Background

Wood-fuel combustion in a RWH involves complex chemical processes which include the pyrolysis and oxidization of volatile, semi-volatile, and solid carbonaceous components of wood fibers. As in all combustion, the burning of wood requires that the four conditions or process elements of combustion (i.e., time, temperature, turbulence, and the air-to-fuel mixture ratio) be optimized for the combustion process to take place efficiently (i.e., complete oxidation of all fuel materials).

From a process control perspective, combustion in a batch-loaded cordwood-burning RWH is made more complex than most applications by the fact that from the time a fire is first started, with paper and kindling, until the last char-ember has ceased burning, all firebox conditions and combustion reactions are changing: Fuel chemistry and physical properties, fuel geometry, air supply, and temperatures, all change dramatically throughout a RWH burn cycle (i.e., the burning of a whole batch-fuel load).

Time is required to allow thorough air and fuel mixing, for energy-releasing chemical reactions, and for heat transfer to occur. If the residence time of heat-generated fuel-gases and oxygen mixing is too short, combustion will not be complete and the transfer of heat from combustion gases to stove and pipe walls will be inefficient. If residence time is too long, gas velocities in the combustion chamber are too low, and the driving mechanism for the mixing of air and fuel gases will be weak. This also leads to incomplete, inefficient combustion.

Temperature is important since the rates of the chemical reactions, which are the essence of the combustion process, increase exponentially with temperature and the driving mechanism for producing flue draft and gas flows through the whole system is dependant on heated flue gases. Generally, high temperatures in the combustion zone ensure complete combustion. In industrial gas, oil, and coal-fired systems, and in internal combustion engines, however, excessively high combustion temperatures can also generate increased nitrogen oxides pollution. RWH combustion processes typically produce relatively low combustion zone temperatures because overall air-to-fuel ratios are more air-rich than utility or commercial boilers and because the typical RWH combustion chamber also serves as a major part of the appliance's heat exchange system. Heat therefore, is radiated and convected away from the combustion zone rather rapidly.

Turbulence is an important factor because wood-derived fuel-gases and air must mix to attain ignition and, in order to sustain the combustion process, the burning and already-burned gaseous materials must mix with fresh fuel-gases and air. Oxygen molecules must forcefully collide with fuel molecules to have heat-releasing chemical reactions in combustible mixtures take place. The frequency of molecular collisions and the force of collision are both governed by temperatures and turbulence. Nearly all industrial combustion chambers, and many residential combustors are "mixing controlled." That is, the rate at which the mixing processes take place, controls the overall burn rate: "if it's mixed, it's burned."

In large utility and commercial boilers, mixing is aided by mechanical blowers or fans. In most RWH appliances, naturally generated "draft" is the primary driving force for mixing combustion gases and for powering air supply and exhaust systems. Natural draft occurs when negative pressures are generated by low density (buoyant), heated combustion gases in the RWH firebox and chimney. RWH natural draft is a relatively weak driving force (generally less than 0.1 inches water column [25 Pascals]) in comparison to blowers and fans and provides the most difficult process control challenges. A major challenge in RWH design is to use the smallest amount of draft so as to reduce heat (i.e., sensible stack) losses up the chimney or to incorporate a mechanical draft system which is reliable and compatible with wood combustion aesthetics and objectives.

Air-to-fuel ratio is expressed as the mass of air (lb or kg) used to burn a unit mass of fuel (lb or kg). Air-to-fuel ratio is important for accomplishing efficient combustion for several reasons. In general, the overall air-to-fuel ratios in virtually all combustion applications are higher than the theoretically-exact, chemical-reactant (stoichiometric) ratios needed. A rule of thumb is that fuels that are more difficult to burn, require higher air-to-fuel ratios. For industrial boilers, the amount of excess-air (above stoichiometric) required to burn natural gas efficiently is about 5%, to burn oil about 10 to 15%, and to burn pulverized coal about 20 to 25%. These amounts represent just enough excess-air to assure that all the fuel molecules find oxygen molecules with which to react. For industrial wood burners, the recommended amount of excess-air is not quite as well defined, though well engineered systems are found to operate in the range of 50 to 100% excess-air. Usually, the "ideal" amount of excess-air can be identified by looking for the "knee" in a curve of carbon monoxide exhaust emissions versus air-to-fuel ratio.

Too much excess-air is detrimental to efficiency, since, as noted above, excessive air leads to low flame temperatures and inefficient oxidation of the fuel; that is, the "combustion efficiency" is affected. Combustion efficiency is a measure of the completeness of the combustion process or the conversion of the available chemical energy in the fuel to sensible heat in the firebox.

Too much excess-air also affects thermal efficiency, or overall efficiency, which is the percentage of the fuel chemical energy which is actually transferred to the space or medium being heated. If excess-air is too high, the increased flow rate carries a higher proportion of the liberated heat energy up the exhaust stack where it is lost. Furthermore, the higher-than-needed flows reduce residence time and reduce the time available for heat transfer to take place. Heat transfer potential of these gases is also reduced because the heat content and hence temperature of the combustion gases is diluted by the non-essential excess-air. Thus, good combustion design requires using only as much excess-air as is necessary.

Although all industrial and residential boilers operate air-rich, small localized pockets of fuel-richness unavoidably occur. These pockets are characterized by the production of carbon monoxide and the formation of solid and condensed aerosols. Flames in these regions usually exhibit an orange color due to thermal radiation from the aerosols. If the flame is well mixed, and thus well aerated throughout, its burning gases will appear blue in color. Sometimes the blue color is difficult to see, however, because it is overwhelmed by red-to-orange thermal radiation emanating from the hot aerosols.

Even though conventional, naturally drafted RWHs usually operate very air-rich, because of poor mixing, most of the excess-air never becomes intimately Even though conventional, naturally drafted RWHs usually operate very air-rich, because of poor mixing, most of the excess-air never becomes intimately mixed with the fuel gases. Thus the flame is yellow in appearance and contains large amounts of solid carbon and unburned condensate aerosols. Even if the excess-air does become mixed with the other gases, the resulting air-rich mixture is frequently too cool to allow completed reactions.

In order for fuel to burn in a RWH, the design and operation has to address the four elements of combustion discussed above. The extent to which all of these elements are optimized throughout a RWH burn cycle governs the efficiency and emission characteristics of the RWH.