Thermochemical processing of organic waste

The article considers the problems of creation of new types of energy based on the processing of organic waste and methods for their transformation into gaseous, liquid and solid fuels. The technological scheme of complex processing of organic waste into liquid and solid fuels is presented. We consider the unit for organic waste processing with the possibility of generating not only electrical energy, but also a valuable product, namely an adsorbent.


Introduction
Currently, the problem of creating new types of fuels based on the processing of organic waste is of immediate interest. The research of complex processing of organic waste into liquid and solid fuels is being conducted at KNRTU [5]. Fig. 1 shows a flowchart of complex processing of organic waste into liquid and solid fuels, in which solid fuels are converted into activated carbon and generator gas of higher quality for supply into the internal combustion engine [1]. Activated carbon, electrical energy, and bio-oil obtained by condensation of the resulting vapor-gas mixture are the end products of the presented scheme of complex processing of organic waste.
The thermochemical processing of organic waste is of the highest scientific interest among all the fuel production stages. Thermochemical processing involves a series of interrelated processes of heat and mass transfer, complicated by chemical transformations, consisting of many parallel reactions in Despite the large number of scientific works, there are no uniform methods for calculating technological processes and devices for continuous pyrogenic processing of organic waste due to the increased complexity of the processes in the field of thermal decomposition of organic matter.

Materials and methods
Studies were carried out using the unit for organic waste processing ( Figure 2) [3]. The unit consists of a vertical retort 1 in which organic waste is transported due to gravitational forces through the zones of heating, drying, pyrolysis, activation, cooling with their transformation into 3 activated carbon, further through the system of the resulting gases separation, the burning device, the gas cleaning system, recuperative heat exchangers. Cooling of the finished product occurs in 2 steps. In the zone of the first stage of cooling 4, where in the lower part, at a height of 15-20% of the total coal, there is a collector 19, through which water is supplied to cool the coal to a temperature of 90-100 °C, and the upper volume of coal, 80-85 %, cooled by the formed water vapor, which, in turn, overheats to a temperature of 800 °C. Next, the humidified and cooled activated carbon is directed to the accumulation zone 5. When the coal is filled in the accumulation zone 5, the vacuum valve 8 is opened and the contents are dropped into the zone of the second cooling stage 6. Then the vacuum valve 8 is closed and the vacuum pump 20 is turned on. When the residual pressure reaches 3-6 kPa in the second stage of cooling 6, the cooled and dried activated carbon is moved by opening the vacuum valve 9 to the unit of unloading of activated carbon 7 and transferred to the capping by a belt conveyor. In the activation zone, the interaction of water vapor with coal residue results in the generation of generator gas, which is converted into electrical energy depending on the type of waste being processed, using an internal combustion engine (ICE) or fuel cells (FC). Liquid fuel is released from pyrolysis gas -bio-oil and non-condensing combustible gases, which are burned in the furnace of the pyrolysis zone [4,5].

The process modeling
The temperature field in the technological zones of the unit is determined by the heat transfer equation [6,7], which describes the change of material temperature in time along the layers. For a one-dimensional layer, it can be written as: The initial temperature in the layer Tini is determined by the final material temperature after the previous zone, the current temperature at the boundary of the material layer is determined by the temperature of the heat transferring surfaces.
If we consider biomass as organic waste, then at a temperature of more than 180 °C, its thermochemical decomposition begins. This is accompanied by the formation of coal, steam-gas mixture and a loss of biomass mass.
The change in mass per unit volume for each of the listed components can be written as follows [8,9]: is the rate constant of chemical decomposition of biomass, s -1 [11,12].
The degree of thermal modification is determined by the formula The change in mass of the steam-gas phase of the pyrolysis zone is determined by the sum of mass flows due to convection and the reactions of thermal decomposition of biomass: where  is the particle porosity, which is determined from, m 3  To determine the velocity of the gas flow we used Darcy's law: where sg  is the gas dynamic viscosity, Pa·s; p K is the gas permeability of the particle, which is calculated according to the formula: The heat outflow during the thermal decomposition of biomass affects the temperature change of the layer. When calculating, we assume that the temperature of the biomass particles does not deviate from the temperature of the resulting pyrogas, and the gas and solid phases are in thermodynamic equilibrium. The temperature of the particle is determined from the equation of energy conservation for the particle: where p  is the particle heat transfer coefficient, W/(m·K) ; 0 q is the specific heat of the chemical reaction, J/kg; sg w is the speed of steam-gas mixture ,m/s; is the current coordinate in the particle, m.
The rate constant of a chemical reaction is determined in accordance with the Arrhenius law: biomass is the pre-exponential factor, s -1 ; Еa is the activation energy, J/mole [13,14]. Initial conditions for solution of the equations (2÷5) and (7) will have the following form: ini Т  and the boundary conditions for expression (7) will be written as: The efficient thermal conductivity coefficient in equation (1) depends on the porosity and temperature of the layer , its value is determined experimentally [15].
The coefficient of thermal conductivity for a particle in equation (7) is defined as the sum of the thermal conductivities of biomass, coal and volatile substances, taking into account the degree of pyrolysis and the heat emission through the pores:

Results and discussions
During the experiments, the kinetic dependence of the change in the initial mass and slurry output was obtained for the following types of biomass: apricot kernels, sunflower husks, walnut shells, wood flour, sawdust and process chips. The average yield of products by weight was 36.6%, in the slurry it was 45.8%.  Deviations of calculated data from experimental data for organic waste in the form of biomass does not exceed 18%.

Conclusions
A technology has been developed for the processing of organic waste into valuable products: electrical energy, liquid fuels, activated carbon. Mathematical modeling of the process of thermochemical processing of organic waste allows one to determine the yield of liquid and solid fuels, to calculate the specific dimensions of the zone of thermal decomposition in a thermal decomposition unit. As a result of modeling, the kinetic dependences of the change in the initial mass of the processed raw material, the output of coal and liquid fuel were obtained.