Investigation of carbon black emissions in tractor diesel powered by biofuels

Full Text

Introduction. Currently, diesels, due to their somewhat better power and economic criteria in comparison with gasoline engines, are widely used in railway, marine, agricultural and motor transport industries. However, due to the significant complication of human interaction with the environment, unforeseen changes occur in ecological systems caused by pollution of the air basin, water and forest resources. Therefore, at present, diesels, especially according to the criteria of exhaust gas toxicity, cease to meet modern requirements [1-4]. To a greater extent, this applies, of course, to their soot content in the exhaust gas. In addition, soot formed during fuel combustion is one of the most toxic components of exhaust gas from tractor diesels [5-9]. Soot particles released into the environment from exhaust gas adsorb carcinogenic polycyclic hydrocarbons on their surface, including extremely dangerous benz(a)pyrene, which can cause tumors and neoplasms in living organisms. Well, everyone knows for sure that soot particles certainly cause chronic respiratory diseases. Therefore, the problem of reducing the soot content in the exhaust gas of tractor diesels was and, unfortunately, still exists today, and requires a solution [10-15].

In energy combustion, from an economic and environmental point of view, soot is an undesirable product. Traditionally, there has been great interest in the issues of soot formation in the field of technological combustion, where soot is the target product of the process. Despite the different target approach in technological and energy combustion, the physical and chemical bases of the process are largely identical. Therefore, all the extensive experience in studying the process of soot formation accumulated in various fields of combustion science can form a unified and reliable theoretical basis for considering this complex multistage process in relation to combustion conditions in tractor diesels running on new (alternative) fuels [16-23].

A significant number of papers have been devoted to the study of the physico-chemical properties of soot [24-29]. Due to the use of modern research methods such as high-resolution electron microscopy, X-ray and spectroscopy, gas chromatography, contact and optical research methods, a fairly complete understanding of the size, structure and physico-chemical properties of soot has been obtained over the past decades [24-29].

The purpose of the work - development of a modern model of soot emission in a tractor diesel powered by ethanol and rapeseed oil (RO) to assess the smokiness of exhaust gases and reduce anthropogenic impact on natural ecosystems.

Methods and means of conducting research. To simulate the processes of formation and burning of soot particles in a tractor diesel engine, the volume of the combustion chamber (CC) was conditionally divided into three zones (the zone of "clean" air not involved in combustion; the zone of thermal decomposition of fuel, in which the processes of formation of soot particles, their growth and coagulation occur, and the zone of combustion products, in which processes of oxidation and gasification of soot particles).

Geometrically, the clean air zone occupies the peripheral volumes of the CC, in which pyrolysis and combustion processes do not occur. The zone of thermal decomposition of fuel and soot formation coincides with the inner area of the fuel flare. The burnout zone of soot particles is located on the surface of the burning steam mantle of the torch. Each zone is characterized by a homogeneous composition and temperature field throughout the volume. A similar technique for modeling soot formation in a diesel cylinder when running on diesel fuel is presented in the works of Baturin, Loskutov, Kozhin [30].

When a tractor diesel engine is running on ethanol and RO, hydrocarbons and gas from the surrounding volume enter the thermal decomposition zone during the evaporation of the fuel. We assume that as the flame front develops, some hydrocarbons burn out without going through the stage of deep pyrolysis, then the rate of change in the concentration of the components of the gas mixture in the reaction zone can be expressed by the equation:

                                    (1)

where dМ_i/dφ – the total rate of change in the concentration of the i-th component of the gas mixture; (dМ_i/dφ)_σ – the rate of change in the concentration of the i-th component associated with the supply of evaporated fuel; (dМ_i/dφ)_k – the rate of change in the concentration of the i-th component due to pre-flame chemical transformations of the fuel; (dМ_i/dφ)_χ – the rate of change in the concentration of the i-th component during the main combustion; φ – the angle of rotation of the crankshaft (RС) of the diesel engine.

The rates of chemical transformations of the reagents of a gas mixture are determined by the equation:

                                                    (2)

where V_p – the volume of the reaction zone; W_j – the rate of the j-th chemical reaction of the kinetic mechanism, n – the rotation speed of the diesel crankshaft, min^-1.

Decarboxylation is the initial stage of the thermal decomposition of RO fatty acid molecules and occurs at lower temperatures, therefore, the rate of the gross reaction of the decomposition of the RO molecule in the zone of high-temperature pyrolysis is limited by the rate of decomposition of the formed olefin.

We will estimate the flame temperature based on the balance of enthalpy of gases in the zone of combustion products. The composition of gases in the combustion zone characterizes the local coefficient of excess air α_Г, which is determined by the expression [25]:

                                                                 (3)

where  ξ_В – the degree of effective use of the air charge, χ - integral heat dissipation function.

The function ξ_В(φ) in accordance with the work [25] is satisfactorily approximated by the dependence

                                            (4)

where  С_В and h – proportionality coefficients;  - the ratio of the current angle of RC to the actual duration of combustion.

The evaporation of each drop of fuel obeys Sreznevsky's law:

                                                       (5)

where d₀, d_k - accordingly, the initial and current diameter of the drop; K - evaporation constant; τ_и - the time from the beginning of evaporation of this drop (the moment it enters the considered zone) to the current moment.

The evaporation function of a drop of mass m from the angle of RC on the basis of Sreznevsky's law will take the form:

                                                   (6)

where m – drop weight; φ_i – the current angle of the RC from the moment the drop hits the CC.

If the fuel supply law is set by an array, then it is convenient to determine the amount of fuel in the cylinder by the equation of the sum of the non-evaporated parts received at different times according to the fuel supply law.

The number of moles of the i-th dM_i substance removed from the pyrolysis zone depends on its volume concentration and is proportional to the proportion of burning fuel, mol/deg RC:

                                                   (7)

where  σ_и – the integral law of fuel evaporation in CC.

Fuel hydrocarbon molecules enter the reaction zone in proportion to the evaporation rate, which break down into smaller molecules and radicals. The final stage of the chain of reactions of pyrolysis of hydrocarbons is the explosive decomposition of acetylene with the formation of chemical soot nuclei. Consequently, the rate of formation of soot nuclei is proportional to the rate of decomposition of acetylene, which is determined by the presented equations of chemical kinetics. The composition of the products in the reaction zone depends on the evaporation rate of the fuel and the local coefficient of excess air.

Let's denote the number of soot particles in the reaction zone N₁ in the burnout zone N₂. Taking into account the rate of decomposition of hydrocarbons and oxidation of acetylene molecules, as well as the removal of components from the reaction zone, which is assumed to be proportional to the rate of heat release, it is possible to determine the number of moles of С₂ that have passed into the solid phase by heterogeneous reactions of soot particle growth. Neglecting the insignificant content of hydrogen atoms and other elements in the soot particle, assuming a constant particle density of 1.9 g/cm³, it is possible to derive an equation relating the number of moles С₂ z_ч in a particle with a diameter of D_C:

                                                   (8)

where  z_ч – mole C₂ in a particle of soot; N_A - Avogadro number.

In order to determine the average diameter of the soot particle, the total number of moles With С₂ z₁ should be divided by the number of particles in the reaction zone N₁. Then the rate of change of the average diameter of the soot particle in the formation zone, taking into account equation (8), is determined by the expression:

                                 (9)

where  D₁ – average mass diameter of particles in the soot formation zone, nm;

In the area of soot formation, partial gasification of carbon atoms occurs by heterogeneous oxidation reactions. The mass burnout rate of soot particles is proportional to the product of the carbon flux on the total surface of the particles in a unit volume. Therefore, the rate of carbon gasification depends on the particle diameter. According to the theory of carbon black gasification presented above, the oxidizing agents in these reactions are mainly carbon dioxide, water and oxygen. The carbon flux from these reactions is summed up. Similarly, the rate of heterogeneous growth of soot particles is proportional to the surface of the particles, therefore, it depends on the diameter.

Particles enter the burnout zone by passing through the flame front without gasification. In the burnout zone, soot particles continue to interact with the oxidizer, mixing with combustion products. The change in particle diameter in the burnout zone is determined by a combination of three processes: burnout, mass removal from the formation zone and coagulation process.

The number of soot particles in the burnout zone, taking into account Brownian coagulation, can be determined by the equation [25]:

                  (10)

where  P₀ – the steric factor; T₂- temperature in the burnout zone.

The first term in equation (10) takes into account the supply of soot particles from the formation zone. As a first approximation, the current volume of the cylinder can be taken as V₂. In a more precise interpretation, V₂ is the volume of CS engulfed in flames. Taking the input of soot particles with an initial diameter of Dн in the amount of Nн in proportion to the evaporation rate of the fuel, the differential equation of the number of particles in the formation zone can be written:

                                   (11)

where  dN_O/dφ – the rate of formation of soot particle nuclei.

The rate of gasification of solid carbon in the soot burnout zone is determined by the equation:

                                                    (12)

where J_c – total carbon flux, g/(m² s); F_y – specific surface area of soot particles, g/m².

The value of J_c is calculated using expressions for all reactions of gasification of soot particles with free air oxygen, carbon dioxide and water vapor remaining from fuel combustion. The composition of the combustion products and the amount of carbon flux are calculated according to separate routines.

The initial number of moles of C₂ is determined in proportion to the proportion of air in the reaction zone and is calculated by the expression:

                                                             (13)

The current mass of soot in the cylinder can be determined by multiplying the number of moles of C₂ in each zone by the molar mass of the growth particle:

                                                           (14)

The number of soot particles in the cylinder was also calculated by algebraic addition of the particles in each zone. The mass concentration of soot in the cylinder was determined as the ratio of the mass of soot to the current volume of the cylinder. The initial soot content in the cylinder is taken in proportion to the volume of residual gases and the concentration of soot measured at the outlet.

Результаты исследований и их обсуждение. Debugging of the mathematical model was carried out according to experimental data on the exhaust smoke of tractor diesel [7, 21, 25, 28] when working on a diesel engine at various load and speed modes. The initial data for the calculation were the experimentally obtained indicator pressure in the cylinder, the average temperature of gases, the rate of heat release (Fig. 1), the rate of evaporation of fuels (Fig. 2).

 

а

 

b

 

c

 

d

 - diesel fuel;  - ethanol and rapeseed oil

Fig. 1. Indicators of the combustion process of a 2F 10.5/12.0 tractor diesel engine at the nominal operating mode: a – indicator pressure; b – average temperature of gases in the cylinder; c – active heat release; d – heat release rate.

 

 - evaporation of diesel fuel;

 - evaporation of rapeseed oil;

 - evaporation of ethanol;

 - heat dissipation diesel fuel;

 - heat dissipation ethanol and rapeseed oil

Fig. 2. Characteristics of fuel evaporation and heat release in the combustion chamber of tractor diesel.

 

The optimal setting angles of fuel injection advance (SAIA) were determined experimentally and amounted to Θ_RO=Θ_e=34^о RC to UDC for RO and ethanol. Cyclic fuel supplies were equal for ethanol to q_e = 52 mg/cycle, for ignition RM to rm =13 mg/cycle [7, 21]. The SAIA of diesel fuel was 30 degrees centigrade to UDC. The cyclic supply of diesel fuel was 42 mg/cycle.

The ignition delay period increased during the operation of a tractor diesel powered by biofuels. This happens for three main reasons: a reduced cetane number, a drop in the evaporation rate of the ignition fuel in the CC due to a deterioration in the mixing process, and a decrease in the temperature of gases in the cylinder during alcohol evaporation. This effect is compensated by the high rate of ethanol combustion and the active supply of heat to the working fluid after UDC. Since the cyclic supply of ethanol at the nominal mode is 20% greater than the cyclic supply of DF and the beginning of the supply of ethanol is shifted by 4 degrees RC from UDC, the volume of the ethanol torch increases by the time combustion begins. The heat of combustion of ethyl alcohol and RM is lower than DF. This leads to a decrease in the calculated temperature of the gases in the combustion zone (Fig. 3).

 

 - in the burnout zone (diesel fuel);

 - in the burnout zone (ethanol and rapeseed oil);

 - in the pyrolysis zone (diesel fuel);

 - in the pyrolysis zone (ethanol and rapeseed oil);

Fig. 3. Determining the temperatures of the processes of carbon black formation and burnout

 

After the initiation of heat release, the removal of soot particles from the formation zone begins and their mixing with particles located in the burnout zone. The mass of the incoming particles is distributed to all particles. Before the start of combustion, the soot particles in the zones did not undergo a change in diameter. With the appearance of a large number of germs, the average diameter of soot particles decreases rapidly (Fig. 4). The maximum number of particles when running on ethanol and RO diesel is 2.5 times higher than when running on diesel fuel. This is caused by an increase in the ignition delay period and the accumulation of a large amount of hydrocarbons in the reaction zone, as well as a low rate of particle coagulation in the reaction zone of ethanol decomposition, which has already been noted above. The movement of the extremes of the functions of the number of particles and the average mass diameter to the late angles of the RC is caused by a displacement of the combustion process.

 

а

 

b

 

c

 

d

 - diesel fuel;  - ethanol and rapeseed oil

Fig. 4. Calculated indicators of soot content in the cylinder of a tractor diesel engine during operation: a – the number of carbon black particles, pcs.; b – the current average mass diameter of carbon black particles in the cylinder, nm; c – the mass concentration of carbon black in the cylinder, g/m3; d – the mass content of carbon black in the cylinder, mg

 

By the time the exhaust valve is opened at φ=140^о RC the mass content of soot in the diesel cylinder stabilizes, and the soot gasification rate gradually decreases to minimum values.

According to the results of the calculation of the soot formation process, we see that the concentration of soot in the cylinder decreases at all angles of the RC. When the diesel engine is running on ethanol and RO in nominal mode, the maximum calculated concentration of soot in the cylinder is 1.93 g/m³ at φ=15,6^о RC. At φ=140^о RC the concentration of soot in the cylinder it drops to 0.091 g/m³. The mass content of soot reaches its maximum value at 26.1 degrees celsius and is 0.216 mg. At the moment of opening the exhaust valve, the soot mass drops to 0.092 mg. When working on DF, the calculated maximum soot content is 0.825 mg at φ=19,2^о RC. The mass of soot when opening the exhaust valve is 0.279 mg, which is three times more than when working on biofuels.

 

Conclusions. Based on the developed mathematical model, the concentration of soot and the main components of the gas mixture in the reaction zone of the CC and the content of soot in the exhaust gas were calculated at various speed and load modes of operation of a tractor diesel powered by ethanol and RO. Valuable information has been obtained on the dynamics of the main stages of soot formation and burnout in the cylinder during operation of a tractor diesel engine on ethanol and RO, showing a decrease of 3.4-3.8 times compared with operation on regular diesel fuel.

About the authors

Vitaly A. Likhanov

Vyatka State Agrotechnological University

Email: lihanov.va@mail.ru
ORCID iD: 0000-0003-3033-7176
SPIN-code: 9474-7629
Scopus Author ID: 57197821797

Professor, Dr. Sci. (Tech.), Head of the Heat Engines, Automobiles and Tractors Department

Russian Federation, 133 Oktyabrsky avenue, 610017 Kirov

Oleg P. Lopatin

Vyatka State Agrotechnological University

Author for correspondence.
Email: nirs_vsaa@mail.ru
ORCID iD: 0000-0002-0806-6878
SPIN-code: 8716-0189
Scopus Author ID: 57197821205
ResearcherId: ААД-8374-2019

Dr. Sci. (Tech.), Professor of the Heat Engines, Automobiles and Tractors Department

Russian Federation, 133 Oktyabrsky avenue, 610017 Kirov

References

Supplementary files