Fired duty

Efficiency. Since only 35 to 50% of fired duty is absorbed in the radiant section, the flue gas leaving the radiant chamber contains considerable energy that can be extracted efficiently in the convection section of the furnace. In the convection section, the feed is preheated along with dilution steam to the desired crossover temperature. Residual heat is recovered by generating steam. The overall thermal efficiency of modem furnaces exceeds 93%, and a value of 95% is not uncommon. 

By using more of the high temperature heat available in the flue gas and reaction products, the fired duty required can be decreased by a significant fraction as compared with the more conventional arrangements. Such concepts are not necessarily new but they have become more interesting as the cost of fuel and feedstock have escalated. 

Fired duty can be reduced by either reduction of dilution steam or combustion air preheating or to a lesser degree by reduction of excess air to the burners. The selection of the optimum steam dilution was discussed previously. 

Air Preheating. The majority of the gas burners installed in cracking furnaces are natural draft burners. As combustion air is introduced at ambient conditions, a part of the heat released from the fuel has to be used to heat up the combustion air to the fire box temperature. Therefore, preheated air reduces the fired duty. Figure 6 shows the fuel consumption as a function of combustion air temperature. For example, preheating combustion air from 0° to 100 °C reduces the fired duty from 100 to 95.5%, which means savings of 4.5% fuel. 

As seen in Table 1, the required heat per carbon atom is less for normal heptane than for methane. It means that the fired duty in the tubular reformer will be slightly less when operating at similar conditions on naphtha instead of natural gas. The higher hydrocarbons are also more reactive than methane with aromatics showing the lowest reactivity approaching that of methane. 

Fig. 8 shows a typical installation of a prereformer, The exit gas from the prereformer can be further preheated to up to 700°C with no risk of pyrolysis or other undesirable reactions such as methane decomposition. In this way, it is possible to replace part of the fired duty of the reformer furnace by external preheating, thus reducing the size of a tubular reformer. The advantages are illustrated in Table 4 comparing advanced reforming of natural gas with the state of the art in the 80 ties for CO-plant reformers. 

Load Power at 610 psig at 150 psig at 60 psig at 3 psig Fired Duty... 

Energy savings—the maximal scope for energy savings in absorbed duty could be 47 (81.8 — 34.8) MMBtu/h or fired duty of 55 MMBtu/h assuming a heater efficiency of 85%. 

The modifications 1 and 2 reduce heater absorbed duty by 33.3MMBtu/h (39.2 fired duty), while modification 3 generates MP steam by 16.2MMBtu/h. The total energy savings is 55.4MMBtu/h. The overall payback for the above modifications is less than 2 years. The composite curves for the retrofit case are shown in Figure 10.18. 

The HTCR reactor consists of a number of bayonet reformer tubes and combines basically the radiant section and the convection section of a conventional HSR in a single piece of equipment. The reaction heat is provided by the flue gas fiowing on the outside of the reformer tubes and by reformed gas fiowing in an upward direction in the bayonet tubes. This results that is about 80% of the fired duty is utilized in the process, and steam export is minimized. 

Convective reformers result in less waste heat. The flue gas as well as the product gas is cooled by heat exchange with the process gas flowing through the catalyst beds, so that they leave the reformer at about 600°C. The amount of waste heat is reduced from 50% in the conventional design to about 20% of the fired duty in the heat exchange reformer. This means that the steam generated from the remaining waste heat just matches the steam needed for the process, so that export of steam can be eliminated. 

This approach allows the relationship between firing duties and catalyst temperatures, if available, (derived from [DMC] controller steady-state gains) to be used, with all of the intercepts being updated as parameters, determined primarily by the measured catalyst temperatures. ITie catalyst temperatures are matched precisely at the solution of each Reconcile case. 

A minimum export steam plant is defined as a plant that optimizes heat recovery in the plant to the maximum extent possible. This is often done when the value of steam is essentially zero or the price of the feedstock and fuel are exceptionally high. Minimum steam export is often achieved by first increasing the SMR process gas inlet temperature and then the combustion air preheat temperature, both of whieh reduce the fired duty of the SMR. Typical temperatures are 1150°F for the process gas and 900°F for the air preheat. These changes reduce the export steam to a low level, but typically not completely to zero. Additional modifications are required to reduce the export steam to an absolute minimum. 

The required heat input, the reformer duty, Q, is the enthalpy difference between the exit and the inlet gas, and it can easily be calculated from enthalpy tables. The duty consists of heat of reaction as well as the heat required to raise the temperature to the level of the reformer exit. In a typical tubular reformer ftirnace, about 50% of the heat produced by combustion in the burners is transferred through the reformer tube walls and absorbed by the process (in an ammonia plant 60% for reaction, 40% for temperature increase). The other half of the fired duty is available in the hot flue gas and recovered in the waste heat section of the reformer for preheat duties and for steam production. In this way, the overall thermal efficiency of the reformer approaches 95%. 

While initially the fired duty reductions appear quite small (0.5-1.4%), this may lead to significant energy savings in fuel costs for the fired heater. Vinayagam [51] states that even a 1% reduction in fuel consumption can provide significant... 

Table 5.20 Key model yields for fired duty case study.

Scenario Total Fired Duty (kj/kg) Aromatic Yield (wt.%) C5+ RON C5-I-Yield (wt.%) Fired Duty Deviation... 

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