Gft .Si GEORGIA FOREST UA3 RESEARCH PAPER I 13 December, 1980 J INDUSTRIAL WOOD ncv-WVCQ aprogissi COMBUSTION SYSTEMS DOCUMENTS UGA LIBRARIES BY WILLIAM S. BULPITT, CAROL L. ATON AND J. FRED ALLEN r RESEARCH DIVISION GEORGIA FORESTRY COMMISSION AUTHORS William S. Bulpitt is Chief of the Wood Energy Systems Branch of the Georgia Tech Engineering Ex- periment Station. He received his B.S.M.E. degree from Georgia Tech in 1970 and his M.S.M.E. from Georgia Tech in 1972. Carol L. A ton is a Research Engi- neer with the Georgia Tech Engineering Experiment Station. She received her Bachelor's degree in Mechanical Engineering from Valparaiso University in 1970. J. Fred Allen is Wood Energy Spe- cialist with the Georgia Forestry Commission, Research Department. He received his B.S.F. degree from the University of Georgia, School of Forest Resources, in 1970. ACKNOWLEDGEMENT Much of the data for this publication came from "A Feasibility Study For Wood Energy Utilization In Georgia" prepared for the Geor- gia Forestry Commission by the Georgia Institute of Technology, Engineering Experiment Station, under the sponsorship of the Coastal Plains Regional Commission. INDUSTRIAL WOOD COMBUSTION SYSTEMS BY WILLIAM S. BULPITT, CAROL L ATON AND J. FRED ALLEN BACKGROUND Since steam power began to be an important source of energy in the eighteenth century, there has been a gradual evolution of hardware to the point we find ourselves today. The original fuel for steam production in this country was, of course, wood. As coal gained more widespread use, wood burning for industrial steam production became almost a lost art. In fact, coal and wood both nearly disappeared from the industrial scene in the southeast with the switchover to natural gas and oil as boiler fuels in the 1940's and 1950's. Wood burning systems were not ignored by all industries, however. During the past 50 years, the forest products industries have seen the gradual development of wood burning boilers from rather primitive pile burners to units that are capable of producing up to 50,000 Ib/hr of steam at pressures that rival the steam pressures found in central utility plants. Boilers may be divided into two gener- al classes -firetube boilers and watertube boilers. As the designations indicate, in the firetube design, the combustion gases travel'through steel tubes passing through a water tank. In the watertube design, the water passes through steel tubes that are heated on the outside by the hot gases from the combustion process. Today many people have the tendency to dis- miss the frretube boiler as an old-fashioned design, but this is not always the case. There are many applications where r^ 3-PASS DRYBACK 4-PASS. DRYBACK Figure 1 FIRETUBE BOILER GAS FLOW PATTERNS the firetube unit has distinct advantages over a watertube unit, particularly in light and medium industrial applications. Figure 1 illustrates several configurations used in firetube boilers while Figure 2 illustrates a circulation pattern typical of a watertube boiler. Besides the firetube and watertube classifications, boiler designations can be made in another manner: boilers can be "package" boilers or "field-erected" boilers. These designations can cause some confusion since virtually all wood burn- A ing units require some field erection. package boiler generally can be shipped over land with normal transportation methods such as flat-bed truck or railcar. The major boiler components are in one assembly and can often be lifted right onto a simple foundation and piped into an existing system. As a result, the package boiler requires far less labor before startup than a field-erected unit. The field-erected unit often requires individual welding of boiler tubes and the entire fabrication of a steel framework. In other words, the boiler is completely built up at the job site from all the component parts, while the package boiler is nearly complete when it leaves the factory. Package boilers in the 100,000 Ib/hr range have been shipped for gas/oil firing, but the larger combustion volumes neces- sary for wood units generally limit the top size for a wood fired package boiler to less than 50,000 Ib/hr. As one might expect, field-erected boilers cost more than package boilers, and construction times are significantly longer. CLASSIFICATION OF WOOD COMBUSTION SYSTEMS There are many different systems available today for burning wood in an industrial setting. Most of these systems fall in- to one of the following general classes: Wood Fired Package Boilers Suspension and Cyclone Burners Fluidized Bed Combustors Pyrolysis Systems Gasification Systems Field Erected Wood Boilers Each of these classes will be considered in further detail. WOOD FIRED PACKAGE BOILERS There, are many manufacturers in the wood package boiler field today. In gen- eral, it can be said that the larger and per- haps better known manufacturers (Babcock and Wilcox, Combustion Engineer- ing, Riley Stoker, Foster-Wheeler, etc.) are not really interested in building small (less than 50,000 Ib/hr) boilers for light and medium sized commercial and indus- trial operations. This gap has been filled quite nicely by a number of smaller manufacturers; and as wood energy use has grown, the market has become more and more competitive. HEAT STEAM BOILER DRUM DOWNCOMER HEADER OR LOWER DRUM Figure 2 WATER CIRCULATION PATTERN IN A WATERTUBE BOILER One of the older wood firing designs is the horizontal return tube (HRT) boil- er. This type of unit was almost consider- ed obsolete for a time, but in recent years A it has seen a resurgence in sales. typical HRT boiler is shown in Figure 3. The HRT is a firetube boiler which is basically a two-pass design. The steel boiler shell is supported by a refractory furnace, which helps these units to burn wet wood chips and residue ranging up to 60% moisture content. Operating pressures of HRT's are generally limited to 300 psi, and steam production is generally limited to 35,000 Ib/hr or less. Some units are able to meet emission regulations using only mechanical collectors. There are several other types of fire- tube wood boilers including compact three-pass types that perform well on furniture plant waste and other dry ma- terial less than 20% moisture content. These boilers may perform with a thermal efficiency up to 80%. Another type of firetube boiler is shown in Figure 4. It is equipped with an underfeed stoker capable of burning wood, coal, or combinations of the two fuels with oil and natural gas backup. Several smaller manufacturers produce watertube boilers suitable for burning wood. One type that has been installed in more than 60 locations uses a refrac- tory-lined cell burner with preheated underfire air. Most of the combustion takes place in the cell, and the hot flue gases are discharged into the watertube section. The first cost of wood burning package boilers can be quite high, as shown in Figure 5. The cost curves in this figure are based on typical "turnkey" jobs installed by various boiler manufacturers and include fuel metering, controls, a limited amount of wood handling equip- ment, and air pollution control devices. HRT BOILER PRIMARY COMBUSTION CHAMBER WOOD FEED CHUTE COMBUSTION GRATES BRIDGE WALL REAR TURNING AREA /-REFRACTORY CURTAIN WALL SECONDARY COMBUSTION GRATES Figure 3 HRT BOILER FOR WOOD FIRING (Courtesy Industrial Boiler Co.) The less expensive range of boilers are generally the o'nes with the least flexibility with regard to the quality of the fuel that may be burned. Even the smallest package boilers require a given amount of solids handling equipment and control systems so that the cost per pound of steam is much higher for the smaller systems. As can be seen from Figure 5, a given size of wood package boiler will generally have a first cost of 3 to 4 times that of a comparable gas/oil boiler. It may be difficult to retrofit an existing steam plant with a wood fired package boiler due to space limitations. Wood systems generally require a greater area than for gas or oil dictating a larger size for a given steam output. The major conclusion to be drawn a- Figure 4 < THREE-PASS WOOD BOILER (Courtesy Combustion Service & Equipment Co.) o WOOD PACKAGE BOILERS GO & --30 CO