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Jun-2013

Selecting the “Best” fired heater

Fired heaters are extensively used in the chemical process industry to heat process fluids and generate steam. Typically, fired heaters consume the largest amount of energy in the process industry.

Jared Remster, David Ma, Omer Hashmi, Goutam Shahani
Linde Engineering North America
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Article Summary
In North America, while natural gas is currently relatively cheap; energy efficiency still has to be carefully evaluated along with both capital and operating costs. Furthermore, available fuel, utilities and steam have to be considered at a given manufacturing complex to develop the best possible long term solution. During the conceptual design phase, detailed examination of various configurations can be very valuable in exploiting opportunities for process optimisation. This will ultimately lead to a simpler and cheaper solution for the refining and petrochemical producer. This paper will outline some important design and cost considerations in selecting the ‘best’ fired heater.
 
Introduction
In North America, vast quantities of ‘shale gas’ have recently become economic to recover due to advances in horizontal drilling and well fracturing or ‘fracking’. Shale gas has become an attractive fuel and chemical feed stock, which has been widely publicised by the media. Currently, the price of natural gas is depressed due to supply exceeding demand. The advent of inexpensive shale gas has been referred to as a potential ‘game-changer’, which could make North America energy independent and revitalise manufacturing particularly the refining and petrochemical sectors.

Fired heaters, which are extensively used in the chemical process industry to heat process fluids and generate steam, consume a large amount of energy. While natural gas is currently relatively cheap; energy efficiency still has to be carefully evaluated along with both capital and operating costs. This paper will outline some important design and cost considerations in selecting the ‘best’ fired heater.
 
Shale Gas
Currently, the price of natural gas is low by recent historical standards. However, this price is determined by a large number of macroeconomic factors including: supply, demand, imports, exports and the price of other fuels such as crude oil.  In addition, climatic factors and the overall level of economic activity have an impact on natural gas prices. Given the large number of independent variables, it is difficult to make precise forecasts about the future price of natural gas. In their annual outlook, the U.S. Energy Administration presents a number of scenarios that are depicted in Figure 1. The two main variables considered are the overall level of economic activity and the estimated ultimate recovery (EUR) per well. It can be seen that there is a fair degree of uncertainty about the long term price of natural gas. However, it is highly unlikely that the ‘real’ price of natural 
gas will approach the peaks experienced in 
the 2008 time frame. Furthermore, there will likely be significant volatility in future natural gas prices, which is consistent with historical behaviour. According to the EIA, the average wholesale price of natural gas fell over 30% in 2012. With so much uncertainty in future pricing, it is important to make long term investment decisions based on the long term outlook for natural gas pricing and not on the current market conditions which can be short-lived.

Fired Heaters
Fired heaters are extensively used in the chemical process industry to heat process fluids and generate steam. Typically, fired heaters consume the largest amount of energy in the process industry.  These pieces of equipment generally last for several decades. As a result, the fired heater has to be carefully selected taking both capital and operating costs into account. Furthermore, available fuel, utilities and steam have to be carefully considered at a given manufacturing complex to develop the best possible long term solution. A simplified example to illustrate these important considerations is presented below.

For the purposes of this analysis a box type heater with vertical tubes was chosen to heat a hydrocarbon fluid. In a typical floor-fired box type heater, the burners are located in parallel rows in the radiant section and the flue gas, which is produced as a result of the combustion, flows upwards through the convection section and then to the stack. Additional process details are presented in Table 1.

Three different possible designs to achieve the required process fluid duty are shown in 
Figure 2.

Case I: Radiant Coil
Case I is the most elementary design needed to achieve the desired process duty. This design has a fixed heating surface area in the radiant section. The process fluid enters the tubes in the radiant section where it undergoes radiant heat transfer and then exits the radiant section at the process outlet. All heat transfer takes place in the radiant section of the heater. The overall thermal efficiency of this heater arrangement is 57.4%.

Case II: Radiant & Convection Coil
Case II entails the addition of a coil section in the convection section of the heater in case I, which provides valuable heat transfer surface area. In this case, the process fluid first enters the convection section and is preheated by the hot flue gas flowing upwards. The process fluid then enters the radiant section tubes where it is heated further by radiant heat transfer before finally exiting at the process outlet of the radiant section. In this arrangement, waste heat recovery is achieved by extracting sensible heat from the hot flue gas. As a result, the hot flue gas is cooled and the process fluid is preheated before entering the radiant coils. The preheated process fluid reduces the required firing duty, which in turn reduces the required fuel usage. The overall thermal efficiency of this fired heater arrangement is 83.8%.

Case III: Radiant & Convection Coil plus Air
Preheater (APH)

In case III, an air preheater is added to case II to further enhance thermal efficiency. This hardware uses additional waste heat from the flue gas to preheat ambient air used in the combustion process. The flue gas is ducted from above the convection section and returned via an induced draft fan to the stack. The addition 
of forced draft and induced draft fans also 
introduces an additional operation cost, as the fans will use electricity. The additional sensible heat of the combustion air reduces the required firing rate once again, resulting in a reduction of fuel usage and an overall thermal efficiency of 92.4%.

The fired duty, fuel flow and other process conditions for each case are summarised in Table 2.
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