Available energy with steam

To evaluate the transport of available energy with steam, the dead state values of enthalpy and entropy for H2O must be determined (note that H2O is liquid at Tg and pg) ... 

Table 14.1. The net calorific value of wood depends on its moisture content here the values are for a typical hardwood. The reduction in available energy with moisture content is due to the need to vapourize and superheat the steam to the same temperature as the flue gases. Typically the net calorific value for a hardwood is 18.2 MJ kg that for a softwood is 19.2 MJ kg (due to its higher lignin content) and that for bark is 19.7 MJ kg (due to extractives).

An inerease in ambient air temperature will deerease the available energy for the generator. This assumes that the fresh feed and eoke burn remains eonstant. The expander horsepower does not ehange, but the air blower horsepower inereases with inereased air temperature, eausing the exeess energy to deerease. Steam and water may need to be added to the flue gas flow at various points in the system to eontrol afterburning. In Figure 4-64, the solid eurves are for a normal flow of steam. The dotted eurves are for inereases in the steam rate by 3.05 times, 4.85 times, and 6.05 times the normal flowrate. 

The operational test of the lube system is, as the name implies, a functional test to check as many of the features as practical under running conditions. The first and last step is a demonstration of the cleanliness of the system. This is followed by a running test of a four-hour duration. The test should simulate the field operation with the compressor in every way practical. All equipment to be furnished with the lube system should be used in the test, including the standby pump start and trip switches. All other instruments should be used to demonstrate their operation. Prior to starting the four-hour run, the system should be thoroughly inspected for leaks and the leaks corrected. If no steam is available for a steam turbine (if one is used), the four-hour run can be made on the electric pump. However, every effort should be made to use an alternate source of energy such as compressed air, to operate the steam turbine. 

Industrial Uses. Large industrial facilities, particularly those using cyclone boilers or fluidized-bed boilers, are potential markets. In addition, several vendors of small- and medium-sized industrial energy and steam facilities are marketing units capable of using TDF. As the availability of TDF expands with new producers entering the market, it is hoped that the industrial use of TDF will also expand (7). 

A simple thermodynamic analysis provides considerably more data to work with. The required task is to separate propylene from propane. On a theoretical basis the ideal work (the minimum availability change) required for this separation is about 400 k BTU s/hr, of which an appreciable fraction is needed to raise the temperature of the products to the final values shown. The available energy (availability, exergy) supplied to this process from the condensing low pressure (20 psig) steam is 18.6 M BTU s/hr. 

The available energy flou through five major sections of sulphuric acid plant is given in figure 2. The major inputs to this system are sulphur and pouer, with demineralised (DM) water uet air, process water and cooling water from environment. The useful outputs from the system are sulphuric acid and steam. Losses to environment include heat losses from various equipments blowdown water steam from deaerator vent warm water and stack gas. 

The production of synthesis gas is an expensive process irrespective of whether one starts with methane or coal. The chemical reason is that the partial oxidation of coal or methane with steam to form CO and H2 is endothermic, and a high energy input is therefore needed. This energy comes from burning additional CH or coal. A prerequisite for an economic process is therefore that CH or coal must be available at low cost. 

To calculate the consumptions of available energy in the combustion process and the heat transfer process, it is supposed that the boiler may be separated into two distinct entities (Figures 4 and 5). The transports of available energy into the combustion process with air, steam, feedwater and stack gases have already been determined. Assuming that the products of combustion have the same composition and total pressure as the stack gases, the... 

The error in energy analyses is that they attribute all the inefficiencies to losses, and then mis-calculate those. As was demonstrated for the coal-fired boiler, the first law efficiency (85%) was a poor approximation to the true efficience (33.8%). (Furthermore, perturbation studies show that the trends in first and second law efficiencies can move in opposite directions. For the coal-fired boiler problem, if the steam conditions were changed to 811°K/6.87 MPa (1000°F/1000 psia) and if the first law efficiency were decreased to 83%, the result would be an increase to 34.3% in second law efficiency.) The major inefficiencies were due to heat transfer (njj = 47.3%) and combustion (Hjj = 73.8%), with the stack losses accounting for only 5.5% of the available energy input with the coal. In contrast, an energy analysis shows the combustion process to be 100% efficient and... 

Costing on the Basis of Available Energy This same procedure will now be followed, but with available energy to evaluate the power flows for the case in which the co-generating power plant exhausts 50 psia steam. For steam produced at 650 psia and superheated to 750°F, the available energy per pound is (in this work,... 

The method used in the previous example, called the extraction method, assumes that the sole purpose of the turbine is to produce shaft power. Therefore, the shaft work is charged for the capital cost of the turbine and for the steam available energy used by the turbine to produce the work. With this rationale, the additional equation is obtained by equating the unit costs of high- and low-pressure steam available-energy, cLp = cHp. The result is that the shaft work bears the entire burden of the costs associated with the turbine process and capital expense. 

Figure 3. Skeleton schematic of 1980 system available-energy flows (megawatts). The unit costs of steam and electricity were calculated with the equality method and for a capital charge of 20.5(106) at an effective after-tax interest rate of 8.5% and an economic life of 20 years.

The cost of process steam is then obtained from a money balance on the turbine. Nevertheless, the commodity of value is available energy—any method which assigns costs on any other basis such as energy or mass is usually invalid. Furthermore, only with available-energy costing can co-generating power plants be analyzed by other methods—equality, extraction, by-product steam— discussed in the preceding paper (2). 

The economic analysis to follow depends upon the evaluation of the various available-energy supplies for feedwater heating and, in turn, the costs associated with those supplies. In particular, the costs of interest, for each case, are those required to take the feedwater from the conditions at the inlet to heater number 4 to the normal temperature of feedwater entering the boiler. These costs include the cost of bleeder steam, which is used to increase the temperature of feedwater in the heater and, under the conditions of Case C, the cost of the additional boiler fuel required when the heater is out of service and the temperature of the feedwater is below normal. The hourly cost of feedwater heating for Cases A and B is given by... 

Af a(j,j is the additional boiler fuel required when the heater is ou of service, cB is the unit cost of steam in the i - 1 bleed line, and cp is the unit cost of boiler fuel (coal). These available-energy flows are calculated in the usual manner and are given in Table II and reference (6). The unit costs are obtained with exactly the same techniques discussed by Reistad and Gaggioli (2)—by applying money balances to each component (boiler, high-pressure turbine,. . . ) of the system of interest to get the dollar flows and then by the subsequent division of the appropriate available-energy flows to obtain the unit costs. 

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