Walls deposition

The tiansition from a choice of multiple fossil fuels to various ranks of coal, with the subbituminous varieties a common choice, does in effect entail a fuel-dependent size aspect in furnace design. A controlling factor of furnace design is the ash content and composition of the coal. If wall deposition thereof (slagging) is not properly allowed for or controlled, the furnace may not perform as predicted. Furnace size varies with the ash content and composition of the coals used. The ash composition for various coals of industrial importance is shown in Table 3. 

The vanadium content of some fuels presents an interesting problem. When the vanadium leaves the burner it may condense on the surface of the heat exchanger in the power plant. As vanadia is a good catalyst for oxidizing SO2 this reaction may occur prior to the SCR reactor. This is clearly seen in Fig. 10.13, which shows SO2 conversion by wall deposits in a power plant that has used vanadium-containing Orimulsion as a fuel. The presence of potassium actually increases this premature oxidation of SO2. The problem arises when ammonia is added, since SO3 and NH3 react to form ammonium sulfate, which condenses and gives rise to deposits that block the monoliths. Note that ammonium sulfate formation also becomes a problem when ammonia slips through the SCR reactor and reacts downstream with SO3. 

Figure 10.13. Vanadia wall deposits in a power plant firing Orimulsion fuel catalyze the premature oxidation of SO2 in heat exchangers. Note that potassium enhances the undesired conversion while a selective poison diminishes the effect to some extent.

It is well known that during liquefaction there is always some amount of material which appears as insoluble, residual solids (65,71). These materials are composed of mixtures of coal-related minerals, unreacted (or partially reacted) macerals and a diverse range of solids that are formed during processing. Practical experience obtained in liquefaction pilot plant operations has frequently shown that these materials are not completely eluted out of reaction vessels. Thus, there is a net accumulation of solids within vessels and fluid transfer lines in the form of agglomerated masses and wall deposits. These materials are often referred to as reactor solids. It is important to understand the phenomena involved in reactor solids retention for several reasons. Firstly, they can be detrimental to the successful operation of a plant because extensive accumulation can lead to reduced conversion, enhanced abrasion rates, poor heat transfer and, in severe cases, reactor plugging. Secondly, some retention of minerals, especially pyrrhotites, may be desirable because of their potential catalytic activity. 

As the reacting gas flows down the channel, it interacts with the channel walls, decomposes, and leaves a film on these walls. If the wall deposit is rapid and heavy, the reactants will deplete so that the gas composition will vary with x. Although this is a technologically important case, it requires a two-dimensional (partial differential equations) description. For the present problem, we will assume that depletion is slow enough for us to neglect, and gas composition will not be a function of x. 

Most of the effort was spent trying to integrate the three simultaneous Y equations. The Y distributions across the channel for a typical condition are shown in Figure 10. The YsiH2 exhibits a peak near the hot wall and is a fairly full profile. This can be attributed to the high diffusion coefficient at these pressures, which allows the SiH2 to readily diffuse toward the cold wall. Deposition on the cold wall is many times smaller than on the hot wall, as evidenced by the smaller value of dYsjH2/dy there. 

PANELIST WOLK They have been looking for deposits of calcium processing Wyoming coal. I think there have been some very small concentrations of oolite, like structures found in the residues that have been looked at. Obviously, some of the calcium ends up on the catalyst, but it is a small proportion. The wall deposits have been checked for calcium concentration and they have been minimal. Whether that is a function of the reactor diameter using the PDU, the turbulence of the bed or really a lack of detailed observation, I don t really know. But it has not proved to be an operating problem with the H-Coal process. The period of time covered with subbituminous coal runs have been as long as thirty days. 

Many of these difficulties can be overcome by choosing an appropriate configuration of the photoreactor system. One such a system is the mechanically agitated cylindrical reactor with parabolic reflector. In this type of reactor, the reaction system is isolated from the radiation source (which could also simplify the solution of the well-known problem of wall deposits, generally more severe at the radiation entrance wall). The reactor system uses a cylindrical reactor irradiated from the bottom by a tubular source located at the focal axis of a cylindrical reflector of parabolic cross-section (Fig. 40). Since the cylindrical reactor may be a perfectly stirred tank reactor, this device is especially required. This type of reactor is applicable for both laboratory-and commercial-scale work and can be used in batch, semibatch, or continuous operations. Problems of corrosion and sealing can be easily handled in this system. 

An impactor is operated such that there is no rebound and internal deposition can be halted by the presence of an electric field. If, when the impactor is operated with the field on, a certain slot velocity gives a downstream concentration of a monodisperse aerosol that is 50 percent of the upstream concentration, what happens when the field is shut off so wall deposition can take place ... 

Since deposition is occurring, the downstream aerosol concentration will drop. To increase this concentration so that C/C0 will again be 50 percent, the impactor flow must be reduced. Thus wall deposition has the effect of raising the value of VStkso compared to the case of no wall deposition. 

Example 10.1 An aerosol flowing through a tube is kept at a constant concentration inside the tube to within 1 mm of the tube wall. If the aerosol is made up of 0.5- xm-diameter spheres and the concentration in the tube is 103 particles per cubic centimeter, estimate the wall deposition rate, in particles per square centimeter per second. Assume T = 20°C. 

Figure 1 illustrates conventional CVD reactors. These reactors may be classified according to the wall temperature and the deposition pressure. The horizontal, pancake, and barrel reactors are usually cold-wall reactors where the wall temperatures are considerably cooler than the deposition surfaces. This is accomplished by heating the susceptor by external rf induction coils or quartz radiant heaters. The horizontal multiple-wafer-in-tube (or boat) reactor is a hot-wall reactor in which the wall temperature is the same as that of the deposition surface. Therefore, in this type of reactor, the deposition also occurs on the reactor walls which presents a potential problem since flakes from the wall deposit cause defects in the films grown on the wafers. This is avoided in the cold-wall reactors, but the large temperature gradients in those reactors may induce convection cells with associated problems in maintaining uniform film thickness and composition. 

Hartl (26) treated vanadium, chromium, and titanium nitrides in argon-5% acetylene RF plasmas. The nitrides were dropped as powders into a vertical plasma torch in which the gas stream was flowing upwards. Product was collected both as wall deposits and as loose powder at the lower end of the torch (i.e., the plasma gas inlet). In the case of vanadium, the products collected were identified by X-ray analysis to be a mixture of vanadium nitride and carbide (VC-VN) with... 

Feed material Powder product Torch wall deposit... 

Leaves dyeing equipment clean no wall deposits. 

The first test with cellulose (Experiment 1.05) used no steam flow. Because the evolved volatile matter cracked on the vycor walls depositing an opaque, black material thereon, the wall temperature of the reactor rapidly climbed to temperatures above 1000°C. Gasification data is presented in Table 1 for the 0.54 g of cellulose fed into the reactor during the test. Subsequent, brief exposure of the reactor to the solar flux with steam (but no biomass) flow cleaned the reactor s walls nicely. 

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