Equilibrium Models of the Gasification Process of Solid Low-reactive Fuel in The Steam-oxygen-air Flow

The goal of this work is to improve the efficiency of the solid low-reactive fuels use in thermal power plants. To achieve the goal we set the following tasks: to develop an equilibrium mathematical model that allows studying the gasification process of solid fuel in the steam-oxygen-air flow; analyze the influence of various mode factors on the composition of produced gas and the main indicators of the technological gasification process of coal particles in the flow, to determine the optimal values of mode factors.

Recently, the development of industry and agriculture has led to a gradual deterioration of the ecological situation.It should be noted that during the last decades there is a trend of the gradual deterioration of the quality of solid fuels, supplied to thermal power plants.This leads to decreased efficiency of boilers and premature wear and failure of boiler equipment.Fuel combustion of the worsened quality in thermal power stations, working with traditional technologies, increases the negative impact of energy companies on the environment.In this regard, the main problem of the energy coal use in the beginning of XXI century is the development of new environmentally friendly technologies for the production of electrical and thermal energy.
One of the directions of the efficient use of low-grade coal is application of gasification technologies.In the 7 work, a method for the gasification of solid fuels in the upward oxidant flow is examined.It should be noted that the method is still little studied, mathematical models for it are not sufficiently developed, characteristics of the processes, occurring in the gasifier, are not defined.The problem of studying the processes occurring during coal gasification is a subject of many studies [2][3][4][5][6][7][8][9][10][11][12][13][14] ; however, the existing models and methods do not allow calculating the basic parameters during the fuel gasification in an upward flow.The currently used methods of calculation, due to the considerable complexity of the gasification process and the lack of sufficient data on the kinetics of the main reactions are of empirical character, as some indicators are selected on the basis of practical data 1,9 .This work is dedicated to the generation of such models.

Basic principles of constructing an equilibrium model
When coal is fed to the gasification chamber, complex processes of heat and mass transfer occur, accompanied by thermochemical transformations involving condensed and gas phases.However, several problems can be solved on the basis of relatively simple models.This is due to intensive mass exchange processes in the volume of the gasifier, and high rates of the chemical reactions at temperatures of ~1000°C.
If during the operation of the gasifier, the time of chemical transformations of fuel components (at all their stages) to the moment of the almost complete utilization of oxygen, contained in the blast, is less than the time of coal particles remaining in the reaction volume (i.e., if the oxygen content in the flow of the reacting mixture at the outlet of the gasifier is close to zero), then the integral material and heat balances of the system are practically independent from the characteristics of individual process steps.In this case, the final composition of the fuel gasification products for all components can be found by using the quasi-static model in the assumption of equilibrium thermodynamic states of the initial components and products of the process.
Since the gasification efficiency (the ratio of the heat of gasification products combustion to the calorific value of coal) is one of the factors affecting the efficiency of the process, it seems feasible to identify the main technological parameters which have the most significant effect on it, on the basis of the mathematical model of coal gasification.
The consumption of coal gasifier is taken as a determining parameter that is why it is convenient to refer the consumption of main components (oxygen, air, steam and gasification products) to 1 kg of fuel as received, this allows achieving a known generalization of the results.
During the model development, the following assumptions were made: -all the oxygen, supplied to the gasifier, is used for the interaction of fuel with carbon to form CO and CO 2 ; -chemical composition of the formed generator gas at the outlet of the gasifier corresponds to the equilibrium composition, which is determined by the reaction CO+H 2 O=CO 2 +H 2 , for a given temperature; -coal gasification in the gas generator are schematically seen as a process occurring at three stages.First stage dry breakdown stage of coal.At this stage, drying of fuel and devolatilization of volatile components occur.The composition of fuel as received (working mass) :   Consider the mode in which water steam and oxygen are fed to the reaction volume in the amount bigger than the one necessary for stoichiometric carbon gasification, but smaller than the one required for its stoichiometric combustion, i.e. 0.5 ≤ α + β ≤ 1.0.Here, a and b are the delivery coefficients of oxygen and steam to the gasification zone, respectively.These coefficients represent the ratio of oxidant, fed to react with fuel carbon to the amount of oxidant, required theoretically (based on the stoichiometric ratios) for complete fuel oxidation by the reactions C+O 2 =CO 2 and C+2H 2 O=CO+2H 2 respectively.
Main reaction equations in this case are as follows: m 1 ×C + 0.   During the calculations we take into account that the dry blast may be supplied to the gasifier with varying degrees of oxygen enrichment, which are determined as the ratio of oxygen, supplied by the air and oxygen blast to the total volume of oxygen-air blast ... where H steam -enthalpy of the steam supplied with the blast, kJ/kg; Q gg -sensible heat of the generator gas, Q gg =H gas Q q3 -chemically bound heat of the generator gas; Q sl -sensible heat of slag: ...( 51) where m sl -slag yield in the gasifier, kg/kg; ñ slslag heat capacity, kJ/(kg×°C); t sl -slag temperature, °C.Q cool -heat loss to the environment, ...(52) where q o , q i -are the densities of the heat flow on the outer and inner cylindrical surfaces of the gasifier annular channel, kJ/(m 2 ×s), F o , F i -areas of these surfaces, m 2 , B -fuel consumption on the gasifier, kg/s.
Analysis of the impact of various factors on the main parameters of the gasification process Based on the above formulas, a programme was drawn up, with which the calculations were carried out on a computer.Fig. 1-3 show the dependences of the composition of gasification products and main process energy characteristics and the consumption of oxygen, water steam and oxygen-enrichment degree of the blast.
The analysis of the graphic dependences (curves) showed (Fig. 1) that with the change in a from 0.4 to 0.9 (β=0.1;X=60 %, C r n =52 %), a decrease in the volume content of ÑÎ occurs from 54 to 14.2 %, H 2 from 21.3 to 3.2 %, CH 4 from 3.32 to 3.28 % with the H 2 S content almost constant -0.52 %.Moreover, due to the increased air supply, there is a decrease in the content of N 2 from 15.6 to 34.4 % and ÑÎ 2 from 5.3 to 44.3 % in the gasification process.Energy characteristics are linear, depending on a.With the growth of a, Q R heat generation in the gasifier significantly increases from 4200 to 15980 kJ/kg.The H GAS enthalpy of the gasification products also increases from 2690 to 4360 kJ/kg.However, the Q SN calorific value decreases from 9100 to 2140 kJ/m 3 and the h gasification efficiency from 0.78 to 0.19.With the change in the b consumption of water steam in the blast from 0 to 0.5 (α=0.5;X=60 %, C r n =52 %) (Fig. 2) the content of CO decreases from 56.2 to 33.7 %, content of N 2 from 20.8 to 17.8 %, CH 4 from 3.5 to 3.0 % and H 2 S from 0.6 to 0.5 % in the gasification products.At the same time, the content of Í 2 increases from 11.9 äî 24.5 % and the content of CO 2 from 7.1 to 20.4 %.With the growth of β, the H GAS enthalpy of gasification products increases from 2430 to 5530 kJ/kg and the Q R heat generation in the gasifier increases insignificantly from 6420 to 6750 kJ/kg.
At the same time, the Q SN calorific value decreases from 8360 to 6900 kJ/m 3 , and the h gasification efficiency decreases insignificantly from 0.67 to 0.65.Thus, changes in the oxygen supply have more significant effect on the change in the gas composition and energy characteristics of the process, compared with the change of the water steam supply at constant values of all other factors (e.g., with the change of a, the content of ÑÎ changes by 3.8 times, H 2 -6.6 times, Q R -3.8 times, Q SN -4.3 times and h -4.1 times, at the same time, with the change in b, the same characteristic change by â 1.7; 2; 1.05; 1.2 and 1.03 times respectively. With the fixed values of the oxidant delivery coefficients into the melt (α=0.5;β=0.1;C r n =52 %) (Fig. 3) with the increase in the α oxygenenrichment degree of the blast from 30 to 100 %, the content of almost all the gas components increases by around 2 times: CO from 32.8 to 56.7 %, H 2 from 10.8 to 21.9 %, CO 2 from 7.7 to 16.4 %, CH 4 from 2.3 to 4.1 % and H 2 S from 0.4 to 0.7 %.At the same time, the content of B N 2 decreases significantly -from 46.1 to 0.3 % in the gasification products.With the growth of X, the Q SN gas calorific value increases from 5300 to 9510 kJ/m 3 , melt temperature), the H GAS enthalpy of gasification products decreases from 5120 to 2210 kJ/kg, but the Q R heat generation in the gasifier and the h efficiency of gasification almost don't change (6520 kJ/kg and 0.67 respectively).
Calculations by the programme allowed determining that the optimum mode with the pure oxygen blast (X=100 %) is the mode characterized by the following parameters: α=0.36; β=0.14, at the same time, the obtained gasification efficiency is maximum η=0.83.The maximum calorific value of gases Q SN =12670 kJ/m 3 is obtained when α=0.5, β=0 and the oxygen enrichment degree of the blast being equal to X = 100 %.

CONCLUSION
We proposed an equilibrium balance mathematical model of the solid low-reactive fuel gasification in the steam-oxygen-air flow.The model allows calculating the volume and composition (CO, CO 2 , H 2 , N 2 and H 2 O) of the formed fuel gasification products per 1 kg of the working mass of fuel, as well as heat generation in a gasifier, enthalpy and calorific value of the generator gases, and the gasification efficiency.The proposed model can become the basis for the methodology development of engineering calculations for coal gasification units in the flow.
We analyzed the impact of various mode factors on the main indicators of coal gasification in the flow.It was found that the change in oxygen consumption has more significant impact on the composition of gases and energy characteristics of the process in comparison with the changes in other factors.We determined the optimal values of operating factors, in which the level of gasification efficiency and the combustion heat of generator model, we take into account that at this stage some fuel components: C r , H r , S r and O r enter into a reaction with each other and form volatile components 2 + 2H 2 = CH 4 ; H 2 + S = H 2 S; Ñ + O 2 = CO 2 ...(1) At the same time, it is necessary to determine what part of C r is used for these reactions (i.e. the part of C r v volatile carbon).At this stage, the products of fuel dry breakdown form per 1 kg of the working mass of coal. 1) Fuel slag with the mass of m sl = m A = A r /100 kg/ kg. 2) Gaseous products, m 3 /kg: -amount of hydrogen sulfide ...(2) When determining the volume of gases, we take into account that a part of fuel hydrogen goes to the formation of hydrogen sulfide by the reaction of H 2 + S = H 2 S ...(3) and the remainder of the hydrogen mass goes for the the formation of methane.Then, the volume of methane, which is formed by the C + 2×H 2 = CH 4 reaction, will amount to ...(4) Part of the carbon (in % of the fuel working mass) goes for the CO 2 formation by the C + O 2 = CO 2 reaction..(5)

Fig. 2 .
Fig. 2. The dependence of the composition of gases (a), the main energy characteristics (b) and the efficiency of gasification (c) on the water steam consumption b (α=0.5;X=60 %, C r H =52 %).

Fig. 3 .
Fig. 3. Dependence of the composition of gases (a) and the main energy characteristics (b) on the oxygen enrichment degree of the blast X (α=0.5;β=0.1;C r n =52 %).
of steam, kg/kg ...(20) (21) Here, we take the delivery coefficient of oxygen and atmospheric air is expressed through α A , and the delivery coefficient of oxygen with the pure oxygen blastà through α O .Thus, α = α A + α O From the (16) equation we can also determine the volumes of gasification products, m 3 /kg: volume of carbon monoxide ...(22) steam ...(26) and the total volume of dry gases ...(27) Then the total volume of gases (including water steam) will be ...(28) Let us write the composition of the gasification products (in volume percent); ...(29) ...(30) ...(31) ...(32) Lower calorific value of dry gases, kJ/m 3 ...(33) Given this, the lower calorific value of dry gases, kJ/kg, will amount to ...(34) The enthalpy of gasification products, kJ/ kg, is determined from the expression , of carbon dioxide and water steam respectively.Determine the heat generation in the gasifier, kJ/kg ...(36) and the efficiency of gasification ...(37) To carry out the calculations it is necessary to determine the Dm value (from the equilibrium condition of the generator gases composition at the outlet of the gasification zone itself).To do this, we write down the expression for the equilibrium constant of the water steam reaction (13): ...(38) where P i and V i -partial pressures and volumes of the i-th gases involved in the reaction respectively.Then we can write ...(39) where one can determine the value of Dm.Third stage -reacting of the product mixture of the first and second stages in a gas atmosphere.At this stage, the first stage products are fed with the volumes: and the second stage products with the volumes: .The equilibrium composition of the generator gas is determined by the equilibrium constant of the reaction CO + H 2 O = H 2 + CO 2 +27700 kJ/kmol of CO at the outlet of the gasifier at a temperature.The formed mixture of gases at the outlet of the generator includes CO, H 2 , H 2 S, CH 4 , CO 2 , H 2 and N 2 .Having considered the main equations and calculated the volumes of reaction products at each of the three stages of the coal gasification process, we can determine the total volume of gases, exiting the gasifier, m³/kg -volume of carbon monoxide ...(40) the volume of gases due to their reaction in the mixture at the third stage.The equation of heat balance with the stationary operation mode of the gasifier is as follows, kJ/kg: ...(47) where Q fuel -chemically bound heat and sensible heat of fuel: ...(48) where r i Q -lower calorific value of fuel, kJ/kg; t f -fuel heat capacity, kJ/(kg×°C); t f -fuel temperature, °C; Q blast -sensible heat of the oxygen-air blast: ...(49) Here, V A and V O -are the volumes of air and oxygen in the blast process, m 3 /kg; c A and c Oare the heat capacities of air and technological oxygen in the supplied blast, kJ/(m 3 ×°C); t A and t O -are the temperatures of air and oxygen, °C; Q steam -sensible heat of the water steam, supplied with the blast, ...(50)