Fuel
To better understand the combustor, it is helpful first to understand the origin and nature of the by products that the combustor burns seasoned wood is composed of the following elements. The rate of release of wood gas is dependent upon many variables: temperature, wood chemistry, wood density, exposed wood surface area, and volume of wood within the firebox. In addition, some of the wood gas is burned before it reaches the combustor. Wood gases are predominantly hydrocarbon compounds. Some of these gaseous compounds combust as they are released, forming flames on the wood surface. Most of the gases that escape the primary burning process are condensable and, if given the proper conditions, will condense on the chimney wall as creosote.

Those unburned wood gases escaping from the chimney as smoke go into the atmosphere as pollution. It is estimated that the chemical energy wasted in the form of unburned wood gases is between 5% and 30%. These troublesome, dirty, energy-laden wood gases provide the fuel for catalytic combustion. Some of the more recognizable compounds are methane, propane, ammonia, carbon monoxide, and methanol. It has been estimated that 150 to 250 identifiable compounds are present during the wood-burning process. Some are very transitional in nature and reform as different compounds when temperature conditions change.

During combustion, the rate of wood-gas release fluctuates wildly. In an appliance burning 2 pounds of wood per hour for 10 hours, a combustor does not experience the same concentrations of gas for each hour. In fact, the wood-gas release rate varies greatly from minute to minute over sections of a 10-hour burn, finally dropping almost to zero. Testing of combustor temperature and firebox temperature shows that the combustor temperature fluctuates due to varying fuel and oxygen levels. During the first six hours, there are high levels of combustible gases. But during the last four hours, the waxes and resins no longer provided significant amounts of combustor fuel. The temperature rise in this area is due mainly to the oxidation of carbon monoxide.

This gas-release variability may cause problems with some retrofits units. A retrofit unit certainly has the potential to burn the wood gases escaping the primary combustion. If there is a significant pause in the release of wood gas, the combustor loses its fuel supply and begins to cool. This pause in gas release may last long enough to deactivate the retrofit device. When the gas is once again released, the catalyst is not at an active temperature and the gases pass through it unburned. These gases may then condense and soot up the catalytic surface, masking the catalyst and rendering it inactive when proper light-off temperature is eventually reached.

Designing a retrofit device with a thermal mass near the combustor or with insulation surrounding the combustor may offset gas-release variability problems. This reduces the chance of the catalyst going out during the described condition. An in-stove design rarely has this problem because there is significant thermal mass around the combustor and a radiating bed of coals that prevents excessive cool-down of the combustor during lean fuel conditions

Two conditions that should be included in the prototype testing of any catalytic device are overloading and air starvation (which is usually a result of overloading). Overloading usually occurs when large amounts of small diameter wood are burned. The small-diameter logs increase the wood surface area, which in turn increases wood gas release. If a stove or retrofit device is under designed, overloading will cause the combustor to become air-starved. In an air-starved state, the system burns only a limited amount of wood gas. A stove or retrofit design should include sufficient air supply to handle larger than normal gas concentrations. The use of soft, pitchy wood in the test described above further worsens conditions.

Oxygen Oxygen, as for any combustion process, is important for catalytic combustion. But for its flameless oxidation process, a specific air/fuel ratio is not as critical as for other types of combustion systems that must maintain a flame.

Excess oxygen left over from the primary burn, or supplemented by secondary oxygen, must be sufficient to support combustion in the combustor under all operating conditions. The best way to test for this is through the use of a continuous-type oxygen analyzer. This will help to determine if an independent secondary-air system should be included in the design. Constant downstream value of greater than 2% will prevent an oxygen-starved condition. Analysis of oxygen levels upstream and downstream of the combustor shows that approximately 4 percentage points of available oxygen are consumed by the combustor. Laboratory stoves that show a level of 10% oxygen upstream from the combustor will show about 6% oxygen when measured downstream.

History shows the effects of an oxygen-starved condition on levels of carbon monoxide and non-condensable hydrocarbons. Oxygen-starved does not mean total absence of oxygen; rather, it means insufficient oxygen for complete fuel burning. This condition can also cause soot to accumulate on the catalytic surface, rendering it useless until the carbon is burned off.

Designers of catalytic appliances using independent secondary air supplies must evaluate the need for preheating the secondary air. Cold outside air sent directly to the combustor can do more harm than good. Locally cooled spots on the combustor can be non-active because they are below the temperature needed to keep the catalyst active. This critical temperature will be covered in a later section.

Turbulence The proper fuel-oxygen mix significantly improves the combustion process. Turbulence is the usual method of mixing gaseous fuel. Catalytic appliances may very well benefit by the use of turbulence-inducing devices upstream from the combustor. Flame shields and thermal masses can be designed to serve a secondary purpose of introducing this turbulence

Flow
Catalytic reaction is dependent on the length of time for which the gas is exposed to the catalyst. This suggests that combustor performance is a function of flow. The unit of measure that describes flow rate is "space velocity." That term allows us to speak about volume flow for any size combustor under any flow condition. It is a measure of how much gas is exposed to the catalyst for a given period of time.

Low burn rates within standard-sized wood stoves produces gas flow rates of 10-20 SCFM (standard cubic feet minute). Using a standard 16 -cell, 5.66 inch diameter, 3 inch long combustor whose gross volume is 75.42 in3 results in space velocities from 14,000/hr. to 28,000/hr. If a combustor of the same configuration but half the Iength were installed, the space velocities would be 28,000/hr. to 56,000/hr., respectively, for 10 and 20 SCFM flows. As the space velocity increases, the residences time the gas within the catalyst bed decreases. Once a range of flow conditions for an appliance has been determined, space velocities should be calculated. Increase in combustor volume should be considered if the maximum space velocity exceeds 50,000 exchanges per hour for a 16-cell combustor or 61,000 exchanges per hour for a 25-cell combustor. This should be used as a guideline. More experimental work is needed in this area to define the optimum volumetric flow rates and space velocities for the combustor.

Heat
To become active, the catalyst must reach a certain temperature, called the "light-off temperature." Light-off temperature is the point at which 50% of the fuel is oxidized within the combustor. Practically speaking, it is the temperature at which the catalytic reaction becomes self-sustaining and the system gives off heat. History shows graphically what conversion efficiency changes take place within the combustor as the temperature is increased An important lesson to be learned from this figure is that oxidation takes place continuously throughout the burning process, but not until light off is reached does the reaction sustain itself.

History also shows that as the catalyst becomes hotter, it becomes more effective. This is true to a point at which little is gained with added temperature. Experimentation with actual wood stoves defines a range of maximum conversion temperatures falling between 1,200 and 1,6000 F. (640 and 8700 C.). Many factors contribute to the differences between experiments and actual woodstove tests. Among them are mixing of fuel and oxygen, flow rates, heat transfer, fuels, varying concentrations of fuels and catalyst age.

The conversion of wood gas closely approximates a mixture of particulates, carbon monoxide and hydrocarbons. It is for this reason that a mixture of palladium and platinum is the recommended catalyst system for a woodstove. A 100% palladium catalyst system will be less expensive, convert carbon monoxide efficiently and have good light off properties but will be less effective in converting heavy hydrocarbons (methane, propane). A 100% platinum catalyst system will convert heavy hydrocarbons efficiently but will have poor light off and carbon monoxide conversion properties. A mixture of platinum and palladium catalyst yields the optimum conversion efficiency over the entire range of elements present in wood gas. The amount of each element is critical to achieve maximum efficiency. Many hours of research, development and testing were spent optimizing this mixture.

Thermal Shock
The cyclical heating and cooling of the combustor and the varying conditions of wood burning create temperature differentials within the ceramic substrate. These temperature differences cause the material to expand and contract at differing rates, resulting in internal stresses that can damage it. If not controlled either by tailoring material strength and expansion properties or by minimizing thermal differentials, the substrate may fail in the same manner as when a cold glass is immersed in hot water.

The ceramic honeycombs used as catalytic substrates are special compositions with high intrinsic strength and low thermal expansion coefficients. Although these components have been tailored for use in harsh thermal environments such as the wood stove, it still is possible, with wide temperature differentials, to exceed the thermal-shock resistance of the ceramic. One solution to thermal cracking of the combustor is the use of a compressive canning system that holds the ceramic together should cracks develope. These masses store radiant heat and act thermal-shock absorbers that reduce extreme heat differences within the ceramic. During heat-up, they slow the heating rate so that the ceramic heats more uniformly. During cooling, they perform the same service, so that the ceramic dissipates its heat more slowly and uniformly.

Flame impingement causes concentrated hot spots at points of flame contact. The extreme temperature differences between these hot spots and cooler areas not touched by flame causes thermal-stress cracks. These cracks result in facial crumbling; eventually they may form full body cracks. Flame-shielding the combustor or placement away from flame concentrations will reduce flame impingement. Controlling thermal shock will extend the life of the catalytic combustor.

Heat Dissipation. The combustor releases most accumulated heat through radiant-heat transfer. Gas passing through the combustor will pick up a relatively small amount of energy because of the low thermal capacity of gases.

The thermal capacity of the combustor depends on its size - more specifically, on its length. Hystory shows us it a greater amount of radiant energy can leave the shorter combustor cell than the longer cell. It is this effect that enables the longer combustor to perform better - it can run hotter. Performance loss as the 25-cell combustor length is decreased. The loss with a 16-cell combustor is even more dramatic because the larger cells dissipate more radiant heat.

Catalytic appliances can operate with shorter combustor lengths if steps are taken to minimize radiant heat loss. Encapsulating the combustor in a ceramic refractory environment (surrounding the combustor with firebrick or insulating brick) is one technique. Radiation reflectors made of stainless steel will help. If these steps are taken, lot only will performance be optimized, thermal shock will be reduced.

Age Many performance factors are affected by combustor age. The substrate is degraded with time primarily due to its thermal history (the temperature peaks and thermal differentials that the ceramic has experienced). There is also some contamination of the ceramic substrate from the chemical by-products of wood combustion, resulting in less thermal-shock resistance.

The harsher the thermal environment, the faster the degradation. Ceramic combustor substrates show age by the amount of thermal-stress-relief cracking. By itself, cracking will not affect combustor performance, but severe cracking will, if it results in a loss of ceramic material. Limiting thermal shock can reduce substrate deterioration. This can be done with thermal-shock absorbers discussed in the previous section

Thermal Shock.
The loss of the catalyst-containing washcoat also reduces combustor performance. The alumina washcoat is deposited on the substrate surface as a coating of finely _ divided particles. Because of this, during the lifetime of a combustor there occurs a certain amount of sintering and: crystalline growth. The stability of the washcoat, along with the thermal history of the combustor will determine how well the washcoat performs. High temperatures will degrade the surface area of the washcoat, thus reducing performance. .

To minimize performance losses from washcoat degradation, temperatures within the combustor should be limited to 1,600° F. (870° C.). This can be done by tailoring combustor length or by controlling wood paralysis. Catalyst degradation occurs as a combustor ages. As discussed in the beginning of this booklet, catalysts tend to "clump." High temperatures accelerate this process. Catalytic systems that limit temperatures last longer. Although appreciable catalyst clumping will not take place below 1,800° F. (1,OOO°C.), limiting combustor operating temperature to 1,600° F. (870° C.) will prolong life. The catalyst also can be poisoned or masked. Poisoning is not a major factor when recommended fuels are used. Building in thermal insulation and thermal masses around the combustor can reduce masking with soot, which happens when the combustor loses ignition.

Masking with fly ash can be controlled to some extent by combustor location and use of a flame shield. A combustor located near an ash pit will accumulate a great deal of fly ash. A location higher up in the stove body will offer the combustor some protection. Flame shields, although designed to protect the combustor from thermal shock due to flame impingement, also serve to collect fly ash. The conversion of carbon monoxide is relatively easy; therefore, one would expect a slight drop during 6,000 hours of operation. Non-condensable hydrocarbons such as propane are difficult to oxidize. Therefore, the decrease in conversion performance during the initial 1,000 hours of use is larger for no condensable hydrocarbons than carbon monoxide. Wood smoke is more closely related to carbon monoxide in terms of its reaction in the presence of a catalyst. Thus, the performance curve for particulates in Figure 9 more closely approximates carbon monoxide than hydrocarbons.

Appliance designers should expect the same type of performance degradation. Even after 6,000 hours, Süd-Chemie Catalytic Combustors should remain 70% effective. Over firing, catalyst abrasion, and substrate degradation are the primary reasons for poorer performance levels.

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