Standalone Boiler Houses for Shaving Peak Heat Loads

I. N. Butakov

We have already shown that the inclusion of standalone boiler houses in a power system, at a given heat load, increases the specific consumption of heat in kilocalories and, consequently, the specific fuel consumption per calorie, regardless of its purpose¹. Nevertheless, including such boiler houses for shaving peaks of the heat load is advisable. In this way it is possible to raise substantially the utilization factor of turbine extractions, loading them more and more fully as the design CHPP temperature \(t_{\text{CHPP}}^{p}\) rises, corresponding to a higher outside-air temperature than the minimum design outside temperature (according to Chaplin),

\[ t_{n}^{p}=0.4\,t_{n}^{\text{sr.k}}+0.6\,t_{n}^{\min}. \]

Figure 1 presents an annual duration curve of the heat load, referring to a certain particular case.

In the absence of peak boiler houses, when heat consumers at an outside-air temperature of \(-30^\circ\text{C}\) were supplied with the heat of steam only from turbine extractions, the utilization factor of the latter was equal to

\[ \frac{\text{pl. }1-3-4-5000-0-1}{\text{pl. }1-5-5000-0-1}, \]

i.e., it proved to be rather low. Increasing this factor would make it possible to raise the delivery of heat from turbine extractions and, consequently, the generation of kWh in heat consumption, correspondingly displacing a quantity of less advantageous condensing kWh in the power system, thereby creating an overall saving of fuel and hence a monetary saving.

Using a simplified formula to determine the quantity of heat required for heating buildings, we obtain, for the design outside-air temperature \(t_{n}^{p}\),

\[ Q_{\max}=xV(t_{vn}-t_{n}^{p})\ \frac{\text{kcal}}{\text{h}}, \]

and for the design CHPP temperature \(t_{\text{CHPP}}^{p}\),

\[ Q_{\text{CHPP}}=x\cdot V(t_{vn}-t_{\text{CHPP}}^{p})\ \frac{\text{kcal}}{\text{h}}, \]

so that

\[ \frac{Q_{\max}}{Q_{\text{CHPP}}} = \frac{t_{vn}-t_{n}^{p}}{t_{vn}-t_{\text{CHPP}}^{p}}. \]

Thus, by assigning different values of \(t_{\text{CHPP}}^{p}\), we shall obtain in Fig. 1 different values of \(Q_{\text{CHPP}}\), as the areas \(1-2'-3'-4'-5000-0-1\), \(1-2''-4''-5000-0-1\), etc., with increasing utilization factors—

of the extractions, such as

\[ \frac{\text{pl. }1—2'—3'—4'—5000—0—1} {\text{pl. }1—5—5000—0—1}, \]

as well as

\[ \frac{\text{pl. }1—2''—4''—5000—[[unclear: digit]]—1} {\text{pl. }1—5—5000—0—1}, \]

and so on, bearing in mind that an increase in \(t^{p}_{\text{TPP}}\) will also cause an increase in the ratios \(\dfrac{Q_{\max}}{Q_{\text{TPP}}}\), i.e., a corresponding rise along the ordinate axis (Fig. 1) of the amount of maximum heating heat \(Q^{i}_{\max}\) to 125, 170, 250 million kcal/hour, depending on the selected value of \(t^{p}_{\text{TPP}}\).

Fig. 1.

The above-mentioned saving of standard fuel and monetary saving when increasing the utilization coefficient of the extractions may be determined in the form

\[ E = \Э^{i}_{m}(b_{k}-b_{m})\cdot \frac{R_{t}}{1000} = \Э^{i}_{m} \left( \frac{860}{7000\cdot \eta_{k}} - \frac{860}{7000\cdot \eta_{m}} \right) \cdot \frac{R_{t}}{1000} = \]

\[ = 0{,}123\cdot \Э^{i}_{m} \left( \frac{1}{\eta_{k}} - \frac{1}{\eta_{m}} \right) \cdot \frac{R_{t}}{1000} \ \frac{\text{rub.}}{\text{year}}, \tag{1} \]

where the generation in kWh for the heat demand is \(E_m^i = \dfrac{Q_{\mathrm{TPP}}^i}{q}\), with

\[ q = \frac{860}{\eta_{\mathrm{oe}} H} (i_1 - \eta_{\mathrm{oi}} H - t_{\mathrm{cond}}) \frac{\mathrm{kcal}}{\mathrm{kWh}}, \]

\(b_k\) and \(b_t\) are the specific consumptions of standard fuel, kg/kWh, in condensing and cogeneration generation of electric power, and \(R_t\) is the cost of a ton of standard fuel in rubles.

Using the annual heat-load duration curve (Fig. 1), for various values of the calculated TPP temperature \(t_{\mathrm{TPP}}^p\) we can determine \(Q_{\mathrm{TPP}}^i\), and therefore \(E_m^i\) for the specified value of \(q\), after which we can easily calculate the amount of saving \(E\). For the particular case of the curve (Fig. 1), the different values of the saving \(E\) are plotted in Fig. 2 (curve No. 1) for various values of the calculated TPP temperature \(t_{\mathrm{TPP}}^p\), with \(R_t = 100\) rubles/t, \(\eta_k = 0.25\), \(\eta_t = 0.8\), \(q = 2600\) kcal/kWh. The fuel consumption by peak boiler houses should not be taken into account, since this consumption would occur under all conditions, of course in larger amounts, because space heating would be supplied from individual or central boiler houses, or even from room stoves.

Fig. 2.

Thus, the inclusion of peak boiler houses, while increasing the specific fuel consumption if the latter’s consumption in them were taken into account, at the same time creates a reduction in the total fuel consumption both as a result of displacing condensing kWh of the power system and as a result of displacing individual and central boiler houses and stoves.

But the monetary saving obtained by formula (1) as a result of including peak boiler houses in the power system is not a net saving, because the increase in the utilization factor of turbine extractions is obtained at the cost of constructing these boiler houses, which require additional capital investments. The capitalization factor (depreciation, repairs) is determined by the formula

\[ R_{\mathrm{pk.\,boil.}}^i = 0.1 \cdot S_{\mathrm{pk.\,boil.}} \left( Q_{\max}^i - Q_{\mathrm{TPP},\max} \right), \]

where \(S_{\mathrm{pk.\,boil.}}\) is the specific cost of peak boiler houses, rubles per Mcal/h, which for the particular example under consideration (Fig. 1) may be taken as

\[ S_{\mathrm{pk.\,boil.}} = 60000 \text{ rubles}, \]

and \(Q_{\mathrm{TPP},\max}\) is the maximum heat value, kcal/h, from

extractions from turbines, for example, for the particular case of the graph in Fig. 1, \(Q_{\mathrm{CHP},\max}=100\) million kcal/hour.

The value of this capital-investment factor must, of course, be calculated on the basis of the maximum heating surface of the peak boilers, according to the maximum heat flow in kcal/hour \((Q^i_{\max}-Q_{\mathrm{CHP},\max})\), and therefore separately for each value of the design temperature \(t^p_{\mathrm{CHP}}\) of the CHP plant.

In addition, as the utilization factor of turbine extractions and of the heating surfaces of the peak boiler houses increases, the heat network will expand, owing to the need to cover an ever larger number of consumers in order to use the additional amount of heat being created. This entails the inevitable lengthening of heat networks and an increase in their diameters, which, of course, will make the networks more expensive. The capital-investment factor for networks will be expressed as:

\[ R^k_{\mathrm{net}} = 0.1 \cdot S_{\mathrm{net}} \cdot Q^i_{\max}\ \text{rubles/year}, \]

where \(S_{\mathrm{net}}\) is the specific cost of heat networks per million kcal/hour of maximum heat consumption, which may be taken as \(S_{\mathrm{net}}=100000\) rubles. Adding \(R^k_{\mathrm{peak.boil}}\) and \(R^k_{\mathrm{net}}\) for the same values of \(t^p_{\mathrm{CHP}}\), we obtain, on the scale of curve No. 1, curve No. 2 in Fig. 2 of the increasing annual capital costs associated with the inclusion of peak boiler houses in the power system. Further subtracting these costs, plotted as ordinates (Fig. 2) for each value of \(t^p_{\mathrm{CHP}}\), from the corresponding ordinates of curve No. 1, we obtain the maximum savings for the particular example under consideration at the temperature \(t^p_{\mathrm{CHP}}=-10^\circ\mathrm{C}\), which is also the optimal design temperature of the CHP plant.

Summarizing all that has been said, it may be asserted that the principal significance of including peak boiler houses in the power system comes down to increasing the utilization factors of turbine extractions, creating an increase in the generation of \(Э^i_t\) kWh for heat consumption and displacing the less advantageous condensing kWh of the power system.

1. Izvestiya TPI, vol. 66, no. 2, 1948. ↩