A gasification system for solid wastes having a thermal reactor and a mechanical gas cleaner

8/7/2019 A gasification system for solid wastes having a thermal reactor and a mechanical gas cleaner

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A gasification system for solid wastes having a thermal reactor and a mechanical gas cleaner, an indirectheat exchange cooler, and an electrostatic precipitator for cleaning and cooling the produced gas. Feed

material is continuously fed to the central section of the thermal reactor above an air introductionmanifold and nozzles and in an upward direction, forming a stratified charge. As feed material movesupward and outward from the reactor center it is reduced to ash. An agitator assures contact between

the hot particulate product and hot gases resulting in gasification of the feed material and netmovement to the sidewall of the thermal reactor, forming ash. The air introduction nozzles serve as agrate. Ash descends along the sidewall to the reactor base for removal. The mechanical cleaner has a

high speed rotating brush-like gas separator element and scraper combination which removescondensed tars and particulates from the produced gas stream. The device is self cleaning in thatcondensed tars and particulates agglomerate on the high speed rotating bristle elements and, upon

reaching adequate size and mass, are thrown off by centrifugal force to the cylindrical sidewall, wherescrapers remove accumulated material which falls to the separator base for removal. An electrostaticprecipitator having a cylindrical brush-like electrode suspended from one end by an insulated arm,

removes remaining particles or aerosols from the product gas.

Gas Producers (Gasifiers)

Design of gasifier depends upon type of fuel used and whether gasifier is portable orstationary. Gas producers are classified according to how the air blast is introduced in thefuel column. History of gasification reveals serveral designs of gasifiers. The mostcommonly built gasifiers are classied as :

y Updraft gas producery Downdraft gas producery Twin-fire gas producery C rossdraft gas producer

y O ther gas producer

Updraft gas producer

An updraft gasifier has clearly defined zones for partialcombustion, reduction, and pyrolysis. Air is introducedat the bottom and act as countercurrent to fuel flow. Thegas is drawn at higher location. The updraft gasifier



C rossdraft gas producer

Crossdraft gas producers, although they have certainadvantages over updraft and downdraft gasifiers, they are notof ideal type. The disadvantages such as high exit gastemperature, poor CO 2 reduction and high gas velocity arethe consequence of the design. Unlike downdraft and updraftgasifiers, the ash bin, fire and reduction zone in crossdraftgasifiers are separated. This design characteristics limit thetype of fuel for operation to low ash fuels such as wood,charcoal and coke. The load following ability of crossdraftgasifier is quite good due to concentrated partial zones whichoperates at temperatures up to 2000 o c. Start up time (5-10minutes) is much faster than that of downdraft and updraftunits. The relatively higher temperature in cross draft gas

producer has an obvious effect on gas composition such as high carbon monoxide, and lowhydrogen and methane content when dry fuel such as charcoal is used. Crossdraft gasifier operates well on dry air blast and dry fuel.

O ther gas producer

Although updraft, downdraft or crossdraft gas producers have been the most commonly builttypes, there is a wide variety of gasifiers which do not really fit into any of these categories andare classified as other gas producers . Some units are built to combine the advantages of crossdraft with downdraft or updraft gas producers.

Combustion

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This article is about the process of burning. For combustion without external ignition, see spontaneouscombustion . For the engine used in mobile propulsion, see internal combustion engine . For the visible

part of a fire, see flame .



"B urning " redirects here. For other uses, see Burning (disambiguation) .

For other uses, see Combustion (disambiguation) .

It has been suggested that Rate of combustion be merged into this article or section. ( Discuss )

This article needs additional citations for verification . Please help improve this article by adding reliable references . Unsourced material may be challenged and removed . (July 2010)

The flames caused as a result of a fuel undergoing combustion (burning)

C ombustion or burning is the sequence of exothermic chemical reactions between a fuel and anoxidant accompanied by the production of heat and conversion of chemical species. The releaseof heat can result in the production of light in the form of either glowing or a flame . Fuels of interest often include organic compounds (especially hydrocarbons ) in the gas, liquid or solid

phase.

In a complete combustion reaction, a compound reacts with an oxidizing element, such asoxygen or fluorine , and the products are compounds of each element in the fuel with theoxidizing element. For example:

CH4 + 2 O 2 CO 2 + 2 H2O + energy

CH2S + 6 F2 CF 4 + 2 HF + SF6

A simple example can be seen in the combustion of hydrogen and oxygen , which is a commonlyused reaction in rocket engines :



2 H2 + O2 2 H 2O(g) + heat

The result is water vapor.

Complete combustion is almost impossible to achieve. In reality, as actual combustion reactions

come to equilibrium , a wide variety of major and minor species will be present such as carbonmonoxide and pure carbon ( soot or ash). Additionally, any combustion in atmospheric air , whichis 78% nitrogen , will also create several forms of nitrogen oxides .

C ontents

[hide ]

y 1 Types o 1 .1 Complete vs. incomplete o 1 .2 Incomplete o 1 .3 Smoldering o 1 .4 Rapid o 1 .5 Turbulent o 1 .6 Microgravity

y 2 Micro Combustion y 3 Chemical Equation y 4 Fuels

o 4.1 Liquid fuels o 4.2 Solid fuels

y 5 Reaction mechanism y 6 Temperature y 7 Instabilities y 8 Rate of Combustion y 9 See also y 10 References y 11 External links

[edit ] Types

[edit ] C omplete vs. incomplete

See also: pyrolysis

In complete combustion, the reactant burns in oxygen, producing a limited number of products.When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water.When elements are burned, the products are primarily the most common oxides. Carbon willyield carbon dioxide , nitrogen will yield nitrogen dioxide , sulfur will yield sulfur dioxide , andiron will yield iron(III) oxide .



Combustion is not necessarily favorable to the maximum degree of oxidation and it can betemperature-dependent. For example, sulfur trioxide is not produced quantitatively incombustion of sulfur. Nitrogen oxides start to form above 2,800 °F (1,540 °C) and more nitrogenoxides are produced at higher temperatures. Below this temperature, molecular nitrogen (N 2) isfavored. It is also a function of oxygen excess. [1]

In most industrial applications and in fires , air is the source of oxygen (O 2). In air, each kg (lbm)of oxygen is mixed with approximately 3.76 kg (lbm) of nitrogen . Nitrogen does not take part incombustion, but at high temperatures, some nitrogen will be converted to NO x, usually between1% and 0.002% (2 ppm). [2] Furthermore, when there is any incomplete combustion, some of carbon is converted to carbon monoxide . A more complete set of equations for combustion of methane in air is therefore:

CH4 + 2 O 2 CO 2 + 2 H2O

2 CH4 + 3 O 2 2 CO + 4 H 2O

N2 + O2 2 NO

N2 + 2 O2 2 NO 2

[edit ] Incomplete

Incomplete combustion occurs when there isn't enough oxygen to allow the fuel to reactcompletely to produce carbon dioxide and water. It also happens when the combustion isquenched by a heat sink such as a solid surface or flame trap.

For most fuels, such as diesel oil, coal or wood, pyrolysis occurs before combustion. Inincomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke withnoxious particulate matter and gases. Partially oxidized compounds are also a concern; partialoxidation of ethanol can produce harmful acetaldehyde , and carbon can produce toxic carbonmonoxide .

The quality of combustion can be improved by design of combustion devices, such as burners and internal combustion engines . Further improvements are achievable by catalytic after-burningdevices (such as catalytic converters ) or by the simple partial return of the exhaust gases into thecombustion process. Such devices are required by environmental legislation for cars in mostcountries, and may be necessary in large combustion devices, such as thermal power plants , toreach legal emission standards .

The degree of combustion can be measured and analyzed, with test equipment. HVAC contractors, firemen and engineers use combustion analyzers to test the efficiency of a burner during the combustion process. In addition, the efficiency of an internal combustion engine can

be measured in this way, and some states and local municipalities are using combustion analysisto define and rate the efficiency of vehicles on the road today.



[edit ] Smoldering

Smoldering is the slow, low-temperature, flameless form of combustion, sustained by the heatevolved when oxygen directly attacks the surface of a condensed-phase fuel. It is a typicallyincomplete combustion reaction. Solid materials that can sustain a smoldering reaction include

coal, cellulose, wood, cotton, tobacco, peat, duff, humus, synthetic foams, charring polymersincluding polyurethane foam, and dust. Common examples of smoldering phenomena are theinitiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, ashort-circuited wire), and the persistent combustion of biomass behind the flaming front of wildfires

[edit ] Rapid

Container of ethanol vapour mixed with air, undergoing rapid combustion

R apid combustion is a form of combustion, otherwise known as a fire , in which large amounts of heat and light energy are released, which often results in a flame . This is used in a form of machinery such as internal combustion engines and in thermobaric weapons . Sometimes, a largevolume of gas is liberated in combustion besides the production of heat and light. The suddenevolution of large quantities of gas creates excessive pressure that produces a loud noise. Such acombustion is known as an explosion . Combustion need not involve oxygen; e.g., hydrogen

burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion.

[edit ] Turbulent



Combustion resulting in a turbulent flame is the most used for industrial application (e.g. gasturbines, gasoline engines, etc.) because the turbulence helps the mixing process between the fueland oxidizer.

[edit ] Microgravity

Combustion processes behave differently in a microgravity environment than in Earth-gravityconditions due to the lack of buoyancy . For example, a candle's flame takes the shape of asphere. [3] Microgravity combustion research contributes to understanding of spacecraft fire safetyand diverse aspects of combustion physics.

[edit ] Micro C ombustion

Combustion processes which happen in very small volume are considered as micro combustion .Quenching distance plays a vital role in stabilizing the flame in such combustion chambers.

[edit ] C hemical Equation

Generally, the chemical equation for stoichiometric burning of hydrocarbon in oxygen is

For example, the burning of propane is

Generally, the chemical equation for stoichiometric incomplete combustion of hydrocarbon inoxygen is as follows:

For example, the incomplete combustion of propane is:

The simple word equation for the combustion of a hydrocarbon in oxygen is:

If the combustion takes place using air as the oxygen source, the nitrogen can be added to theequation,as and although it does not react, to show the composition of the flue gas:



For example, the burning of propane is:

The simple word equation for the combustion of a hydrocarbon in air is:

Nitrogen may also oxidize when there is an excess of oxygen. The reaction is thermodynamicallyfavored only at high temperatures. Diesel engines are run with an excess of oxygen to combustsmall particles that tend to form with only a stoichiometric amount of oxygen, necessarily

producing nitrogen oxide emissions. Both the United States and European Union are planning toimpose limits to nitrogen oxide emissions, which necessitate the use of a special catalyticconverter or treatment of the exhaust with urea .

[edit ] Fuels

[edit ] Liquid fuels

Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It isthe vapour that burns, not the liquid. Therefore, a liquid will normally catch fire only above acertain temperature: its flash point . The flash point of a liquid fuel is the lowest temperature atwhich it can form an ignitable mix with air. It is also the minimum temperature at which there is

enough evaporated fuel in the air to start combustion.

[edit ] Solid fuels

The act of combustion consists of three relatively distinct but overlapping phases:

y P reheating phase , when the unburned fuel is heated up to its flash point and then fire point .Flammable gases start being evolved in a process similar to dry distillation .

y D istillation phase or gaseous phase , when the mix of evolved flammable gases with oxygen isignited. Energy is produced in the form of heat and light. Flames are often visible. Heat transferfrom the combustion to the solid maintains the evolution of flammable vapours.

y Charcoal phase or solid phase , when the output of flammable gases from the material is too lowfor persistent presence of flame and the charred fuel does not burn rapidly anymore but justglows and later only smoulders .



A general scheme of polymer combustion

[edit ] Reaction mechanism

Combustion in oxygen is a radical chain reaction where many distinct radical intermediates participate.

The high energy required for initiation is explained by the unusual structure of the dioxygenmolecule. The lowest-energy configuration of the dioxygen molecule is a stable, relativelyunreactive diradical in a triplet spin state . Bonding can be described with three bonding electron

pairs and two antibonding electrons, whose spins are aligned, such that the molecule has nonzerototal angular momentum. Most fuels, on the other hand, are in a singlet state, with paired spinsand zero total angular momentum. Interaction between the two is quantum mechanically a"forbidden transition ", i.e. possible with a very low probability. To initiate combustion, energy isrequired to force dioxygen into a spin-paired state, or singlet oxygen . This intermediate isextremely reactive. The energy is supplied as heat . The reaction produces heat, which keeps itgoing.

Combustion of hydrocarbons is thought to be initiated by hydrogen atom abstraction (not protonabstraction) from the fuel to oxygen, to give a hydroperoxide radical (HOO). This reacts further

to give hydroperoxides, which break up to give hydroxyl radicals . There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species includesinglet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl . Such intermediates are short-lived and cannot be isolated. However, non-radical intermediates are stable and are produced inincomplete combustion. An example is acetaldehyde produced in the combustion of ethanol . Anintermediate in the combustion of carbon and hydrocarbons, carbon monoxide , is of specialimportance because it is a poisonous gas , but also economically useful for the production of syngas .



Solid fuels also undergo a great number of pyrolysis reactions that give more easily oxidized,gaseous fuels. These reactions are endothermic and require constant energy input from thecombustion reactions. A lack of oxygen or other poorly designed conditions result in thesenoxious and carcinogenic pyrolysis products being emitted as thick, black smoke.

[edit ] TemperatureAssuming perfect combustion conditions, such as complete combustion under adiabatic conditions (i.e., no heat loss or gain), the adiabatic combustion temperature can be determined.The formula that yields this temperature is based on the first law of thermodynamics and takesnote of the fact that the heat of combustion is used entirely for heating the fuel, the combustionair or oxygen, and the combustion product gases (commonly referred to as the flue gas ).

In the case of fossil fuels burnt in air, the combustion temperature depends on all of thefollowing:

y the heating value ;y the stoichiometric air to fuel ratio ;y the specific heat capacity of fuel and air;y the air and fuel inlet temperatures.

The adiabatic combustion temperature (also known as the adiabatic flame temperature ) increasesfor higher heating values and inlet air and fuel temperatures and for stoichiometric air ratiosapproaching one.

Most commonly, the adiabatic combustion temperatures for coals are around 2,200 °C (3,992 °F)(for inlet air and fuel at ambient temperatures and for = 1.0), around 2,150 °C (3,902 °F) for oil

and 2,000 °C (3,632 °F) for natural gas .

In industrial fired heaters , power plant steam generators, and large gas-fired turbines , the morecommon way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air . For example, excess combustion air of 15 percent means that 15 percentmore than the required stoichiometric air is being used.

[edit ] Instabilities

Combustion instabilities are typically violent pressure oscillations in a combustion chamber.These pressure oscillations can be as high as 180 dB, and long term exposure to these cyclic

pressure and thermal loads reduces the life of engine components. In rockets, such as the F1 usedin the Saturn V program, instabilities led to massive damage of the combustion chamber andsurrounding components. This problem was solved by re-designing the fuel injector. In liquid jetengines the droplet size and distribution can be used to attenuate the instabilities. Combustioninstabilities are a major concern in ground-based gas turbine engines because of NOx emissions.The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion



temperature and thus reduce the NOx emissions; however, running the combustion lean makes itvery susceptible to combustion instabilities.

The R ayleigh Criterion is the basis for analysis of thermoacoustic combustion instabilities and isevaluated using the R ayleigh Index over one cycle of instability: [citation needed ]

where q' is the heat release rate perturbation and p' is the pressure fluctuation. [4][5] When the heatrelease oscillations are in phase with the pressure oscillations, the R ayleigh Index is positive andthe magnitude of the thermo acoustic instability increases. On the other hand, if the R ayleighIndex is negative, then thermoacoustic damping occurs. The R ayleigh Criterion implies that athermoacoustic instability can be optimally controlled by having heat release oscillations 180degrees out of phase with pressure oscillations at the same frequency. This minimizes theR ayleigh Index. [citation needed ]

[edit ] Rate of C ombustion

R ate of combustion is the amount of mass of a material that goes through combustion over anamount of time. It can be expressed in g/s or kg/s.

[edit ]

A gasification system for solid wastes having a thermal reactor and a mechanical gas cleaner

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