Top-fired furnaces

The differential equations (7.138) and (7.139) for the top-fired furnace, however, describe a boundary value problem with the boundary conditions (7.140) and (7.141) that can be solved via MATLAB s built-in BVP solver bvp4c that uses the collocation method, or via our modified BVP solver bvp4cf singhouseqr. m which can deal with singular Jacobian search matrices referred to in Chapter 5. On the other hand, the differential equation (7.142) is a simple first-order IVP. 

The treated feed gas mixed with process steam is reformed in a fired reformer (with adiadatic pre-reformer upstream, if used) after necessary super-heating. The net reforming reactions are strongly endothermic. Heat is supplied by combusting PSA purge gas, supplemented by makeup fuel in multiple burners in a top-fired furnace. 

Their thermal efficiency is not very different and in a top-fired furnace can be as high as 95 %. The enthalpy difference between inlet and exit, often referred to as reformer duty, is made up of the heat required to raise the temperature to the level at the tube exit and the enthalpy of the reforming reaction. In a typical tubular steam reforming furnace, about 50% of the heat generated by combustion of fuel in the burners is transferred through the reformer tube walls and absorbed by the process gas (in a conventional ammonia plant primary reformer 60 % for reaction, 40% for temperature increase). 

The heat is used for the various process duties as shown in the example of Figure 46 (top-fired furnace). 

Figure 46. Example of the heat recovery in leh convection section (top-fired furnace, plant with auxiliary boiler)...

Model development for steam reformers Modelling of side fired furnaces Modelling of top fired furnaces Modelling of methanators Modelling of the catalyst pellets for steam reformers and methanators using the dusty gas model Computational algorithm... 

Schematic temperature and heat flux profiles for a top-fired and a sidewall-fired reformer for identical process outlet conditions are seen in Figure 3.5 below. The top-fired furnace has a high heat flux at the inlet, whereas the sidewall-fired furnace has a more equally distributed heat flux profile. The top-fired furnace has an almost flat tube temperature profile, whereas in a sidewall-fired furnace the tube-wall temperatures increase down the reformer. The terrace-wall fired reformer has profiles similar to the sidewall-fired reformer, whereas the bottom-fired reformer has a larger heat flux in the lower part of the reformer.

The eonvection heat transfer from the gas volume has of course only eontributions from the two neighbouring tube and furnace wall zones. The heat release in a volume zone Qcomb is from the eombustion. For a sidewall-fired reformer it can be ealeulated eonsidering the burner as a point source [352] [358] or simply as a duty heat release in the nearby gas volume assuming a well-stirred combustion. For a top-fired furnace an appropriate duty heat release model must be used. 

In spite of good results using a simplified burner model it is evident that CFD is the proper tool to model a furnace, but a simplified realistic model is necessary in the daily work. A comparison between a full CFD model and the simplified burner model is shown in [330], where CFD has been used to calculate exact flow and temperature fields in the sidewall-fired furnace using appropriate burner models. It is evident that modelling of flue gas exit flows in the top of a sidewall-fired furnace and also the flow distribution in a top-fired furnace along the walls requires CFD. 

The Foster Wheeler reformer furnace is a Terrace-Wall furnace. The unique feature of this side-fired design is the burner location (Fig. 27 and 28). The burners are directed at the walls of the furnace, which radiate heat to the tubes. As with the top-fired reformer, the process gas enters the top and passes to the bottom. Unlike many top-fired furnaces, the terrace-walled furnace tubes have... 

Figure 9.1. Schematic arrangement of a top-fired furnace for the primary reformer in synthesis gas production. Reprinted from Catalyst Handbook, 2" ed., by kind permission of M. Twigg.

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

Reformers are fired to maintain a required process gas outlet temperature. Most modern reformers are top fired. In a top-fired reformer, the burners are located at the top of the furnace and fire downward. Process gas flows downward through catalyst-filled tubes. This flow of process gas and flue gas allows the highest flue gas temperature when the in-tube process gas temperature is lowest and the lowest flue gas temperature when the in-tube process gas temperature is highest. This results in tube-wall temperatures that are uniform over the tube s length and since the average tubewall temperature is lower this reduces tube cost and increases tube life. 

Industrial steam reformers are usually fixed-bed reactors. Their performance is strongly affected by the heat transfer from the furnace to the catalyst tubes. We will model both top-fired and side-fired configurations. 

Fig. 5.1. Spent sulfuric acid regeneration flowsheet. H2S04(f) in the contaminated spent acid is decomposed to S02(g), 02(g) and H20(g) in a mildly oxidizing, 1300 K fuel fired furnace. The furnace offgas (6-14 volume% S02, 2 volume% 02, remainder N2, H20, C02) is cooled, cleaned and dried. It is then sent to catalytic S02 + Vi02 —> S03 oxidation and H2S04 making, Eqn. (1.2). Air is added just before dehydration (top right) to provide 02 for catalytic S02 oxidation. Molten sulfur is often burnt as fuel in the decomposition furnace. It provides heat for H2S04 decomposition and S02 for additional H2S04 production. Tables 5.2 and 5.3 give details of industrial operations.

The feed is desulfurized and mixed with process steam before entering the steam reformer. This steam reformer is a top-fired box type furnace with a cold outlet header system developed by Krupp Uhde. The reforming reaction occurs over a nickel catalyst. Outlet reformed gas is a mixture of H2, CO, C02 and residual methane. It... 

Today contractors and licensors use sophisticated computerized mathematical models which take into account the many variables involved in the physical, chemical, geometrical and mechanical properties of the system. ICI, for example, was one of the first to develop a very versatile and effective model of the primary reformer. The program REFORM [361], [430], [439] can simulate all major types of reformers (see below) top-fired, side-fired, terraced-wall, concentric round configurations, the exchanger reformers (GHR, for example), and so on. The program is based on reaction kinetics, correlations with experimental heat transfer data, pressure drop functions, advanced furnace calculation methods, and a kinetic model of carbon formation [419],... 

In many modern top-fired reformers the heat flux calculated for the inner tube wall surface is around 60 000 W/m2, although in some designs it cam be as high as 75 000 W/m2. The maximum heat flux may be 1.4 to 2 times higher. In side-fired and terraced-wall furnaces, where the mean fluxes are generally in the range of 60 000-85 000 W/m2, the difference between mean and maximum flux is much smaller, as shown in Figure 37 [444],... 

Top-fired reformers are preferred for large capacities. It is possible to accommodate 600 to 1000 tubes is one radiant box. Very large furnaces for methanol plants, in which there is mostly no secondary reformer and the furnace bears the whole reforming duty, have been built by various companies. Top-fired reformers in worldscale ammonia plants usually have between 300 and 400 tubes. The sytem has several advantages ... 

K), nj is the effectiveness factor for reaction j (to be computed from the diffusion reaction equation at each point in the reactor), (—AHj) is the heat of reaction for reaction j (KJ/Kg mole), Rt is the catalyst tube radius, m, U is the overall heat transfer coefficient in (KJ/mP.K) and To> is the wall temperature (K) which is determined through the coupling between the above model equations for the catalyst tube and the model equations for the combustion chamber (the furnace). A distributed parameter model for the combustion chamber is being developed by (CREG) for both top fired and side fired furnaces. 

Example 2.4 A proposed natural-gas-fired furnace will need a heat transfer coefficient of 16 Btu/ft hr°F. (a) Determine the needed mean furnace gas temperatures with 18", 36", 54", and 72" heights of the furnace ceiling above the tops of the load pieces (gas blanket thicknesses), (b) Compare probable NOx emissions. 

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