Legislation on quality and harmful emissions. Solid, liquid and gaseous fuels.

Legislation on quality and harmful emissions.

Solid, liquid and gaseous fuels.

DOWNLOADS ARTICLE IN PDF

With reference to liquid, gaseous and solid fuels in publication 2018-19.03, we have carried out research on European legislation concerning the quality of fuels and the emission of harmful pollutants into the air. We have been interested in the technical analysis of a fuel by carrying out research on the relevant quantities with which they are characterized: calorific value, combustion temperature, power on temperature, ignition temperature (spontaneous combustion), combustion air, viscosity, calorific value; we carried out studies on the theoretical method for the determination of the calorific value and of the combustion air and of the efficiency of an engine plant, with interest also to some experimental methods.

1 - INTRODUCTION
With the aim of improving the quality of the atmosphere with regard to sulfur dioxide and other polluting gases, the Community has had to take steps to progressively reduce the sulfur content of the diesel used for the propulsion of vehicles, including aircraft and ships and diesel for heating, industry and ships from 1975 with Directive 75/716/EEC which constitutes a first step towards reducing the level of sulfur of liquid fuels and concerns only gas oils; with this directive two types of gas oils are defined, type A and type B, in it is given a definition of certain types of fuels containing sulfur with the intention to reduce its content. With Article 1 of the same directive in paragraph 1,diesel oil is defined any petroleum product which, due to its distillation limits, is part of the average distillates intended for use as fuels or fuels and of which at least 85% by volume, including distillation losses, distills at 350 ° C; (are also included, in the same definition of gas oils, any petroleum product as defined in subheading 27.10 C I of the Common Customs Tariff, 1 January 1974 edition). In the same article, with diesel type A are defined any low sulfur diesel oil whose use is not subject to restrictions in the Member States; diesel oil Type B any diesel oil intended for use: in areas where the levels of air pollution due to sulfur dioxide, measured at ground level, are sufficiently low, or in areas where the participation of diesel fuel to air pollution due to sulfur dioxide is not significant. The same Article in paragraph 2 excludes the applicability of the definition in paragraph 1 for gas oils used in power stations, used for ships used for maritime navigation, contained in the fuel tanks of vessels used for inland waterways or motor vehicles at the time transition from one area to another or from a border between a third State and a Member State. The limits of sulfur content for gas oils are defined in Article 2 (1) and Member States shall take the necessary measures to ensure that: (a) Type A gas oils can be placed on the internal market of the Community only if the content of sulfur compounds, expressed as sulfur, (which they contain) is not more than 0,5% by weight, with effect from 1 October 1976, and 0,3% by weight, with effect from 1 October 1980; (b) Type B gas oils may be placed on the internal market of the Community only if the sulfur compounds content, expressed as sulfur, which they contain, does not exceed 0,8% by weight, with effect from 1 October 1976 and at 0,5% by weight from 1 October 1980. Article 5 states that Member States shall determine the areas in which the use of type B diesel is permitted. They shall inform the other Member States and the Commission on their decisions, as well as on the criteria followed for their selection.

Directive 75/716/EEC is first amended by Directive 87/219/EEC and then replaced by Directive 93/12/EEC. With the Directive 87/219/EEC various modifications are made including the reduction of the sulfur content possible in diesel oils. Further reductions are made by Directive 93/12/EEC; this last directive is first amended by Directive 98/70/EC and definitively repealed by Directive 2009/30/EC currently in force, the same directive amending Directive 98/70/EC on the quality of petrol and diesel fuels.

With the directive 93/12/CEE the European Commission in order to reach the levels of emission of particles fixed in the specific community directives (for example the directive 2008/50/CE, of 21 May 2008, currently in force and relative to the quality of the ambient air and for cleaner air in Europe under investigation in forthcoming research), Member States shall prohibit the marketing in the Community of diesel fuels with sulfur compounds, expressed as sulfur (hereinafter referred to assulfur content), exceeds:
- 0,2% by weight from 1 October 1994,
- 0,05% by weight from 1 October 1996.
Member States shall ensure the progressive availability of diesel fuels referred to above (paragraph 1 (1), Article 2) with a maximum sulfur content of 0.05% by weight. Member States shall prohibit the marketing in the Community of other gas oils, or those destined for different uses, from those referred to above (paragraph 1, Article 2 of Directive 93/12/EEC), with the exception of kerosene for aircraft, the sulfur content of which is exceeds 0.2% by weight from 1 October 1994. With the same directive the European Commission has laid down the obligation for Member States to take the necessary measures to carry out random checks on the sulfur content of gas oil on the market. The reference method adopted for the determination of the sulfur content of the commercialized gas oils is that defined by the ISO 8754 method. The statistical interpretation of the results of the controls in order to establish the sulfur content of the commercial gas oils must be carried out according to the ISO standard 4259.

With the Directive 98/70/EEC the European Commission intends to reduce the polluting emissions produced by the exhaust gases of motor vehicles and with the intention to reduce the disparities between the legislative or administrative provisions of the Member States on the specifications of fuels traditional and alternative used in vehicles with positive-ignition engines and compression-ignition engines; it intends to reduce the obstacles between trade in the Community which may have a direct impact on the establishment and functioning of the internal market, as well as on the international competitiveness of the European automobile and refining industries. Legislative provisions are dictated both for imitating polluting emissions and for defining appropriate and harmonized measures for the purpose, taking into consideration that primary atmospheric pollutants, such as nitrogen oxides, unburnt hydrocarbons, particulate matter, carbon monoxide, benzene and other harmful exhaust gases contributing to the formation of secondary pollutants such as ozone are contained in relevant quantities in exhaust gases and evaporative emissions from motor vehicles, thus creating a substantial risk to health, directly or indirectly. of man and the environment. With Directive 98/70/EEC and pursuant to Article 4 of Directive 94/12/EC of the European Parliament and of the Council, the Commission has defined a new approach for emission reduction policies, implemented since the year 2000, he also examined, among other things, the extent to which the improvement in the quality of petrol, diesel and other fuels would reduce atmospheric pollution. Directive 94/12/EC on the measures to be taken against air pollution by emissions from motor vehicles amends Directive 70/220/EEC with which the Commission establishes the first provisions concerning the approval of motor vehicles with positive ignition with reference to atmospheric pollution from exhaust gases. With Directive 94/12/EC in Article 4, provisions are laid down on Community measures against air pollution caused by motor vehicles. The Commission establishes that the measures will be designed in such a way that their effects meet the Community requirements for air quality criteria and the objectives associated with them; undertakes to carry out an assessment of the cost / effectiveness aspects of each measure taking into account, inter alia, the contributions that could be made to improve air quality: traffic management, for example regarding a breakdown adequate environmental costs, - the promotion of urban public transport - new propulsion technologies (eg electric traction), - the use of alternative fuels (eg biofuels), - the measures will be proportionate and reasonable with respect to the objectives pursued. The Commission's proposals take into account the above methodology and aim at the substantial reduction of pollutant emissions with regard to vehicles, in accordance with the same Directive 94/12/EC, and include in particular the following elements: requirements of this Directive (Articles 1 to 3) on the basis of the evaluation of the potential of the traditional engine and post-combustion technology, of possible improvements to the test procedure, for example, cold start, starting at low or winter temperatures, duration (for example in the conformity tests), steam emission; of the type approval measures involving stricter inspection and maintenance requirements, including, for example, on-board diagnostic systems. The improvements, I intend to refer to the aforementioned directive, are also based on the assessment of the possibility of checking the conformity of vehicles in circulation, the possible need for: i) specific limits for HC and NOx as well as a cumulative limit value and ii) measures for to include pollutants that are not yet regulated. The additional element in the Commission's proposals for the measures to be taken by Directive 94/12/EEC against atmospheric pollution from motor vehicle exhaust is the Measures complementary techniques in the context of specific directives, including: improvements to the fuel quality of emissions of hazardous substances (in particular benzene) from vehicles, stricter requirements for the inspection and maintenance program.
Limit values reduced rates, which are the subject of Directive 94/12/EC, have not been applied before 1 January 2000 for new type approvals. The Council has introduced tax incentives by defining concession conditions based on these limit values.

Directive 70/220/EEC and Directive 94/12/EC are subsequently repealed by Regulation (EC) 715/2007 as amended in 2008 by Regulation 692/2008/EC; the aforementioned standards have been studied in the publication 2018-19.06 (H Research magazine number 2018-19, publication date: June 2018).
Directive 98/70/EEC establishes, for reasons of health protection and the environment, the technical specifications for fuels to be used in vehicles with positive-ignition engines and compression-ignition engines (diesel); defines ecological specifications for petrol and diesel fuels, the said legislative provisions, considering that the use of oxygen and a substantial reduction of aromatics, olefin, benzene and sulfur may allow fuels of better quality from the point of view of quality air. For the purposes of this Directive, the following definitions apply:
1) petrol: volatile mineral oils intended for the operation of internal combustion and spark ignition engines, used for the propulsion of vehicles and included in CN codes 2710 00 27, 2710 00 29, 2710 00 32, 2710 00 34 and 2710 00 36;
2)diesel fuel: the gas oils specified in CN code 2710 00 66 and used for self-propelled vehicles (motor vehicles with positive or spontaneous ignition, by compression).
The Annexes I, II, III and IV of this Directive define the ecological specifications of fuels available on the market both for motor vehicles with positive ignition (petrol) and vehicles with compression ignition (diesel) engines; both Directive 98/70/EEC and the Annexes are amended by Directive 2009/30/EC, Directive 2014/77/EU and Directive 2015/1513/EU.

2 - COMBUSTIBLE

The main factor characterizing the fuels, liquids, solids and gasses is the calorific value: the quantity of heat that develops from the combustion of one kilogram of fuel; in the international system measured in kJ (kiloJoule) or in MJ (megaJoule). The kcal (kilocaloria) is very common, where 1 kcal = 4.187 kJ.

Solid fuels

The most used solid fuel for the machines is coal, mainly made up of carbon, in a percentage that varies according to quality and from other substances: hydrogen, oxygen, sulfur (aromatic), and small quantities of mineral substances (silica, alumina, iron oxide, etc.) which constitute the solid residue of combustion (ash). Coals (solid fuels) are distinguished in anthracite, litantrace, lignite and peat; the distinction is linked to the age of coal formation: for example the age of the tree trunk from which the coals are produced. The peat-type carbons are of more recent epoch, the anthracite type coals of the most remote epoch. The percentage of carbon, the main constituent, therefore decreases from the highest values ​​in anthracite (over 90% because in it the process of carbonization has been almost complete) to the lowest in peat (50% - 60%). The higher the percentage of carbon, the higher the amount of heat developed in combustion.

Below are the data collected regarding the elemental compositions and higher calorific powers Hs of the coals

	Carbon %	Hydrogen %	Oxygen %	Nitrogen %	Hs
Peat	50 - 60	4,5 – 5,8	28 - 48	0,75 - 3	12600-16700
Lignite	60 - 75	5	20 - 35	0,75 - 2	15100-25100
Litanthrace	75 - 90	4,5 – 5,5	5 - 15	0,75 – 1,8	29300-36800
Anthracite	90 - 95	2,5	3	0,50 - 1	34800-35600

There are other practical classifications of coals, for example based on the percentage of volatile hydrocarbons present, on long or short flame (organic compounds of carbon and hydrogen), or on the percentage of bituminous hydrocarbons on which the agglomerating power depends (fats, seeds-fats), epoch of the main component (timber) or from the place of provenance. The most commonly used coals for the boilers are the short flame half-fat litantrace with a sufficiently homogeneous agglomerate which, therefore, do not tend to crumble like the thin ones and eliminate the inconvenience of a flame that is too long, such as cooling due to the contact with the pipes; (at the same calorific value the long flame distributes the heat in a larger area, reducing the heat flux per unit of surface and therefore the temperature reached). Even if the increase in the cost of oil has favored the use of coal, liquid fuel is still the most used.

Liquid fuels

Oil is a liquid composed mainly of water, oils and fats (hydrocarbons) and other numerous substances; the composition is quite different in relation to the quality and place of origin (Arab countries, United States,Canada, Mexico, and others). On average it is composed of 80% to 90% of C carbon, of 10% to 15% of H2 hydrogen, of 20% to 5% of O2 oxygen, of 0.10% to 1.6% of S sulfur, 0.10% to 1.6% from N2 nitrogen, from 0.10% to 1% from water and 0.10% from ash. The oil coming from the producing countries is subjected to refining with a fractional distillation process through which the substances of different qualities are separated. The distillation takes place in special columns, or towers, where the crude oil is brought after a preheating to 400 °C, in the lower part of the tower the heavy oils are extracted that distill at a temperature between 300 °C and 400 °C, and residues (bitumen). From this phase we obtain the heavy naphtha, (fuel oil) of density ρ = 800-900 kg/m³ used in steam generators and in big two and four stroke diesel engines, while residues, bitumen, in lubricants. At higher heights of the distillation towers, medium oils are extracted, distilling between about 200 °C and about 300 °C, used for the production of gas oils, kerosene, used in fast diesel engines and gas turbines. At the top of the towers are finally taken light oils that distill at temperatures of less than 200 °C used for the production of gasoline that have the characteristic of being volatile and then form a highly flammable mixture with the air. Gasolines are widely used in automotive engines operating according to the Otto cycle, also referred to as carbureted engines, or with switch on commanded; the gasolines are also subjected to other processes such as reforming, a process used to increase the octane number of a hydrocarbon mixture; the octane number is an index of the detonation resistance (or anti-knock characteristic) of petrol or other fuels.

The petrol normally on the market for cars has an octane number of about 95, measured with the Research (RON) method in which the detonating power is tested with the engine in cold conditions, or it is about 85 if measured with the Motor method (MON ), a more severe test, where the test engine is the same as the RON system, but is tested under load, with the engine having a higher rotation speed, moreover the ignition advance is higher than the RON method , to test the power detonating (antiknock power). Both tests are based on the use of a four-stroke engine with a variable compression ratio; from the variation of this last one brings the gas in measurement to detonation. Subsequently the test is repeated without varying the compression ratio and using a sample mixture in which it is possible to vary the known octane number. When the detonation is reached, the octane number of the petrol is defined as the octane number of the sample mixture detonated by varying the known octane number. By definition we attribute a number of octane nil to n-heptane, any alkane: organic compounds consisting only of carbon and hydrogen, for this reason belonging to the largest class of hydrocarbons having crude formula C_nH_{(2n + 2)} as in case of n-heptane with crude formula C₇H₁₆ or to any mixture of several compounds corresponding to this formula (structural isomers) such as the linear isomer, more properly called n-heptane, which at room temperature is presented as a colorless liquid from the pungent odor, is a very flammable compound, irritating to the skin, dangerous to the environment, harmful. It is attributed an octane number of 100 to the isothane, a traditional name generally used to indicate 2,2,4-trimethylpentane, a branched aliphatic hydrocarbon belonging to the series of alkanes. At room temperature it is a colorless liquid with a typical smell of petrol, in which it is present, it is also moderately volatile and very flammable; it is used as a solvent and as a reference hydrocarbon for the measurement of the octane number, because from studies carried out on the engines it appears that highly branched hydrocarbons have a much lower pre-detonation (or head beating) than linear aliphatic hydrocarbons.

A gas having octane number 95 has the same resistance to detonation as a 95: 5 mixture of isooctane and n-heptane.

The "normal" car petrol (no longer on the market in Italy) has an octane number of about 84 - 86, while the "super" petrol (no longer on the market in Europe) has a higher octane number, equal to about 98 - 100 (after reduced to 97). Currently several oil companies also offer fuels with octane number 98 - 101, considering that the greater the octane number, the higher the "no-detonating power", fuel quality that reduces the risk of detonation of the bootable gasoline for the simple compression caused by the piston with the possibility of achieving a higher compression ratio.

The collected data indicate that in the liquid fuels derived from oil the lower calorific value assumes values​​ranging between 41.870 - 42.000 kJ/kg (10.000 - 10.300 kcal/kp). Relevant quantities for liquid fuels are also the flammability temperature, the temperature that must reach a liquid fuel so that the emitted vapors form with the air a flammable mixture in the presence of trigger. The value of the flammability temperature is very important also for safety reasons; it is low in gasolines, 20 °C, while it grows considerably in gas oils, in naphtha: 70 - 140 °C. From the above values ​​it is clear that gasolines are particularly suitable for forming mixtures with the air and for this reason they are mainly used in to Otto cycle engines (controlled ignition) but they are also dangerous for the easy formation of explosive mixtures. The naphtha and the gas oils to form a mixture with the air must be preheated, which is why they are mainly used in compression-ignition engines and are considered to be less dangerous than gasoline. Another characteristic that characterizes liquid fuels is the ignition temperature defined as the temperature at which the liquid ignites in contact with the flame and with an adequate presence of air; this is a temperature slightly above the flammable temperature. Finally, the viscosity of the liquid fuels is defined as the quantity that characterizes the resistance to sliding through the pipes, holes; it is determined with special apparatuses called viscometers, based on the measurement of the time necessary for the passage through a pre-defined orifice of the fluid under test and of another liquid compared, both at a certain temperature. There are different types of viscometers including that of Redwood, Saybolt and Engler. In this last one normally flows through a capillary (the orifice) by gravity 200 ml of oil and an equal quantity of distilled water at a temperature of 40 °C; the system is equipped with thermal insulation with the outside and under the indicated conditions a common oil normally takes 600 seconds to drain while the distilled water takes 60 seconds thus allowing to define the ratio (600/60) = 10, said viscosity of the oil, measured in Englar and indicated as follows: 10 °E to 40 °C. The instrument standardized according to the ISO DIN standards consists of a small-sized brass vessel, immersed in a thermostatic tank, provided with a calibrated hole (orifice) placed at the bottom center. The viscosity decreases with the increase in temperature and it is therefore common for the manufacturers to equip the machines with electric resistors to heat the fuel (especially heavy naphtha) before entering the pipe and the pulverizers (for example injectors in the combustion chamber) or in the injection line that feed the engine systems.

The size defined as the ignition temperature is also important: the temperature at which a mixture of fuel and comburent (for example gasoline and air) must be brought so that the combustion reaction triggered at one point can propagate throughout the mass of the mixture. Obviously even at slightly lower temperatures it is possible that the combustion reaction triggered at one point propagates throughout the comburent fuel mixture. It is useful to define the activation energy Ep of the combustion according to the distance between the molecules, called the reaction coordinate:

When the reacting molecules of the mixture come into contact with each other, they collide, the kinetic energy is transformed into potential energy; when the molecules are sufficiently distant from each other they do not react and are at a certain energetic level indicated in the figure above with "reagents". In order for the combustion reaction to occur, an activated complex must be formed which splits due to combustion in the products; an event that requires a certain amount of kinetic energy to the reacting molecules indicated by Ea in the figure above and said activation energy. From the analysis of the statistical distribution of the molecules speed shown in the figure below it is possible to observe that with the increase in temperature there is an increasing number of molecules with a speed equal to or higher than the V₀ speed necessary for creating the activated complex and then starting the combustion by propagating throughout the mass of the combustible combustive mixture. Since the kinetic energy is transformed into potential energy in the collision, and being the first increasing with the speed of the molecules, there is a velocity V₀ so that the transformation into potential energy is sufficient to trigger the activated complex. The number of molecules influences the activation because if the energy produced in the collision is low, even if from reacting molecules with a speed equal to or greater than the speed V₀, it is dispersed not being sufficient to trigger the combustion.

When the number of N molecules is sufficiently high such that the energy produced exceeds the potential energy dispersed, during combustion of the reacting molecules, the combustion which is propagated throughout the comburent combustible mixture is triggered. Since the temperature determines the above phenomenon, once the temperature T₃ is exceeded, the fuel in contact with the air triggers spontaneous combustion without the need for trigger (self-ignition). The temperature value T₃ is defined as ignition temperature which assumes the following values: for coke 500 °C, for 340 °C heavy mineral oils, for 250 °Cpetrol, for 230 °C diesel oil.

Gaseous fuels

The most widely used gases are natural gas, which is mainly made up of methane (obtained from technological processes of compressing air in combination with other substances, from which mainly carbon, hydrogen and nitrogen derive from the compressed air forming the new element: gaseous fuel), coking and blast furnace gases. The blast furnace gas is a mixture of gas obtained as a by-product of cast iron in a blast furnace. Its average composition is: 60% N₂, 24% CO, 12% CO₂, 4% H₂ and ash. In the lower part of the blast furnace, air enriched with preheated oxygen is insufflated at about 1000 °C. This comes in contact with the coke that ignites with CO and CO₂ production according to the Boudouard balance (2COCO₂+C). The gas rises from the blast furnace, reducing iron oxide to metal iron, but not all CO is oxidized to CO₂. The gases coming out of the furnace (blast furnace gas) are still rich in CO and can therefore be used as fuels, after the separation of the ashes. The cokery gas is a mixture of gas obtained by dry distillation of some types of litantrace. Litantrace is distilled to produce metallurgical coke, the resulting gas is used as fuel, as a reagent or as a source of hydrogen; the composition is very similar to that of city gas, but with a higher hydrogen content. Methane is a simple hydrocarbon (alkane) consisting of a carbon atom and four of hydrogen; its chemical formula is CH₄, and is found in nature in the form of gas. Below are the data collected on the average composition of some gaseous fuels with percentages expressed in volume (gas analyzers indicate data in volume percentages):

%	Natural gas	Blast furnace gas	Cokery gas (from litantrace)
CH4	95,6	0,5	34
CO	-	26	8
H2	-	3	50
Cm Hn	2,6	-	4
O2	0,2	-	-
N2	1,5	56	2
CO2	0,1	9,5	2
H2O	-	5	-

3 - CALCULATION OF COMBUSTION AIR AND CALORIFIC POWER

Among the various elements that make up fuel, the phenomenon of combustion is interested from carbon, hydrogen, sulfur and oxygen. The elementary reactions of a complete combustion are represented as follows:

C + O₂ = CO₂

2H₂ + O₂ = 2 H₂O

S+ O₂ = SO₂

The oxygen requirements can be easily obtained from the atomic and molecular masses of the elements indicated above:

H (1, 2) – O (16, 32) – C (12, 12) – S (32, 32),

rom which it derives, for example, that

32 kg of oxygen for 12 kg of carbon (rapport 8/3)

16 kg of oxygen for 2 kg of hydrogen (rapport 8)

32 kg of oxygen for 32 kg of sulfur (rapport 1)

therefore for a molecule of CO₂ are needed 32 kg of oxygen and 12 kg of carbon are necessary (the atomic mass of oxygen and 12 kg, the molecular mass of the molecule, consisting of two atomic masses of oxygen is 32 kg). Same for the water molecule H₂O, where a molecular mass of hydrogen H₂ binds to a molecular mass of oxygen O and therefore the stoichiometric ratio is 8 (16/2); the oxygen present is combined with the hydrogen (alloy) by subtracting the O/8 amount for which only H-O/8 actually participates in the combustion (hydrogen which remains free and therefore does not bind with oxygen). By indicating the mass percentages with C, H, S and O, the amount of stoichiometric oxygen required for combustion is analytically expressed by the following relation:

  

Considering then that the mass of air is made up of approximately 23 parts of oxygen and 77 parts of nitrogen (there is also 1% of rare gases), the stoichiometric mass necessary for the complete combustion process is given by the following relation:

  

For the combustion air it is also necessary to know the volume to determine the correct size of the pipes, the various sections of air passage in the system, to size the capacity of any fans. In this regard, the normal volume of a mole of gas measured in cubic meters is used and indicated with Nm³ (normal cubic meters), defined as the volume of a mole of gas at a temperature of 0 ° C and at a pressure of 760 mm of column of mercury equal to (1.013 bar); the normal volume of a mole is equal for all the gases and is equal to 22.40 m³(number of kg equal to the molecular mass of the substance), therefore the specific volume O of the molecular mass oxygen 32 is given by the following relation:

   

where m_o is the percentage of oxygen necessary for combustion. By replacing the previous relation to mo and considering that the parts of oxygen in the air are 21, we obtain:

 

The stoichiometric quantities determined by the above relationship would be sufficient in an ideal situation of perfect and homogeneous mixing, where each fuel molecule can come in contact and react completely with the corresponding oxidizing molecules (the oxidizing agent in the combustion, mainly oxygen) within the time available for combustion. In practice it is not possible to achieve the ideal condition described above, it is necessary to use a larger amount of air to ensure complete combustion of the fuel, in order to reduce waste, costs and reduce atmospheric pollution; the expression of the amount of real air to be used in combustion is usually expressed as follows:

or:

e represents the excess air coefficient and has a value greater than unity.

The excess of air is very variable depending on the type of fuel (solid, liquid, gaseous) and the manner in which combustion takes place in relation to the technical applications of use. In steam boilers it assumes values ​​between 1.60 to 1.80 with coal in pieces, while it goes down to 1.10 - 1.20 for liquid and gaseous fuels and for coal dust which all allow a better mixing with the combustion air (10% - 20% more than the stoichiometric values ​​calculated from the theoretical model). In gas turbines the amount of air actually engaged reaches 300% - 400% more than the theoretical air with the coefficient and which takes on values​​from 3 to 4; the excess of air is not due to combustion but to limit the maximum temperature of the cycle by diluting the combusted gases with excess air. The experimental method most used to determine the excess air present in the combustion involves the knowledge of the percentage of the carbon dioxide present in the exhaust gases measured by special analyzers. Note the percentage of carbon dioxide in combustion and the composition of the fuel it is possible to calculate the coefficient and through the following report for liquid and solid fuels:

where y is the percentage by volume of CO₂.

4 – CALCULATION OF CALORIFIC POWER

The analytical determination of the calorific value of a fuel can be carried out by adding the quantity of heat supplied by the various components with the simplified hypothesis of separate combustion of the same (Dulong's law); the formula used known as Dulong's formula is an empirical relationship for calculating the higher calorific value, which combines the main combustion reactions considering the energy emitted by each of them. For a solid or liquid fuel it is possible to write, (with C, H, O the percentages of the mass of the various components contained in a kilogram of fuel determined by the elementary chemical analysis (1 °)) are indicated:

 

or

 

The above relation, like the one below, uses the simplified logic of summing the amount of heat that develops in combustion every kg of the relative component multiplied by the percentage of mass present in a kilogram of fuel.

The lower calorific value can be written as follows:

 

or

 

The term subtractive is given by the evaporation heat of the water in kJ or kcal for the mass of water produced in the combustion increased from the humidity a present; in particular it results that 1 kg of hydrogen plus 8 kg of oxygen give 9 kg of H₂O. The determination of the calorific power with the above relationships is acceptable in approximate way, for a more precise measurement experimental methods with calorimeters devices are used; the most used are the Berthelot-Mahler calorimetric bomb for liquid fuels and the Junkers calorimeter for gaseous fuels.

(1°) Note: in analytical chemistry, the elemental analysis represents the determination of which chemical elements and in what quantities they combine to form a given material or substance. The quantity of the individual elements present is usually expressed as a percentage. One of the classic methods used for the elemental analysis of organic compounds exploits their combustion. In this way, elements such as carbon, hydrogen and nitrogen respectively form carbon dioxide, water and nitric oxide, from which it is possible to trace the original content of the chemical element sought by gravimetric analysis. Modern instruments and techniques have led to the development of automatic analyzers.

Gravimetric analysis is performed by two mainly methods,

precipitation analysis: the component is separated by transforming it into a compound not much soluble which, after appropriate treatments, is weighed;

analysis by volatilization: the component is determined by exploiting the volatility of one of its constituents (for example crystallization water in the determination of hydrated salts) or that of one of its derivatives (for example, carbon dioxide in the determination of carbonates).

By determining the loss of the weighed mass of the sample, data are obtained to ascertain the quantity or mass of the component under test (by analyzing the chemical reaction that forms the sample). This type of analysis has a margin of error of about 5%.

Below is the calorific value of wood:

The higher calorific value of the wood depends to a maximum of 15% to the species of the plant; it in the commercial energy vectors is very variable and depends on the origin of the material and the treatments subsequently suffered, therefore the values in the table are purely indicative, however it is possible to refer to the data obtained by the Oak Ridge laboratories: Lower and Higher Heating Values of Gas, Liquid and Solid Fuels Filed on 20 February 2013 in the Internet Archive.

5 - EXPERIMENTAL DETERMINATION OF CALORIFIC POWER.

Two types of calorimeters are mainly used to determine the calorific value of fuels: the Berthelot-Mahler calorimetric bomb for solid and liquid fuels and the Junkers calorimeter for gaseous fuels.

Before proceeding with the study two calorimeters it is necessary to recall the definition of calorific value of a fuel: defined as the quantity of heat developed by 1 kg of fuel whether it is in the solid state that liquid or from 1 Nm3 (normal cubic meter, defined in paragraph 3) of gaseous fuel, in the course of the whole combustion. Full combustion means that during which all the carbon, in whatever form it is found in the original fuel, is transformed into carbon dioxide, C0₂, all the hydrogen in water H₂0, all the sulfur in sulfur dioxide SO₂ and all the nitrogen in elementary nitrogen N₂.

 

Berthelot-Mahler calorimetric bomb

The calorimetric bomb, also known as the Mahler bomb, is a calorimeter with which it is possible to determine the amount of heat developed in the combustion of solid or liquid substances (combustion heat). The calorimetric bomb consists of a small, hermetically closed, strong-walled steel container in which a small porcelain capsule is placed in which the substance to be examined is placed. Inside the calorimetric bomb pure oxygen is introduced into pressure and the combustion is triggered by an electric resistance "immersed" in the substance to be examined. By passing electrical current through the electrical resistance, it becomes red hot and causes the rapid burning of the compound. The heat of the combustion reaction is absorbed by a known quantity of distilled water, in which the "bomb" is immersed.

Known, by means of calibration, the thermal capacity C of the calorimetric bomb and taking into account that the specific heat of the water is equal to 4,184 J·g-1·°C-1, applying the fundamental law of thermology (°)we can to determine the quantity of heat emitted during combustion.

(°) The fundamental law of thermology expresses the quantity of heat that must be given (or subtracted) from a body of mass m to raise or lower its temperature from the initial value t1 to the final value t2. The specific heat C_sp is an intrinsic and characteristic property of every type of substance.

This value can be calculated by applying the following formula:

in the which:

m = mass in grams of the distilled water in which the calorimetric bomb is immersed (g);

C = heat capacity of the calorimeter (J· °C^-1);

T1 = initial water temperature (°C);

T2 = final water temperature (°C);

Hs = amount of heat emitted during the combustion reaction (J).

In this way the higher calorific value is determined, ie the calorific value obtained if the water present at the reaction temperature of the combustion is in the liquid state without taking into account the quantity of heat engaged for the passage of state from liquid to gaseous, condition that in practice it does not happen because, the water present in the fuel evaporates by engaging part of the amount of heat developed during combustion, part that if subtracted from the same amount of total heat developed (higher calorific value obtained from the measurement of temperatures T₁ and T₂) allows the determination of the lower calorific value. For the determination of the latter, (lower calorific value), which occurs when the water is in the gaseous state at the reaction temperature of the combustion, it is necessary to know the quantity of water generated during combustion. The lower heating power differs from the higher calorific power precisely because it takes into account the quantity of heat (produced by combustion) engaged for the passage of liquid-to-gaseous state of the water present at the combustion reaction temperature, plus any water present in the fuel (liquid fuels). In practice, the lower calorific value is of greater interest because in most cases the flue gases leave the equipment still warm and therefore with the steamed water.

The changeover from Hs to Hi in the calorimetric bomb occurs as follows: the bomb is extracted from the casing containing the water, a bottle with inert gas is connected to the inlet valve, generally nitrogen, absolutely anhydrous (dry). We connect to the outlet valve a tube containing a strongly hygroscopic substance (for example: anhydrous lime chloride); causing the gas to flow into the bomb, it will carry with it the water vapor molecules that are suspended inside and will cause further evaporation of the other water molecules. The gas that carries the water vapor exits the outlet valve ending up in the small tube containing the hygroscopic substance that absorbs the water. In this way all the water present in the bomb evaporates and is transferred to the hygroscopic substance which increases in weight; when all the water in the bomb is withdrawn, the hygroscopic substance ends up absorbing. The increase in weight of the hygroscopic substance gives us the quantity of water that has developed during the combustion of the mass of fuel. The lower calorific value is calculated from the higher calorific value through the following formula:

 

where n = quantity of condensed water and 600 is the latent evaporation heat of 1 kg of water (kcal / kg).

 

Calibration of the calorimetric bomb and determination of the thermal capacity of the calorimeter

The thermal capacity of the calorimeter takes into account the fact that also its component parts (container, thermometer, stirrer, etc.) absorb heat.

It is therefore necessary to predetermine the thermal capacity of the calorimeter, ie to determine the amount of heat needed to raise the temperature of this system by 1 ° C.

It is possible to determine the thermal capacity of the calorimeter by having the reaction perform in the apparatus whose calorific value is known. Alternatively, the heat capacity of the calorimeter is determined by passing, through the resistance, a known quantity of electric current that is dissipated as heat inside the instrument.

The Junkers calorimeter

The Junkers calorimeter is used to determine the specific heat of a gaseous fuel. The operating principle is similar to that of a calorimetric bomb; it basically consists of a combustion chamber in which the gas is burned, contained within a container in which water flowing from the bottom upwards is introduced and which is heated by combustion. Several thermometers are placed at various heights to monitor the temperature.

A measured quantity of the gas for which the specific heat is to be determined is sent to the burner at a certain pressure which is measured by a pressure gauge. After combustion of the gas, the products deriving from combustion migrate upwards and, through pipes, fall downwards and come out of the calorimeter. The temperature at which the gases deriving from the combustion come out is recorded, which should be close to the ambient temperature; this implies that all the heat deriving from the combustion has been absorbed by the water. Any water formed by the condensation of the steam is collected in a container. In particular, the inlet temperature of the cooling water T₁ is measured and the outlet temperature of the same is measured, the temperature T₂; (to be observed that after a certain period of time, keeping the combustion unchanged, the temperature T₂ reaches the steady state value and does not change). The system in the figure is such that the combustion fumes release all the heat to the water before leaving it. In this system the water formed in the combustion condenses with the cooling of the gases coming out of the system in the condenser. The higher calorific value is defined as follows:

where:

 higher calorific value (kcal/Nm³), therefore it refers to the volumetric flow of gaseous fuel,

G_wa= flow of cooling water flowing inside the calorimeter during the measurement (m³/t) where t is the time (minutes, seconds, hours),

volumetric flow rate of the gaseous fuel (Nm³/t) where t is the time (minutes, seconds, hours),

 Specific heat of water flow (J·g-1·°C-1).

Known the amount of water that has formed in the combustion and being the higher calorific value calculated without considering the amount of heat produced by combustion and engaged for the passage of water status from liquid to steam (as if the water present at the temperature of reaction of the combustion in the liquid state without having undergone the change of liquid-gas state), in fact the measurement of the temperatures used for the calculation are a consequence of the total absorption of heat by the water, it is at this point that the power lower calorific can be calculated, value obtained from following report:

where n = quantity of condensed water, 600 is the latent evaporation heat of 1 kg of water (kcal/kg).

Note: the enthalpy possessed by a thermodynamic system (usually indicated by H) is a status function defined as the sum of the internal energy U and the product of the pressure p for the volume V:

for an isocorobaric transformation (at constant volume and pressure), the variation of enthalpy coincides both with the heat (Q) and with the variation of internal energy (ΔU) that occurred during the process. Due to the fact that it is not normally possible to know the absolute value of the internal energy of a system or a substance, during a given thermodynamic transformation only the variation of enthalpy (ΔH) and not its absolute value can be measured; for example, in determining the variation of internal energy due to temperature variation, the part not dependent on temperature and which remains constant, is zero in the difference between the two initial and final energy levels caused by the temperature variation, which could be difficult to determine if you want to calculate the initial, absolute internal energy value. It is possible to define a reference value that will identify a starting energy state or even identify it as a zero energy state and refer to the calculations at this reference level, thus calculating not the absolute value but the variation of the state size required for the project.

6 – EFFICIENCY OF A ENGINE PLANT

In order to determine the efficiency of an engine plant and therefore not of the single operating machine or of the single driving machine but of the group of operating machine plus driving machine, it should be considered that this consumes primary energy available in nature and can be in the form of energy potential and, or kinetics of fluids such as for hydraulic and wind power plants (for example, power plants with hydraulic turbines or wind turbines). In thermal engine plants, instead, chemical fuel energy is consumed (excluding nuclear plants not of interest for this research); for the overall efficiency of the engine plant, reference is made to the mechanical power P_ma used on the crankshaft and the thermal power corresponding to the fuel consumed. Measuring P_ma in kW, indicating with  the fuel flow consumed, measured in kg/s and indicating with Hi the lower calorific value measured in kJ/kg, the global efficiency  is calculated by the following relation:

By measuring the calorific value in kcal/kp and the fuel flow rate in kp/h the relationship above becomes:

where 860 [kcal / kWh] is the thermal equivalent of kWh.

The Ng yield in thermal engines can be broken down into the four-yield product:

The combustion efficiency  takes into account the following dynamics: in a motor system there is a phase of heat development and subsequent adduction to the fluid (gas or steam) that operates actively in the plant. The heat development phase is characterized by a combustion that can be internal or external to the operating fluid (internal for internal combustion engines, gas turbines, external for example for steam turbines), under no circumstances can it be complete as well as being affected by other minor losses related to heat loss to the environment outside the machine. We can therefore consider that in practice not all the thermal power H_i theoretically available is actually received from the operating fluid, but only an aliquot Q1 <H_i dynamics that allows the definition of the combustion efficiency  or more generally of the development of heat and its adduction to the operating fluid:

The limit efficiency  takes into account the following phenomena related to the cycle in which the motor fluid evolves: the fluid to which heat is transferred from the combustion evolves in the driving machine operating according to a determined thermodynamic cycle (for example Rankine or Hirn for those in steam, of Joule for gas turbines, Otto cycle for internal combustion engines), a logic that requires an ideal behavior as well as the same fluid also of the complex of machines (operating and traction) that build the engine system. This means that the evolving fluid has specific heat constant with the temperature and that the walls of the machine should be perfectly adiabatic during the compression and expansion phases and perfectly permeable to the heat (adiabatic) during the heat exchange between the fluid and the environment (and the other way around). Logic that would requires for the construction of a plant too far from the technical and practical possibilities; at to weigh is certainly the impossibility of obtaining a fluid with ideal behavior because of the two physical-chemical characteristics on which it is not possible to influence in any way: the variability of specific heat with temperature, and the variability of the molecular species of the motor fluid due to internal combustion in internal combustion engines, (the engine fluid obtained from the combustion of the fuel-air mixture). On the contrary, one could admit the ideal behavior of the engine system, assuming the limit condition in which all losses are eliminated. Under these conditions it is possible to refer to a limit cycle relative to a perfect machine in which a real fluid evolves (in which the viscosity effects are assumed null) obtaining from the hypothesis mentioned that the adiabatic compression and expansion phases, for the machine in the perfect conditions of perfect isolation, are also isentropic; the viscosity hypothesis is a consequence of the perfectly insulated machine as this, although strongly influenced by the design and the dimensions of the machine as well as by the various parts, the relative effects are null in the hypothesis of a limit cycle in which the machines are supposed to be ideal and therefore supposing the hypothesis of null losses, including those related to viscosity (considered null). By indicating with Pi the useful power obtainable from this cycle, the following ratio is defined as limit efficiency:

where  is the thermal power actually received from the fluid.

The internal efficiency  eliminates the hypothesis of a perfectly real machine and therefore the real fluid operating in it undergoes further inevitable losses; in particular, the power transferred to the mechanical parts of the machine is not that of the limit cycle P_l but a power of P_r minor; the following report defines the internal performance of a machine:

and takes into account all the losses related to the real operation of the machine, representing an index of goodness of the engine system. The product between the limit yield and the internal efficiency defines the real thermal efficiency  which, as indicated in the following report, is provided by the ratio between the power actually transferred to the mobile parts of the machine and the thermal power received by the fluid acting.

The mechanical efficiency  takes into account the frictions that are generated between all the organs equipped with relative motion and the unavoidable auxiliary mechanisms such as pumps, fans and others, necessary for the operation of the machine and driven by the crankshaft, loads that reduce the power supplied P_r on the shaft itself for the operating machines and therefore there is a mechanical power P_malower than P_r. The mechanical efficiency is defined by the following ratio:

In conclusion, the breakdown of the overall efficiency in the product of the four efficiencys defined above is as follows:

replacing it with the actual thermal efficiency, becomes:

The subdivision of the global yield from the division of losses into four categories or classes has a general validity and is absolutely rational, being exact if the heat adopted in the real cycle coincides with that relating to the limit cycle; a fact that is not always verified, just think of the gas turbines, for example, where the heat used in the actual cycle is always smaller and therefore the above formulas must be appropriately corrected.

From the expressions of the global efficiency without subdividing into the four yields, defined above, it is possible to calculate a parameter of great interest in thermal engine plants, the specific fuel consumption c_swhich represents the fuel flow rate, expressed in kg/s and in kp/h, which is consumed per unit of power, ie per kW; its expression of calculation turns out to be:

   

multiplying by the conversion factor 3600 is expressed as c_s in kg/(h kW),

   

or also

   

Heat is often used as the heat needed to produce the mechanical work unit defined by the relationship

the expression of c_s defined above is obtained

 

The knowledge of the specific consumption allows to obtain the global efficiency without having to know the lower calorific value of the fuel used. It is obvious that the specific consumption will be as low as the higher the yield and expresses a very important technical data as indicative of the degree of perfection and economy (also in terms of pollution: a engine plant, that burns less fuel for the work required certainly pollutes less) of a thermal engine system. The formulas studied here remain valid also for a reduced engine installation to a single device, the prime mover, as in the case of alternative internal combustion engines.

Sources for research: Directive 75/716/EEC, sulfur content fuels | Directiva 87/219/EEC,amending directive 75/716/EEC | Directive 93/12/ECC, sulphur content of certain liquid fuels | Directive 98/70/EC, quality petrol and disel | Directive 2009/30/EC, amending directive 98/70/EC | ISO 8754 |ISO 4259,2-2017 | Directive 94/12/EC, measures atmospheric pollution vehicles | Directive 70/220/EEC, motor vehicle type-approval | Directive 2014/77/EU, amending Annexes I and II directive 98/70/EC |DIRECTIVE (EU) 2015/1513, amending directive 98/70/EC | Clipboard, course of Machines, Federico II University of Naples, Mechanical Engineering, Professor Renato della Volpe | Renato della Volpe, Machines, Liguori Editore, Naples, 1994 | Sources: Mario Alnin, Steam Generators, Liguori Editore, Naples 1967 |