Forced Draft Firing vs Induced Draft

Paper from the AFRC 2017 conference titled Forced Draft Firing vs Induced Draft

Combustion of any fuel requires providing oxygen (usually in the form of atmospheric air) and fuel in the proper amounts, and adequately mixed. (While combustion with pure oxygen is appropriate for some processes, the scope of this paper is for fuel and air combustion. Therefore, for simplicity, in the balance of this paper, the term "air" will be sufficient to describe the oxidizing agent in combustion.) There are several ways to bring the constituents of combustion together. One way, as in a bonfire, is simply to allow the fuel and air to find one another. (Generally, this method is not practical for an industrial process, so will not form part of the discussion in this paper.) Another way is to use a mechanical source to cause the flow of both air and of fuel (i.e. a fan or blower). A final way of combining air and fuel is to rely on natural pressure imbalances generated by temperature differences or velocity differences in order induce flows of either air or gas.; Each of these approaches has obvious advantages and obvious drawbacks. This paper focuses primarily on the relative merits of the latter two mixing strategies mentioned above.

Forced Draft Firing vs Induced Draft AFRC 2017: Bloom Engineering Introduction Combustion of any fuel requires providing oxygen (usually in the form of atmospheric air) and fuel in the proper amounts, and adequately mixed. (While combustion with pure oxygen is appropriate for some processes, the scope of this paper is for fuel and air combustion. Therefore, for simplicity, in the balance of this paper, the term "air" will be sufficient to describe the oxidizing agent in combustion.) There are several ways to bring the constituents of combustion together. One way, as in a bonfire, is simply to allow the fuel and air to find one another. (Generally, this method is not practical for an industrial process, so will not form part of the discussion in this paper.) Another way is to use a mechanical source to cause the flow of both air and of fuel (i.e. a fan or blower). A final way of combining air and fuel is to rely on natural pressure imbalances generated by temperature differences or velocity differences in order induce flows of either air or gas. Each of these approaches has obvious advantages and obvious drawbacks. This paper focuses primarily on the relative merits of the latter two mixing strategies mentioned above. Classification and definition of drafts The pressure imbalances described in the introduction are used to generate a flow of a fluid, known as a draft. Obviously, a fluid will flow from an area of high pressure to an area of lower pressure any time a pressure gradient exists. Clever combustion engineers exploit this fact to move fluids through a system. There are several ways to effect this, generally classified as forced and natural draft. First: defining and clarifying some terminology. There is understandably some confusion about the terms natural draft, forced draft, induced draft, etc., due mostly to the fact that there is not general agreement as to what they mean. Therefore, for the purposes of this paper, the following definitions and differentiations shall apply. A less dense fluid will rise in comparison to a denser fluid, which is why oil (less dense) floats on water (more dense). The density of a fluid (liquid or gas) is inversely related to the bulk temperature meaning that with increased temperature, the fluid becomes less dense. (Hence, a hot air balloon works because the heated air in the balloon is less dense than the atmospheric air, causing it to rise.) Design of a chimney exploits this concept. Warm less dense gases as the bottom of a flue gain buoyancy (due to increased temperature) and rise through the chimney. As it does so, a pressure gradient develops, and gases continue to flow up the chimney following the pressure gradient. This phenomenon is known as a "natural draft" (i.e. it occurs naturally with no external impetus). In contrast, a draft requiring some outside force is called "forced draft". Commonly a forced draft system will have an external fan or blower that will create a pressure difference, and thus the draft. In a combustion system, fans usually provide air and in some cases pull waste gases out of the combustion chamber. An important subset of forced draft is "induced draft". Here, a high-pressure fluid passes through a ventrui (restriction) generating a negative pressure thus entraining another fluid. A common application of this principle is a carburetor. A flow of relatively higher-pressure air passes through an engineered restriction, thus lowering the pressure and drawing in a flow of atomized gasoline. (The flow of air induces a flow of gasoline to provide a combustible mixture.) Figure 1 gives a schematic representation how an induced draft works. Figure 1 - Schematic of an induced draft burner Special Considerations for an induced draft combustion system Air Fuel Ratio The carburetor, as previously mentioned, is a simple device used to obtain a combustible mixture of air and fuel by creating an induced draft. However, there is a reason that carburetors have fallen out of favor for common use in automobiles. They work well at a precise point for an exact air/fuel mixture and a specific firing rate. However, if any parameter changes (e.g. air temperature or density, fuel source or pressure, etc), the carburetor performs less efficiently. Furthermore, it is not easy to alter the characteristics of the combustion system (flame shape, capacity, etc.) should the process change for some reason, a corresponding change in the combustion system is not easily accomplished. Proper combustion requires a certain mass of air for a given mass of fuel. An induced draft system provides a given volume of air for a given volume of fuel. As long as no conditions change, the volume delivered by the pressure difference induced by the flow will also remain constant. If, however, some physical parameter changes, the mass delivered will change, and it will not be possible to achieve optimal combustion. Take, for example, a system that uses ambient air (assumed to be 60°F) for combustion. When the temperature of the air changes, the density (and thus the amount of mass in a given volume) changes. Figure 2 shows the profound volume changes produced by a modest change of temperature (±35°F, which is not uncommon for a reasonably temperate climate). Based on the temperature, the combustion system could receive ±7% air compared to the design. The more startling result appears in Figure 3. Here the percent fuel use corresponding to the relevant temperatures appears. On a moderately cold day, for example, the system could use as much as 7% more fuel compared to the design conditions, and on a very hot day, as much as 12%!! It doesn't take many hot days before the fuel bill becomes very large. Air Mass vs Temperature 108% 106% % Mass of Air 104% 102% 100% 98% 96% 94% 92% 0 20 40 60 80 100 80 100 Air Temperature [°F] Figure 2 - Air mass vs temperature Fuel Use vs Air Temperature 114% 112% Fuel Use 110% 108% 106% 104% 102% 100% 98% 0 20 40 60 Air Temperature [°F] Figure 3 - Fuel use vs Air Temperature Uniformity Some applications require stringent uniformity windows. It is usually possible to stay within these windows either by tailoring the flame shape, or by providing sufficient waste gas volume to ensure that cold spots do not develop. It is very difficult to accomplish either of these strategies with an induced draft burner. Their very design means that there is no way to deliver more waste gas, and it is also not possible to alter the flame shape in order to avoid hot spots making temperature uniformity very difficult with an induced draft burner. Emissions - Nox Careful consideration of flue gas emissions continues to be increasingly important. Nitrogen Oxides (NOx) are particularly problematic, and present importance challenges to the combustion engineer. These oxides are key players in the formation of ground level ozone and smog, with all of the welldocumented adverse effects on human health. While there are ways to mitigate the effects of NOx, through post-combustion processing, it is often more cost effective and less maintenance intensive to manage NOx at the point of formation. Because flame temperature has a disproportionate effect on the formation of NOx, finding a way to lower the peak flame temperature is an effective way to cut NOx formation dramatically. Flame temperature is at a maximum when the exact amounts of air and fuel are well mixed and present at the same physical location. One of the most common ways to lower the flame temperature, while still efficiently using the heat available from the fuel, is to delay the mixing through staging of the fuel or air. Doing so causes regions within the flame envelope that are fuel rich or fuel starved. Of course, as a whole, the flame envelope has the appropriate amounts of fuel and air, but because they are not mixed as well, the temperature is much lower, and NOx formation is much reduced. Figure 4 schematically shows how combustion staging works. Remember, the correct amounts of air and fuel allow complete combustion, but the delayed mixing means that the flame temperature is sufficiently reduced to impact NOx formation. Figure 4 - Schematic representation of staged combustion By the very nature of its simplicity, an induced draft burner cannot provide proper staging and in fact has almost instantaneous mixing of the air and fuel. That, of course, provides very high flame temperatures and the concomitant excess NOx production. It is, of course, possible to remove the offending NOx with post combustion scrubbing, but these processes can be expensive and maintenance intensive. Safety As we saw above, induced draft combustion systems can have inherent efficiency concerns based on the fact that the flow of air is completely dependent on the flow of fuel. Furthermore, because the air always "follows" the fuel, as the burner increases its output, the air lags behind, causing an instantaneous excess of fuel. Not only is this situation inefficient, it is also potentially dangerous. Depending both on how quickly the air responds to the increase in fuel input, and how quickly the fuel moves in the first place, there could be a delay of a few seconds where the combustion system is running with excess fuel. The inherent dangers of this situation should be obvious. Lacking the proper amount of air to combust, not all fuel will burn in the combustion chamber. There remains the possibility that it may combust when the fuel laden waste gas finds enough oxygen as it leaves the exhaust system. Even more dangerous, though, is the very common occurrence of generating dangerous levels of carbon monoxide (CO) which is toxic even at very low concentrations. Additionally, due to the changing flame characteristics offered by an induced draft burner, flame detection and safety may be tricky. As just discussed, a flame can vary from fuel rich to fuel deficient as a system changes its heat demand, which can make it difficult for a proper flame signal. Fuel Flexibility While there have already been hints about the lack of flexibility available with an induced draft burner, the final point to make is that the design of an induced draft burner is very specific to the fuel used; an induced draft burner requires careful engineering specific to a precise fuel. Figure 1 makes it apparent that a change in fuel would change the characteristics of the flow, and thus the pressure, and thus the performance of the burner. For example, if a burner used a calorically poor fuel as designed, a switch to a richer fuel would give a diminished flow, and thus a less intense vacuum at the venturi and consequently, less entrained air. Special considerations for forced draft burners In fairness, there are important considerations for a forced draft combustion system. Generally, there is more physical equipment (valves, switches, etc.) and the controls strategy is consequently more involved. Furthermore, there is obviously a fan or blower with its attendant controls, utility connections and maintenance. Finally, the skill level for operators and maintenance staff are generally correspondingly higher due to the added complexity of the system. In most cases, though, the benefits available from a forced draft system (emissions reductions, better uniformity, operational and fuel flexibility, improved efficiency, enhanced safety, etc.) outweigh the additional challenges. Examples and Case Studies B019505 OO 805-019/605-300 Conclusions The operational and performance benefits available from a forced draft combustion system are pronounced and proven. With proper engineering and careful execution, it is possible to design a system that gives greater efficiency, better safety and enhanced performance as compared to a simple induced draft solution.