas is mainly used in blast stoves, power boilers for heating of coke oven batteries, soaking pit furnaces and rolling mills. Besides, it can be also used in gas natural pressure turbines.

The second (after the top gas) most important source of carbon monoxide on ferrous metallurgy is the converter gas. Its heating value is 8,4 – 9,2 MJ/m³ with a temperature of 1400 – 1600⁰C.and this gas is also used as heat waste.

Coke oven gas is the third largest source of combustible waste of steel and iron industry. It should be noted the overall high waste output in coke production (about 80% of primary energy consumption). It is the energy potential of the coke oven gas, that can be used as the heat and combustible waste.

The main consumer of coke oven gas as fuel is metall production. First of all it is used for open-hearth furnaces and heating units heat treatment of metal. If there is enough it is also used in heaters coke oven batteries. In this case, the combustion products at the output of the generator has a temperature of 260-3500⁰C, taking heat 15-20% let down. In some enterprises, it is spent on getting the hot air to heat the coal tower gate in the winter.

The chemical pulping process uses a complex combustion system called a recovery boiler to generate process heat and electricity as well as to recover the processing chemicals in an almost closed cycle. The recovery boiler is a very complex device, which is actually operated as a gasifier - combustor. After evaporation of the majority of the water, the very high solids black liquor is sprayed onto a mass of char in the bottom of the boiler.

The lignin pyrolysis produces reducing gases and char. These react with the spent pulping chemicals to produce sodium carbonate and sodium sulfide. Ash and other minerals in the wood feed turn up as non-process elements and have to be removed from the cycle. The gases from the char bed pass to an oxidizing zone in the furnace and burn to produce process steam (and electricity) as well as provide radiant heat back to the char bed for the reduction chemistry to take place. The product chemicals are molten, drained from the char bed to collectors, and then poured into water to produce green liquor.