Heating Furnaces with Semi-Producer Gas

N. N. Nyurkin and I. A. Makarov

A Complete-Combustion Furnace, a Semi-Gas Furnace, and a Gas Producer

In the thermal economy of the refractory industry (in furnaces and dryers), complete-combustion furnaces, semi-gas furnaces, and gas producers are used.

In furnaces equipped with complete-combustion furnaces, the intensity of heat transfer to the material being heated depends exclusively on the difference between the temperatures of the combustion products and the material. The direct output of the furnace is close to zero. In furnaces equipped with semi-gas furnaces and gas producers, when semi-gas (or gas) is burned in the working space of the furnace, the intensity of heat transfer to the material depends on the difference between the temperature of the combustion flame and the material. Direct output is of exceptional importance. A rational arrangement of gas burners (gas inlets) in the furnace makes it possible to regulate the length of the flame and to equalize the temperature of the gaseous medium of the material being fired in the furnace.

These circumstances (direct output, temperature equalization) have led to the predominant spread of gas producers and semi-gas furnaces as compared with complete-combustion furnaces. The difference between these three methods of fuel combustion consists in the following: when fuel is burned in a complete-combustion furnace, combustion products enter the furnace from the firebox with a high heat content (physical heat), but the chemical energy of the gas is absent (the calorific value of the combustion products is zero); when fuel is gasified in a producer, producer gas enters the furnace with the lowest heat content but with the maximum calorific value; when fuel is gasified in semi-gas furnaces, gas enters the furnace which, in composition and thermal properties, occupies an intermediate position between a complete-combustion furnace and a gas producer.

Table 1 gives a comparative characteristic of the three methods of burning one and the same type of fuel in furnaces.

Table 1

When comparing the thermal properties of producer gas and semi-gas, the advantages lie with producer gas; however, in the absence of a centralized gas-producer station and other types of gaseous fuel (blast-furnace gas, coke-oven gas), the advisability of equipping furnaces with semi-gas fireboxes is unquestionable.

In the present article the thermal characteristics of semi-gas are analyzed, which can be obtained by calculation; the conclusions from the method and from the calculation results can be used in designing and establishing operating regimes for semi-gas fireboxes.

THE USUALLY APPLIED METHOD FOR CALCULATING SEMI-GAS, AND ITS CONVENTIONALITY

The usually applied method for calculating the composition and thermal properties of semi-gas is based on the following assumptions:

1) The content of \(CO\) in the semi-gas, obtained by the reaction

\[ C + \tfrac{1}{2}O_2 = CO, \tag{1} \]

is the result of combustion, with a deficiency of air, of such an amount of the carbon of the fuel as is the difference between the amount of elementary carbon in the fuel and the amount of carbon burning to \(CO_2\) and determining the high temperature of the semi-gas.

2) The constitutional moisture of the fuel (Dulong moisture), wholly or in part, depending on the kind of fuel, participates in the gas reaction proceeding to completion,

\[ H_2O + CO = CO_2 + H_2. \tag{2} \]

In this case, the carbon monoxide \(CO\) participating in the reaction is borrowed from the previously obtained \(CO\). In the semi-gas, consequently, the content of \(H_2\) increases, \(CO\) decreases, and \(CO_2\) increases.

3) Free hydrogen (elementary hydrogen of the fuel minus Dulong’s bound hydrogen) enters into the composition of the semi-gas.

4) Depending on the semi-gas temperature arbitrarily adopted in the calculation, as it leaves the firebox (usually the semi-gas temperature is taken from 900° to 1100°C), the heat content of the semi-gas (physical heat) at the adopted temperature is determined by 80% of the amount of heat released during partial combustion of the carbon of the fuel in the firebox itself according to the reaction

\[ C + O_2 = CO_2. \tag{3} \]

Having set up the equation for the heat content of the gases as a function of the amount of carbon burned to \(CO_2\) in the firebox, the amount of carbon burned to \(CO\) is determined by solving the equation, and the composition of the gas is computed algebraically.

5) The working moisture of the fuel passes into the semi-gas in the form of water vapor.

6) Hydrocarbons of the type \(CH_4\), \(C_mH_n\) are absent from the semi-gas.

This method was adopted by analogy with the algebraic method for calculating producer gas proposed by Prof. V. E. Grum-Grzhimailo.

Thus, for one and the same kind of fuel, the composition of the semi-gas, its calorific value, and its heat content (physical heat) depend on the semi-gas temperature preliminarily adopted in the calculations; obviously, the higher the adopted semi-gas temperature, the more \(CO_2\) there will be in the semi-gas, and the lower its calorific value will be (since the content of \(CO\) will correspondingly decrease as a result of the decrease in the amount of carbon burning to \(CO\) according to the reaction \(C + \tfrac{1}{2}O_2 = CO\)).

The conventionality of the calculation method for semi-gas set forth consists in the fact that: 1) the method does not reflect the dynamic equilibrium of the reaction \(CO_2 + C \rightleftarrows 2CO\) as a function of the temperature of the gasification hearth and the time of contact of the gases with the incandescent fuel bed; 2) the working moisture of the fuel, in the form of steam, supposedly does not participate in the equilibrium reaction \(CO + H_2O \rightleftarrows CO_2 + H_2\), but is mechanically transferred into the composition of the semi-gas.

As a result of this, the composition of the semi-gas usually contains less \(CO_2\) than under practical operating conditions of a semi-gas furnace; the calculated \(CO\) content is greater than the practical value; the calculated \(H_2\) content is greater than the practical value. Methane \(CH_4\) is always present in practical semi-gas, whereas according to the calculation it should not be present. The heat losses adopted in the calculation, amounting to 20% of the heat released during the combustion of \(C\) to \(CO_2\) in the furnace—apart from losses in slag, carry-over, etc.—are conventional, irrespective of the furnace design.

The indicated conventional assumptions require a very cautious evaluation of the calculated composition of semi-gas. Therefore, in designing new furnaces, the composition of semi-gas practically obtained in furnaces under experimental test conditions should be taken as the basis; however, theoretical calculations should help reveal the influence of individual factors that contribute to improving the composition of semi-gas.

COMPOSITION AND THERMAL CHARACTERISTICS OF SEMI-PRODUCER GAS FROM THE CONDITIONS OF PHYSICOCHEMICAL EQUILIBRIUM

In addition to the view presented in Section II on the question of the method for calculating semi-producer gas, it is expedient to apply the conditions of physicochemical equilibrium to the calculation of semi-producer gas. For this purpose, 5 groups of calculations of the thermal characteristics of semi-gas were carried out for fuel of one and the same composition, assuming different conditions of semi-gas formation.

The calculations were performed for fuel from the Khakass deposit of Eastern Siberia, of the following composition:

\[ \begin{aligned} C^p &= 59.2\%\\ H^p &= 4.12\%\\ O^p &= 9.21\%\\ N^p &= 1.75\%\\ S^p &= 0.75\%\\ A^p &= 14.00\%\\ W^p &= 11.00\% \end{aligned} \qquad \begin{aligned} &\text{Volatiles on combustible mass}\\ &V^2 = 41\%\\ &\text{or on working mass}\\ &V^p = 30.8\% \end{aligned} \]

\[ 100\%. \]

The initial conditions laid as the basis for calculating the thermal characteristics of semi-gas are:

For the I and II groups of calculations:

a) the moisture of the fuel and the constitutional moisture of dolong react with \(CO\), forming \(CO_2\) and \(H_2\) according to the reaction

\[ CO + H_2O \rightleftarrows CO_2 + H_2, \tag{4} \]

b) free hydrogen passes into the semi-gas,

c) the heat content of semi-gas at \(1000^\circ C\) in the first case and at \(1100^\circ\) in the second case is due to the expenditure of 70% of the heat released in the gasification zone; the remaining 30% are heat losses.

For the III group of calculations the products of dry distillation were taken into account in advance, and it was assumed that:

a) 10% of the elemental oxygen of the fuel is converted into carbon dioxide according to the reaction

\[ C + O_2 = CO_2, \tag{5} \]

b) 50% of the elemental oxygen of the fuel is converted into water vapor according to the reaction

\[ H_2 + {}^{1}/_{2}O_2 = H_2O, \tag{6} \]

and the remaining amount of oxygen (40%) participates in the formation of carbon monoxide in the products of dry distillation according to the reaction

\[ C + {}^{1}/_{2}O_2 = CO. \]

The remaining free carbon burns to \(CO_2\), the latter being reduced to \(CO\) according to the equilibrium equation

\[ CO_2 + C \rightleftarrows 2CO - 38800. \tag{7} \]

Calculating the equilibrium constant \(K = \dfrac{P_{co_2}}{P_{co}}\) for a temperature of \(1000^\circ C\) from the Boudouard equation

\[ \ln K = 21.4 - \frac{21000}{T}, \tag{8} \]

which for this temperature is \(K = 133\), we obtain the amounts of \(CO_2\) and \(CO\) at the moment of equilibrium in the gasification zone (the zone of the combustion and reduction hearth). But in the gas phase the temperature decreases from \(1000^\circ C\) to 820 as a result of evaporation of the fuel moisture and chemical reactions. At a temperature of \(820^\circ C\), a new equilibrium system is formed in the gas phase

\[ CO + H_2O \rightleftarrows CO_2 + H_2 + 10260. \]

Calculating the equilibrium constant \(K = \dfrac{P_{co}\cdot P_{h_2o}}{P_{co_2}P_{h_2}}\) for a temperature of \(820^\circ C\) from the Van’t Hoff equation

\[ \ln K = -\frac{2226}{T} - 0.0003909\,T + 2.4506, \tag{9} \]

which (constant), for this temperature, is \(K = 0.896\), we obtain the final composition of the semi-gas at the moment of equilibrium; this composition of the semi-gas is given in Table II (group III). Calculations by this method are far from exact, since the reaction rates are not taken into account.

For the IV group of calculations, the time of contact of carbon dioxide with incandescent carbon during the passage of \(CO_2\) through the fuel layer was taken into consideration. With a gas velocity of \(0.8\ \text{m/sec}\), the time for the gases to pass through the fuel layer may be taken approximately as \(1.5\ \text{sec}\).

In the zone of the combustion hearth of the furnace, the reaction takes place

\[ C + O_2 = CO_2, \]

which is practically completed instantaneously. The rate of this reaction—the disappearance of free oxygen—is \(0.0036\ \text{sec}\). The carbon dioxide obtained is reduced to \(CO\) according to the reaction

\[ C + CO_2 \rightleftarrows 2CO. \]

The rate of the reaction in the direction of the formation of \(CO\) is expressed by the equation

\[ K_1 \tau = \frac{2.3}{\frac{1+m}{2}} \lg \left( \frac{1}{m - \frac{1+m}{2}x} \right), \tag{10} \]

(see Haslam and Russell, Fuel and Its Combustion, p. 78).

Here \(\tau\) is the time of contact of \(CO_2\) with \(C\), in sec,

\(m\) is the content of \(CO_2\) in the gas at the beginning of the reduction reaction, at \(\tau = 0\),

\(x_1\) is the content of \(CO\) in the gas after the lapse of time \(\tau\),

\(K\) is the specific reaction rate.

To determine \(x\), it is necessary to know \(K\) and \(m\).

The value of \(K_1\), according to Clement, Adams, and Haskins (Haslam and Russell, Fuel and Its Combustion), for a hearth temperature of \(1200^\circ\), under the conditions of gasifying anthracite, is

\[ K_1 = 0.237; \]

obviously, the values of \(K_1\) taken for anthracite are more suitable than the values of \(K_1\) for coke. The value of \(m\) at \(\tau = 0\), i.e., at the initial moment of the carbon dioxide reduction reaction, is 1. Then, with \(\tau = 1.5\) sec, solving the equation with respect to \(x\), we obtain

\[ x = 0.30, \]

i.e., in the composition of the reaction gas, \(CO_2 = 70\%\) and \(CO = 30\%\).

The reduction of carbon dioxide is accomplished by 30%.

Taking into account the nitrogen of the air, we obtain the final composition of the gas in the combustion hearth:

\[ \begin{aligned} CO &= 7.10\% \\ CO_2 &= 16.4\% \\ N_2 &= 76.5\% \end{aligned} \]

If the products of dry distillation are taken into account by a calculation analogous to that carried out in group III, then the final composition of the gas will be expressed in volume percent as

\[ \begin{aligned} CO &= 7.13 \qquad & H_2S &= 0.12 \\ CO_2 &= 14.35 \qquad & N_2 &= 66.44 \\ H_2 &= 7.90 \qquad & & \overline{100\%}, \\ H_2O &= 4.06 \end{aligned} \]

which is also given in column IV of Table II.

However, if a heat balance of the furnace is drawn up and it is assumed (as usual) that 5% of the calorific value of the fuel is accounted for by losses from mechanical incompleteness of combustion (fall-through, raking out, in the slag), and 30% of the heat liberated in the combustion hearth by heat losses through the masonry of the furnace, then the temperature of the combustion hearth would be \(1450^\circ C\). But in the combustion hearth of a semi-gas furnace such a temperature cannot be allowed. A reduction in temperature is achieved by saturating the air blast with moisture. In semi-gas furnaces, water is usually always poured into the ashpit of the furnace for this purpose. The influence of water in the ashpit is clarified in the following fifth group of calculations.

The V group of calculations is based on the following assumptions:

1) All the provisions adopted in the IV group of calculations also apply in this group.

2) The excess heat liberated as a result of exothermic reactions is expended on the endothermic reactions of decomposition of water-

steam into the fuel bed, heated to a high temperature. Owing to this, the temperature of the combustion zone is lowered from 1450° (see group IV of the calculations) to 1200°C. By means of simple calculations, the excess heat is determined as the difference between the quantity of heat released by the reactions forming \(CO_2\), \(CO\), \(H_2O\), and the quantity of heat used to heat the gas to the specified temperature. Dividing the excess quantity of heat by the heat of the reaction of decomposition of water vapor,

\[ C + H_2O = CO + H_2 - 30173, \]

one determines the quantity of decomposed water vapor (calculated for the initially adopted unit weight of fuel, for example, per 100 kg).

3) But the decomposition of water vapor does not proceed completely; an equilibrium system is established between \(H_2O\), \(CO\), and \(H_2\), and therefore the quantity of water vapor supplied is not equal to the quantity of decomposed water vapor; the former is greater than the latter. The decomposition (reduction) of water vapor takes place 2.18 times more slowly than the reduction of carbon dioxide \(CO_2\), under the same temperature conditions and with the same contact time of \(CO_2\) and \(H_2O\) with incandescent coke. In our particular case \(CO_2\) is reduced by 30% (see group IV of the calculations), hence the decomposition of \(H_2O\) occurs in the amount

\[ \frac{30}{2{,}18}\,100 = 13.8\%. \]

The remaining quantity of water vapor

\[ 100 - 13{,}8 = 86{,}2\% \]

passes through the coke undecomposed. Knowing the absolute quantity of decomposed water vapor from the excess heat, one determines the quantity of undecomposed water vapor and, consequently, the total quantity of water vapor that must be supplied to the gasification zone in the fuel bed.

Thus, the composition of the semi-producer gas (wet) has been calculated for the gasification zone in the fuel bed. The composition of the semi-gases is given in item \(A\) of group V of Table II.

4) In item \(B\) of group V of Table II, the composition of the semi-gas is given with allowance for the products of dry distillation and the gas obtained in the combustion zone. The determination of the quantity and composition of the dry-distillation gases was indicated earlier. The calculation of the gas composition was made for wet gas.

In item \(C\) of group V, the composition of the semi-gas at a temperature in the combustion zone of 1200°C is given, calculated for dry gas.

The gas compositions under items \(B\) and \(C\) are gas compositions calculated on the basis of the conditions of physicochemical equilibrium at 1200°C. Under other temperature conditions, the compositions of the wet and dry gases will be different, but the method remains unchanged.

5) The question arises: how is water vapor to be supplied to the furnace? In semi-gas furnaces, water is poured into the ash pit, saturating the primary air.

For our case it is necessary to have air completely saturated with water vapor at 78°C. With cold primary air such saturation conditions are impossible; the saturation is considerably less, and the deficient quantity of water vapor is obtained as a result of evaporation of water in the furnace ash pit by means of the heat of the cooling fuel slag, which, as the fuel burns out, descends along the stepped grates into the furnace ash pit.

All the calculations performed for fuel of one and the same composition are given in Table II.

Table II

Thermal characteristics of semi-producer gas, calculated for various conditions of its formation.

FUEL LAYER IN THE REACTION ZONE OF THE FIREBOX

In the reaction zone of the firebox (combustion of \(C\) and reduction of \(CO_2\) and \(H_2O\)), the heat required for completion of the endothermic reactions

\[ C + CO_2 = 2CO - 38800\ \text{cal/mol} \]

and

\[ C + H_2O = CO + H_2 - 30173\ \text{cal/mol} \]

must be accumulated by the fuel layer in the course of the exothermic reaction \(C + O_2 = CO_2 + 97650\ \text{cal/mol}\).

This continuously accumulated heat is also continuously consumed. The fuel layer must have a constantly high temperature—the temperature of the reaction zone—and be sufficient to compensate for the heat of the endothermic reactions.

Obviously, the quantity of fuel heated to the temperature of the reaction zone is in a definite dependence on two factors: first, on the temperature of the zone, and second, on the composition of the gas (of course, the inverse dependence also exists).

If we denote by \(Q_1\) the heat of the reaction of reduction of \(CO_2\), calculated per 100 kg of coal being gasified; by \(Q_2\) the heat of the reaction of decomposition of \(H_2O\), calculated per 100 kg of coal being gasified; by \(G\) the weight of fuel, per

heated to the temperature of the reaction zone, per 100 kg of gasified fuel, then

\[ Q_1+Q_2=Gct, \]

\[ G=\frac{Q_1+Q_2}{Ct}\ \text{kg}/100\ \text{kg of gas. f.} \tag{11} \]

The height of the fuel layer heated to the temperature of the reaction zone depends on the intensity of gasification, i.e., on the loading of the grate surface.

If the loading of the grate surface is \(B/R\ \text{kg}/\text{m}^2\cdot\text{h}\), then, with the bulk density of the fuel \(\gamma\), the thickness of the fuel layer is expressed as

\[ H=\left(\frac{B}{R}:\gamma\right)\cdot\frac{G}{100}\ \text{m} \tag{12} \]

Let us show the application of the derived relations by a concrete example for the gas composition according to item \(B\) (or \(C\)) of group V, Table II.

\[ Q=\frac{1}{2}x_{co}\cdot Q_{co}. \]

Here \(x_{co}\) is the number of moles of \(CO\) as a result of the reaction

\[ C+CO_2=2CO, \]

\[ Q_1=\frac{1}{2}\,1.345\cdot 38790=26000\ \text{cal}/100\ \text{kg of fuel}, \]

\[ Q_2=\frac{1}{2}y_{(co+h_2)}\cdot Q_{(co+h_2)}. \]

Here \(y_{(co+h_2)}\) is the number of moles of \(CO\) and \(H_2\) as a result of the reaction \(C+H_2O=CO+H_2-30173\).

\[ Q_2=\frac{1}{2}(1.3+1.3)\,38800=39000\ \text{cal}/\text{kg of fuel}. \]

We determine \(G\)—the weight of the heated fuel in the reaction zone per 100 kg of gasified fuel:

\[ G=\frac{Q_1+Q_2}{Ct}. \]

Here \(C\) is the heat capacity of the fuel,
\(t\) is the temperature of the reaction zone.

\[ G=\frac{26000+39000}{0.25\cdot1200}=\frac{65000}{300}=217\ \text{kg}/100\ \text{kg of fuel}. \]

The obtained relation shows that the value \(G\), the amount of heated fuel, must be the greater, the greater the reduction of \(CO_2\) and \(H_2O\), and the lower the temperature of the reaction zone.

The height of the fuel layer

\[ H=\left(\frac{B}{R}:\gamma\right)\cdot\frac{G}{100}\ \text{m}; \]

for our example, if we take

\[ B/R=80\ \text{kg}/\text{m}^2\ \text{h}\quad \text{and}\quad \gamma=700\ \text{kg}/\text{m}^3, \]

then

\[ H=\frac{80}{700}\cdot\frac{217}{100}=0.25\ \text{m} \longrightarrow 250\ \text{mm}. \]

If \(B/R = 100\ \text{kg}/\text{m}^2\cdot\text{h}\) is adopted, then, with the same composition of semi-gas and the same temperature, the height of the fuel layer is expressed as

\[ H=\frac{100}{700}\cdot\frac{217}{100}=0.31 \longrightarrow 310\ \text{mm}. \]

In what follows, the determination of the structural dimensions of the semi-gas firebox by calculation (theoretical method), taking into account the process of drying and dry distillation of the fuel, will not present any particular difficulties.

Questions concerning the design of the semi-gas firebox and the methods for calculating the geometric dimensions of the firebox will be discussed in the next work.

CONCLUSIONS

1) The article presents a comparative analysis of firing furnaces with semi-gas, producer gas, and with the aid of complete-combustion fireboxes.

2) The existing algebraic (statistical) methods for calculating semi-gas are conditional; the article presents a physicochemical method for calculating the composition of semi-gas under various initial conditions.

3) The article presents a method for calculating the thickness of the fuel layer in the reaction zone of a semi-gas firebox.

The specific computational analysis performed makes it possible to draw the general conclusion that the results of the theoretical calculation are in close agreement with production-experience data.

LITERATURE

1. Handbook for Furnace Designers. Stalproekt. ONTI, NKTP. Moscow, 1935.
2. Prof. Chernobaev—Fuel, Its Combustion, and General Methods for Calculating Factory Furnaces. 1936.
3. Eng. Isserlis—Shaft Furnaces. ONTI, NKTP. Kiev, 1936.
4. Haslam and Russell—Fuel and Its Combustion. ONTI, Moscow, 1934.