Hybrid Thermal Oxidizer Systems and Methods

Hybrid thermal oxidizer systems and methods for combusting waste gas and heating utility oil using an efficient transfer of heat from fuel gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The subject matter of the present invention is described with specificity, however, the description itself is not intended to limit the scope of the invention. The subject matter thus, might also be embodied in other ways, to include different steps or combinations of steps similar to the ones described herein, in conjunction with other present or future technologies. Moreover, although the term “step” may be used herein to describe different elements of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless otherwise expressly limited by the description to a particular order. While the following description refers to the oil and gas industry, the systems and methods of the present invention are not limited thereto and may be applied in other industries to achieve similar results.

Referring now toFIG. 3, one embodiment of a hybrid thermal oxidizer is illustrated for use in an LNG facility. A fuel gas stream301enters the burner302at the same time a combustion air stream304enters the burner302. The burner302combusts the fuel gas stream301and the combustion air stream304in a combustion chamber306. Impurities from a preheated waste gas stream307enter the combustion chamber306through inlet opening308and are burned with the combustion air stream304and the fuel gas stream301at about 1742° F. to break them down into an exhaust gas in the same manner as described in reference toFIG. 1. The preheated waste gas stream307, however, enters the combustion chamber306at a much higher temperature of about 900° F. than the waste gas stream entering a conventional thermal oxidizer. In this manner, less fuel gas stream301is required to burn and break down the impurities in the preheated waste gas stream307through combustion. The combustion air stream304entering the burner302may be regulated with a valve312so that if the temperature in the combustion chamber306drops below or goes above a predetermined value such as, for example, about 1742° F. when detected by a temperature sensor310, the flow of combustion air stream304into the burner302may be increased or decreased using the valve312. Likewise, the fuel gas stream301entering the burner302may be regulated with a valve303so that if the temperature in the combustion chamber306drops below or goes above a predetermined value such as, for example, about 1742° F. when detected by the temperature sensor310, the flow of fuel gas stream301into the burner302may be increased or decreased using the valve303. In order to maintain the combustion air stream304ahead of the fuel gas stream301for safety reasons, the combustion air stream304entering the burner302may be regulated with the valve312so that if the oxygen in the combustion chamber306drops below a predetermined value such as, for example, about 2% when detected by an oxygen sensor311, the flow of the combustion air stream304into the burner302may be increased using the value312.

A waste gas stream314enters a gas preheater318through inlet opening316where it passes through a coiled tubing and exits the gas preheater318through outlet opening320as the preheated waste gas stream307at about 900° F. The waste gas stream314may enter the gas preheater318at a temperature of about 122° F. The waste gas stream314should not be heated above a predetermined auto ignition temperature of the hydrocarbons in the waste gas stream314when the hydrocarbons in the waste gas stream314are more than 50% of a lower explosion limit. A lower explosion limit is the concentration of a gas or vapor in air capable of producing a flash fire in the presence of an ignition source.

A quench chamber322is positioned between the combustion chamber306and the gas preheater318to control the temperature of the exhaust gas exiting the combustion chamber306before it enters the gas preheater318. A quench air stream324enters the quench chamber322through inlet opening326, which is controlled and regulated by a quench air valve328and a temperature sensor321to maintain a predetermined temperature in the quench chamber322of about 1400° F. In this manner, the temperature of the exhaust gas from the combustion chamber306can be controlled to about 1400° F. before passing through to the gas preheater318. Controlling the temperature of the exhaust gas before it enters the gas preheater318is necessary in order to avoid damaging the gas preheater318. If, for example, the waste gas stream314entering the gas preheater318is interrupted for a while due to unexpected reasons, then the exhaust gas from the combustion chamber306may be controlled to a temperature of about 1400° F. in the quench chamber322before it passes through the gas preheater318at about the same temperature without damaging the coiled tubing therein. Otherwise, the exhaust gas exiting the combustion chamber306at about 1742° F. would directly enter the gas preheater318at about the same temperature and most likely damage the coiled tubing therein because the gas preheater318cannot handle such an elevated temperature due to high thermal expansion stresses. If, however, the waste gas stream314entering the gas preheater318is consistently uninterrupted at about 74,132 lbs/hr, then exhaust gas exiting the combustion chamber306at about 1742° F. is cooled in the quench chamber322to about 1400° F. and loses some of its heat in the gas preheater318, to the waste gas stream314passing therethrough. The exhaust gas exits the gas preheater318at about 1097° F.

The exhaust gas exiting the gas preheater318enters a waste heat recovery module330. A utility oil stream332enters an upper portion of the waste heat recovery module330through inlet opening334, passes through a coiled tubing therein and exits the waste heat recovery module330through outlet opening336. The utility oil stream332is used in a separate process for the LNG facility and, in this manner, is heated to about 475° F. using heat from the exhaust gas exiting the gas preheater318at about 1097° F. The heat from the exhaust gas in the waste heat recovery module330therefore, passes around the coiled tubing containing the utility oil stream332, which exits outlet opening336as a preheated utility oil stream338.

Heat from the exhaust gas passing through the hybrid thermal oxidizer300is therefore, used to efficiently produce a preheated waste gas stream307and a preheated utility oil stream338. The exhaust gas exits exhaust stack340through an opening341into the atmosphere at about 424° F. or less. In order to control the temperature in the waste heat recovery module330, a valve342and a temperature sensor331are used to regulate exhaust gas through outlet opening344thus, bypassing the waste heat recovery module330and entering exhaust stack340through inlet opening346at a temperature of about 1097° F. Regulation of the valve342therefore, controls the temperature of the preheated utility oil stream338to about 475° F. The temperature in the waste heat recovery module330may also be indirectly regulated by valve303. If, for example, the utility oil temperature falls below about 475° F., even after full closure of valve342, the fuel gas stream301may be increased through the valve303to increase the utility oil temperature to about 475° F.

EXAMPLE

In the example below, table 1 summarizes the cost of using a regular Thermal Oxidizer (Regular TOx) and a fired heater. Table 2 summarizes the savings associated with using a Hybrid Thermal Oxidizer (Hybrid TOx) according to the present invention.

In table 1, the fired heater fuel cost assumptions are 85% thermal efficiency for a 30 MM Btu/hr heater with a fuel usage of about 35.3 MM Btu/hr. The fuel cost is estimated at $4/MM Btu (no inflation/fluctuation considered), which results in about $1,236,900 per year. The Regular TOxfuel cost assumptions include a 40 MM Btu/hr Thermal Oxidizer with a fuel usage of about 40 MM Btu/hr. The fuel cost is estimated at $4/MM Btu (no inflation/fluctuation considered), which results in about $1,401,600 per year.

In table 2, the Hybrid TOx fuel cost assumes that no additional fuel consumption is required to heat the hot oil when the Hybrid TOxis operating under normal conditions to burn a waste gas stream.

In addition to the fuel cost savings, the Hybrid TOxalso produces fewer noxious emissions (“NOxEmissions”). In table 1, the NOxEmissions for a conventional fired heater assume:NOxemitted by a 30 MM Btu/hr heater, lbs/MM Btu/hr 0.035Efficiency of the heater=85%NOxemissions eliminated, lbs/MM Btu/yr=0.035*35.29*8,760=10,820In table 1, the NOx Emissions for a Regular TOxassume:NOxemitted by a 40 MM Btu/hr TOx, lbs/MM Btu/hr=0.073NOx emissions, lbs/MM Btu/yr=0.073*40*8,760=25,580

In addition to the significant and substantial cost savings and environmental impact by reducing noxious emissions by approximately 10,820 lbs/yr, eliminating the use of a separate fired heater will provide cost savings by eliminating the maintenance and operational costs associated with a fired heater. Moreover, construction costs and space are reduced by eliminating the requirement of a separate fired heater.

While the present invention has been described in connection with presently preferred embodiments, it will be understood by those skilled in the art that it is not intended to limit the invention to those embodiments. It is therefore, contemplated that various alternative embodiments and modifications may be made to the disclosed embodiments without departing from the spirit and scope of the invention defined by the appended claims and equivalents thereof.