PROSPECT OF USING THE PYROLYSIS PROCESS TO PRODUCE HIGH-QUALITY LIQUID FUELS

However, the formation of oxygen-containing compounds with traces of the formation of hydrocarbon compounds with the presence of nitrogen or other elements in the structure is not excluded. It is evident that with a change in the pyrolysis temperature from 600 °C to 800 °C, the yield of the gasoline fraction in the composition of a wide fraction of hydrocarbons (from the potential possible) changes from 15.09 % to 6.37 % and the content of the diesel fraction also decreases. We call this process of pyrolysis conditionally pyrolysis without access of air or oxygen to the reaction zone, although in the reactor working capacity of 6 liters the free space from loading of raw materials of pyrolysis - geometrically fractions of cotton stems was occupied by air, which in terms of this air volume - 4 liters to oxygen is - 1 liter or 0.2 g-mol. It should be noted that although in the tables for drawing up the balance the yield of pyrolysis gas is presented in mass fractions. This mass value is calculated, and during the experiment the yield of gaseous products of pyrolysis are determined by volume.

In order to find the optimal process parameters to ensure the maximum yield of light hydrocarbon fractions, studies and experiments were continued under various conditions of the pyrolysis process. It is known that one of the most widely used in chemical, petrochemical and biochemical technology is pyrolysis with partial oxidation. In this case, in a pyrolytic reactor, oxygen supplied to the zone of thermal destruction of high-molecular compounds partially oxidizes some compounds. In this case, the so-called oxidative destruction occurs with the release of a certain amount of heat due to oxidative processes, which is advisable to ensure the maximum degree of destruction of high-molecular complex structures, because when using, therefore, pyrolytic processes often turn to oxidative pyrolysis. In our case, we limited ourselves to connecting the air flow to the reaction zone at a rate of 10 l / 10 min in order to prevent complete oxidation of the pyrolysis raw materials. However, we were unable to determine the amount of gas formations, since at the outlet of the gas mixtures we had a mixture of air with pyrolysis gas. Therefore, we limited ourselves to presenting them with the formation by summing up the gas formations with losses. However, it should be noted that the resulting gas mixture burned with a bright blue flame on the torch. As can be seen from Tables 1 and 2, the results of oxidative pyrolysis allow changing the boiling range of the liquid fraction. It is evident that the beginning of boiling of liquid fractions at a process temperature of 475 °C in the case of using oxidative pyrolysis increases by several points, and the end of boiling decreases by 12 points, against during pyrolysis without oxygen access to the reaction zone.

Changes in the boiling range of the liquid fraction are also observed when carrying out pyrolytic destruction at 800 °C. If in the case of conventional pyrolysis the onset of boiling of the liquid fraction is noted at 36 °C, then in the case of oxidative pyrolysis, the temperature of the onset of boiling of the liquid product rose by 3 points higher and had a value of 49 °C, and at the same time the value of the end of boiling of the pyrocondensate also rose from 316 °C to 348 °C, which we explain by the formation of oxygen-containing hydrocarbons with significantly increased boiling point values. Changes in the yields of liquid fractions and solid residue are also observed. The indicators of the head part of liquid hydrocarbons also changed. It is noteworthy that, in this case, a significant increase in the yield of the light fraction of the pyrocondensate is observed. However, at high temperatures, the yield of the solid part also increases and is 29% versus 26% during pyrolysis without combining partial oxidation processes. This phenomenon is explained by an increase in the slow cooking process versus the thermal destruction process. The possibility of the influence of a certain part of the high-molecular hydrocarbon part on the process cannot be ruled out.

The difference in the reduction of the yield of solid residue in contrast to the simple slow pyrolysis process and pyrolysis with partial oxidation is that, apparently, in the previous cases, the slow pyrolysis process first proceeds with the formation of higher molecular weight hydrocarbons and then thermal dehydrogenation, ensuring the formation of pyrocarbon.

According to optimistic forecasts, hydrocarbons of biological origin will become the only source of energy and energy carriers for humanity, replacing all others. Therefore, at the initial stage of using pyrocarbon of biological origin, we considered it expedient to use hydrocarbons of biological origin in a certain ratio in the process of obtaining solid carbon-hydrocarbon fuel.

In world practice, numerous technologies of physical and thermochemical conversion of biomass for the purpose of obtaining energy or valuable secondary energy sources and products (syngas, biogas; synthetic liquid fuels - biodiesel, bioethanol; solid biochars, sorbents and carbon nanomaterials) have been mastered or are at the stage of research and demonstration projects [1].

Of particular interest is the development of innovative technologies for the thermochemical conversion of plant biomass and carbon-containing waste to obtain high-quality secondary fuels of gaseous, liquid and solid consistency, with low ash content and high calorific value and competitive medium-calorific and high-calorific fuel.

The use of the pyrolysis process, which allows, depending on the conditions, to obtain high-quality liquid fuel, is promising.

The latter could be effectively used for co-combustion with local low-grade brown coals, for example, in fluidized bed furnaces. Existing biomass pyrolysis plants are characterized by low productivity and efficiency. In addition to direct use as boiler and household fuel, wood fuel pellets (WFP) and wood fuel briquettes (WFB) are also of interest as a semi-finished raw material for further thermochemical conversion to produce generator gas and charcoal.

Various carbon-containing wastes are increasingly used as alternative renewable raw materials for pyrolysis and gasification processes: the organic part of municipal solid waste (MSW), solid sewage sludge (SSW), hydrolytic lignin (HL), etc. This type of raw material is characterized by "negative cost", since it requires considerable costs for management and disposal. The production of secondary biofuel from it simultaneously solves this problem.

Optimization of existing and development of new methods and devices for thermochemical processing of wood and waste require a deep understanding of the mechanisms and patterns of conversion processes and their relationship with operating conditions and the composition of raw materials.

Let us dwell on two types of thermochemical conversion. Torrefaction (roasting) of wood fuel is a process of its soft pyrolysis at a relatively low temperature (200–350°C), the purpose of which is to increase the calorific value by removing moisture and decomposing highly reactive components of organic matter, mainly hemicellulose, which have a relatively low energy value [2, 3]. Technologically, the torrefaction process can be carried out.

•  in a windproof stationary layer, for example, on a hot tray;
•  in an infiltrated moving layer, for example, on a conveyor belt, in the environment of “juice” water vapor released from the processed raw material at the preliminary stage of drying or in a screw conveyor;
• in a fluidized bed using water vapor (including "juice"), combustion products of part of the original biomass or gaseous products of its torrefaction as a fluidizing agent.

The organization of the circulation of the steam-gas mixture in a closed circuit between the zones of drying, pyrolysis and cooling of raw materials in combination with the regeneration of the combustion heat of pyrolytic gases allows the implementation of energy-saving environmentally friendly torrefaction schemes. The main product is solid torrefied wood ("gray" biocoal) - hydrophobic, resistant to decay, highly reactive semi-coke with a moisture content of 0.1-5%, ash content of ~1%, a relatively low content of fixed carbon of 35-40% and a combustion heat of up to 21 MJ/kg. A promising gaseous torrefaction agent is carbon dioxide CO₂, the use of which in a closed cycle with the utilization of excess amounts allows reducing emissions of this greenhouse gas into the atmosphere.

Unlike torrefied wood, which contains a significant amount of residual volatile mass elements (hydrogen, oxygen), biochar consists mainly of solid fixed carbon. Biomass pyrolysis occurs at higher temperatures (350–650 °C) and is a process of thermal decomposition of organic matter without oxygen access to form condensable (resins, H₂O) and non-condensable gases (H₂, CO, CH₄, CO₂), as well as a solid product – biochar (in particular, charcoal), the ratio between which is a function of temperature, pressure and raw material heating rate.

During charcoal production, slow pyrolysis of large pieces and trunks of wood is carried out at heating rates of 5–7 K/min. The duration of the charcoal combustion cycle ranges from 8–12 hours in small mobile furnaces with a retort volume of 4–10 m³ to 7–14 days in concrete stationary furnaces with a working volume of hundreds of cubic meters [3, 4]. The mass yield of coal is 20–33%. In the world, similar equipment produces from 26 to 100 million tons of charcoal per year with a growth trend of about 3% per year. The maximum yield of biochar, close to thermodynamic equilibrium, with a significantly shorter process duration was obtained by the method of "rapid carbonization" at elevated pressure (up to 0.5–1.5 MPa) at the University of Hawaii (USA) [4, 5].

At the same time, the role of increasing pressure remains unclear, since theoretically its effect under these parameters should not be significant.

CONCLUSION

This paper presents the results of calculations of the equilibrium composition of torrefaction and pyrolysis products of absolutely dry (ADW) and wet wood, wood fuel pellets and carbonaceous municipal solid waste depending on the main process conditions - temperature (T = 200-650 °C), pressure (p = 0.02-1.2 MPa) and the ratio of mass flow rates of the gas conversion agent (water vapor) and biomass. The calculations were performed using the NASA CEA program by the Gibbs energy minimization method for an equilibrium mixture of gas and condensed components [6]. The calculated raw materials are coniferous wood and wood fuel pellets, municipal solid waste and granules based on them, solid sewage sludge and hydrolytic lignin, the composition and effective molecular formulas of the combustible mass of which are given in Table 1 (low nitrogen and sulfur contents were not taken into account in the calculations). The moisture content of wood and waste varied within the range of 0–60%, with the exception of wood pellets and household waste, the moisture content of which was taken as constant and equal to 8%.

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