Methane, adiabatic combustion

During the strike, the sulfur plant was shut down for minor repairs. I had to supervise its start-up. Mainly, I had to reheat the adiabatic-combustion chamber to 1800°F, before restoring the flow of H2S. This was done by burning a controlled amount of methane or natural gas, with a carefully regulated flow of air. The idea was to slowly heat up the combustion chamber with hot flue gas by 100 to 200°F per hour. This slow reheat was needed to avoid cracking the refractory bricks, because of uneven heating. To carry away a portion of the heat of combustion of the natural gas, we used pipeline nitrogen. 

The net heat that is liberated in the overall combustion reaction is called heat of combustion, and is equal to the heat released by the exothermic reactions minus the heat absorbed by the endothermic reactions. The heat of combustion is partially stored in the combustion product as sensible energy, thus increasing their temperature, and partially lost to the environment. The maximum temperature that the combustion products can ever reach is given for the ideal conditions of no heat losses or adiabatic combustion. This is the called adiabatic temperature and it sets the theoretical maximum limit for the temperature of a flame (for methane/air mixtures this is 1950°C at atmospheric conditions). The tabulated values of adiabatic temperature for different gaseous fuels can be found in [1,2],... 

As an illustration of the above exposition we consider the adiabatic combustion of methane and ask for the final temperature that may be achieved in this process. The relevant reaction is written as... 

A natural gas having the volumetric composition of 90% methane, 8% ethane, and 2% nitrogen at 1 atm and 25°C is used as fuel in a power plant. To ensure complete combustion 75% excess air is also supplied at 1 atm and 25°C. Calculate (i) the lower and higher heating values of the fuel at 25°C and (ii) the theoretical maximum temperature in the boiler assuming adiabatic operation and gaseous state for all the products. 

FIGURE 9.23 Adiabatic flame temperature for stoichiometric combustion of methane in mixtures of oxygen with nitrogen and oxygen with carbon dioxide, computed using NASA s Chemical Equilibrium Analysis (CEA) program [52]. 

Use GRI-Mech (GRIM30. mec) and a laminar premixed flame code to calculate the burning velocity of a methane-air mixture at 1.0 atm. Repeat the calculation, replacing the nitrogen in the combustion air with helium. Compare flame speeds and adiabatic flame temperatures. 

The temperature that a self-sustaining reaction could reach can be estimated, on the high side, by calculating the adiabatic flame temperature (McQuarrie and Simon, 1997). The adiabatic flame temperature is routinely calculated in physical chemistry. An example with the combustion of methane gas will illustrate the salient points. The balanced chemical reaction for the complete combustion of methane is, again... 

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

Since the maximum attainable temperature is sought, we assume complete adiabatic (Q = 0) combustion. With the additional assumptions that the kinetic- and potential-energy changes are negligible and that there is no shaft work, the overall energy balance for the process reduces to AH = 0. For purposes of calculation of the final temperature, any convenient path between the initial and final states may be used. The path chosen is indicated in the diagram. With one mole of methane burned as the basis for all calculations,... 

The flame temperature increases significantly when air is replaced with oxygen because N2 acts as a diluent that reduces the flame temperature. Figure 1.13 is a plot of the adiabatic equilibrium flame temperature for CH4 combustion, as a function of the oxidizer composition, for a stoichiometric methane combustion process. The flame temperature varies from 3600 to 5000°F (2300 to 3000 K) for air and pure... 

We shall develop next a single-channel model that captures the key features of a catalytic combustor. The catalytic materials are deposited on the walls of a monolithic structure comprising a bundle of identical parallel tubes. The combustor includes a fuel distributor providing a uniform fuel/air composition and temperature over the cross section of the combustor. Natural gas, typically >98% methane, is the fuel of choice for gas turbines. Therefore, we will neglect reactions of minor components and treat the system as a methane combustion reactor. The fuel/air mixture is lean, typically 1/25 molar, which corresponds to an adiabatic temperature rise of about 950°C and to a maximum outlet temperature of 1300°C for typical compressor discharge temperatures ( 350°C). Oxygen is present in large stoichiometric excess and thus only methane mass balances are needed to solve this problem. 

Methane reactants within noncatalytic channels remain unburned and they must be combusted in a fully catalytic stage or in a homogenous flame. A fully catalytic monolith would cause the catalyst to reach adiabatic temperatures and to deactivate. Thus, the only practical option is to complete the combustion in a homogenous combustion process. 

Catalytic oxidation reactions on noble metal surfaces are sufficiently fast and exothermic that they can be operated at contact times on the order of one millisecond with nearly adiabatic temperatures of 1000°C. At short contact times and high temperatures complete reaction of the limiting feed is observed, and highly nonequilibrium products are obtained. We summarize experiments where these processes are used to produce syngas by partial oxidation of methane, olefins by partial oxidation of higher alkanes, and combustion products by total oxidation of alkanes. The former are used to produce chemicals, while the latter is used for high temperature catalytic incineration of volatile organic compounds. 

Estimate the maximum amount of work that can be obtained from the controlled combustion of methane in air at 1 bar as a function of the methane-to-air ratio. Assume that the process is as follows. First the methane is burned adiabatically so that the adiabatic flame temperature is obtained. Next a Carnot cycle is used to extract heat from the combustion products until they are cooled to 25 C. Note that the work cannot all be extracted from the combustion gases at the adiabatic flame temperature, but rather is extracted over a range of temperatures starting at the adiabatic flame temperature and ending at 25°C. 

It therefore appears preferable to convert the methane by air, so as to introduce the nitrogen required. This operation is performed at a comparable temperature, in order to maintain the required thermal levels of the successive operating sequences and to avoid excessively disturbing the stream compositions. This is done in the presence of nickel-, based catalysts similar to those employed in the primary reforming reactor, to guarantee the conversion of low hydrocarbon contents in a dilute medium. Post-combustion is thus carried out adiabatically, between 8S0 and 1000 C, at a pressure that is also close to that of the initial steam reforming. 

Comments The adiabatic flame temperature depends on the fuel (in this case methane) but also on the composition of the reacting mixture. The highest temperature is achieved when combustion is done in pure oxygen under stoichiometric conditions (why ). This calculation is left as an exercise. 

In order to simplify the system by reducing the number of turbo machines, the cathode off-gas is used for the combustion process. This measure leads to a high air ratio of 4.4 and a low adiabatic temperature of about 640 K. With regard to CH4 emissions, complete methane combustion requires a temperature of 723-773 K. 

---