Principles of Pyrolysis and Gasification

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5. Principles of Pyrolysis and Gasification
5.1 Introduction

In a basic chemistry textbook pyrolysis would be likely to be defined as decomposition by application of heat, or in similar words to those. Some inorganic compounds can be pyrolysed, but in the context of waste management it is of course organics which are of interest.

Pyrolysis and gasification as technologies were developed for application to coal, the former in the late eighteenth century and the latter in the early nineteenth. There is a degree of overlap of meaning between pyrolysis and gasification. When a coal is pyrolysed – ‘carbonised’ would be a more common choice of word in the coal industry – there are three classes of product: solid, liquid and gas, collectively referred to as pyrolysate. Since gas is amongst the products, pyrolysis of coal could be, and frequently is, described as ‘partial gasification’. By contrast when air and/or steam is passed through a bed of coal all of the organic content becomes gas and this is ‘total gasification’. Temperatures of coal carbonisation will be in the range 500 to 1000°C.

5.2 Heat balance in pyrolysis
The process of pyrolysis of an organic material can be represented in general terms as shown in the shaded area below.

Continuing with coal as the starting material but bearing in mind that the principles apply equally to other organics such as MSW and cellulosics, all three classes of pyrolysate will be combustible and have fuel potential. The solid from coal pyrolysis is either coke or char: the difference is in the mechanical strength and degree of swelling on carbonisation and chemically each approximates to pure carbon. The liquid part contains tars and oils. These will comprise hydrocarbon liquids and, depending on the oxygen content of the starting material, oxygenated hydrocarbons. A low-temperature (≈ 100°C) pyrolysis product of wood is methanol, which is why ‘wood alcohol’ is one synonym for methanol. The tar from a bituminous coal will contain little oxygen, that from a low-rank coal (lignite) an appreciable quantity.

Gaseous pyrolysate will be flammable by reason of hydrogen, carbon monoxide and possibly methane within it. There might however be non-flammable components which will act as a diluents, notably carbon dioxide. One would expect very little carbon dioxide from the pyrolysis of a bituminous coal but significant amounts from pyrolysis of materials themselves having a high oxygen content.

Gaseous pyrolysate is soon removed from the hot pyrolysis zone and will not undergo the further reactions schematised above. At least for coals (which are the focus of the discussion at present) little hydrogen is released on pyrolysis as molecular hydrogen H₂. The product of pyrolysis in response to supply h₂ of enthalpy will therefore have a similar C:H ratio to the original structure and yield on burning a product of similar proportions CO₂ and H₂O. Hence the difference between h₂ and h₃ is small and if it is neglected and the enthalpy is given the single symbol h_f we have:

enthalpy change for combustion of the initial compound or structure = h₁ - h_f

and:

enthalpy change for combustion of the compound or structure modified by pyrolysis = (h₁ + ∆h) – h_f

Clearly the second of these is the larger. What is explained here for a single ‘compound or structure’ will of course apply to many such in pyrolysis of coal, MSW or whatever. So when the combined pyrolysis products have greater potential for heat release than the starting material the difference has been taken from the heat applied during pyrolysis: it is as simple as that!

5.3 Reactions taking place during total gasification

These include:

C + 0.5O₂ → CO

which gives a flammable gas as product. More commonly (in fact just about always) the oxygen is atmospheric, therefore the equation should really be written:

C + 0.5O₂ (+ 1.88 N₂) → CO (+ 1.88 N₂)

and the effect of the nitrogen is of course to lower the calorific value. The above gas has composition molar or volume basis:

CO: 1/2.88 = 35% N₂: 1.88/2.88 = 65%

and is known as Siemens gas, the simplest form of producer gas. Carbon monoxide has a molar heat of combustion [1]([1] SI Chemical Data Book John Wiley, any available edition.) of 282 kJ mol^-1 so, having regard to the fact that 1 m³ of any gas or gas mixture at 1 bar and 25°C contains 40 moles, the calorific value of the Siemens gas is:

40 × 0.35 × 0.282 MJ m^-3 = 4 MJ m^-3

Another reaction of importance in gasification is:

C + H₂O → CO + H₂

The above gas for fuel use is called blue water gas (on the basis of the colour of the flame) and has a calorific value of 11 MJ m^-3. Such a gas can however be used to make such compounds such as methanol in which case it is known as a synthesis gas. Before cracking technologies for oil products were developed in the early 20th Century synthesis gas was the primary means of organic chemical manufacture. In today’s world there is much production of liquid fuels by this means. Production of1synthesis gas is a classical technology as we have seen, and the novel content of any current application is often the catalysis by means of which the desired end product, which might be gasoline, is obtained.

A coal gasifier is classified on a MW_th basis, that is:

heat which the gas can release on burning/time required to make the gas

and this will be examined against the chemistry given above. Let us suppose that an equimolar mixture of CO and H₂, such as is shown in one of the equations above, is produced from coal waste at a rate of 10⁶ m³ per day. The calorific value of a binary mixture of CO and H2 in any proportions is 12 MJ m^-3 to the nearest whole number. The rating of the gasifier in MW_th is then:

The time in the formulation refers of course to gasification, so the gasifier rating in MW_th must not be equated to the heat-release rate of the manufactured gas fuel after ignition in air.

5.4 The role pyrolysis in combustion

Clearly, if it is intended to pyrolyse a substance the atmosphere must be inert so as to preclude combustion. However, when a substance such as MSW, wood waste or TDF is burnt there will be overlapping combustion and pyrolysis. Heat released at the early stages of combustion feeds back to unburnt material and stimulates pyrolysis. Tars and gases are thus released into the flame and burn there. Pyrolysis in combustion will not be total, so some of the material will burn ‘unpyrolysed’. The extent of pyrolysis in combustion depends inter alia on heating rate.

The overall process is:

waste substance + air → combustion products

and the enthalpy change is, by the First Law of Thermodynamics, independent of the path. How much fuel burns as pyrolysis product or as ‘unpyrolysed’ material does not therefore affect the heat of reaction.

5.5 Plasma gasification

An electric plasma results when a normally non-conducting medium such as air bears a current. Temperatures of the plasma are in the range 3000 to 10000K, very much higher than those obtainable in combustion processes. Applications are many and include welding and steel making. Plasma gasification of waste [2]([2] http://www.safewasteandpower.com/process_plasma-gasification.html) consists heating a carrier gas which enters the waste and breaks it down. The product gas is of moderate calorific value (4 to 5 MJ m^-3) and suitable for fuel application and the slag will often have a potential use. Examples of plasma gasification of wastes will be given in a subsequent chapter.

5.6 Concluding remarks

This chapter, whilst concerned with ‘principles’, has focused on coal and appropriately coal waste featured in the calculation immediately above. The gasification of coal waste is of immense importance at the present time in Pennsylvania where piles of such waste having been in existence for a century or more are being dismantled and the coal substance gasified. The sites previously occupied by the coal waste piles are then landscaped and this is to the obvious benefit of residents.

Pyrolysis a.k.a. partial gasification has also been explained in outline, and further comments on this are needed before reader proceeds to the later chapters. We have seen that pyrolysis of any organic waste results in solid, liquid and gaseous products. Each has fuel potential, and in a commercial pyrolysis process much will depend on the suitability of the respective products for fuel use. Can the liquid be used as a fuel, or blended with a conventional fuel, in any widely used combustion appliance? Can the gas be used to supplement or extend natural gas without modification to burners? Is the solid sufficiently strong mechanically to be used as a metallurgical reductant in place of coke? Many more such questions have to be addressed when pyrolysis of a waste is undertaken on an industrial scale, and the feasibility depends on the nature and saleability of the three classes of product. There is however one point of major importance: if the material being pyrolysed is carbon-neutral so are its pyrolysis products. This gives them an intrinsic advantage over their counterparts from coal carbonisation and makes further processing more viable. If for example the liquid component from pyrolysis requires hydrogenation for fuel use the cost of that might well be more than offset by carbon credits generated if the liquid so hydrogenated is used in place of a petroleum product. It is only since about the time of the Kyoto Protocol that substitution of unconventional fuels for conventional ones on the basis of carbon credits accruing has become prevalent and for this reason production of fuels from waste materials themselves carbon-neutral is a growth industry.

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