Use of Plantation-grown Biomass for Power Generation

The conversion of wood energy feedstocks to energy beginning with harvesting and ending with export of power to the electric grid is depicted in Fig. 3.1, Process diagram: plantation-grown biomass to power generation. Post-harvest fuel preparation, transport, storage, and fuel preparation and handling are key steps linking fuel production with conversion. These process steps influence the efficiency of the conversion facility through feedstock quality (moisture content, size, uniformity) as well as overall system cost. Figure 3.1 also shows feedstock handling and long-term storage losses. These losses can be significant and must be accounted for in determining total feedstock requirements for the conversion facility. Harvest, transport, storage, and fuel preparation techniques and requirements are discussed in the following section. A second section provides a brief overview of conversion processes.

Harvesting, Transport, Storage, Handling and Fuel Preparation

Harvesting is a significant cost and a technical barrier to commercialization and use of plantation-grown biomass for power generation. In the industrialized countries, considerable efforts have been expended to develop equipment for harvesting plantation-grown trees. Results of studies conducted during the mid-1980s conclude that cost-effective harvesting requires equipment be appropriately sized and be able to cut and handle large numbers of relatively small diameter trees. Conventional forest harvesting equipment tends to be inappropriate because it is designed for single-stemmed severance of large trees. Such equipment is also high-powered and expensive relative to the value of plantation-grown trees.

Much of the work on harvesting systems in the industrialized countries has been based on the development of feller bunchers, often as attachments to standard tractors. These harvesters have three functions: severing or cutting, accumulating, and offloading.41 For example, the prototype Hyd-Mech FB-7 continuous feller-buncher uses two counter-rotating saws for severing. Accumulating arms push severed trees off into holding areas. Once the holding areas are filled (8 to 10 trees), the trees are dumped alongside and parallel to the feller buncher. Other equipment (forwarder, grapple skidder, or tractor with grapple) is then used to move the piled trees to a landing area. Here the trees can be chipped and blown directly into a trailer or van for transport or simply loaded in whole form and hauled to a conversion facility. In the latter situation, chipping or size reduction is done at the conversion facility.

Stokes and Hartsough analyzed the productivity and cost of three systems for harvesting a small diameter and large diameter plantation stand. 42Their analysis was based on studies conducted on 7.6 cm sycamore stand in south Alabama and a 10.2 to 15.2 cm eucalyptus stand in central California. The systems included a continuous feller-buncher, a three-wheel feller-buncher, and chainsaw harvesting. A grapple skidder or tractor with winch and a whole-tree chipper were also configured into a balanced harvesting system. Their results are displayed in Fig. 3.2a and Fig. 3.2b.43

Fig. 3.2. A comparison of productivity and cost of harvesting systems for a large diameter (15.2 cm) and small diameter (7.6 cm) plantation stand.

The first panel shows how tree diameter influences harvest system productivity. For the continuous feller-buncher system, productivity was significantly higher when harvesting the larger diameter stands. In contrast, a chainsaw harvesting system was more efficient for the small diameter stand. The second panel shows a similar relationship -- the continuous feller-buncher is more cost-effective for the larger diameter stands with chainsaw harvesting more cost-effective for small diameter stands. Although not specifically shown in Fig. 3.2, the Stokes and Hartsough data indicate that chainsaw felling is about 2.5 times more productive than the continuous feller-buncher for the small-diameter trees. However, bunching and skidding the cut trees to a landing greatly diminishes the productivity of the chainsaw system. A comparison of the production rates achieved on other felling machines is shown in Table 3.1. These data also show a fundamental relationship between tree size (spacing or planting density) and productivity. A research team at the University of Hawaii (Manoa) has modeled this relationship and used it to identify optimum spacing and rotation age for short-rotation eucalyptus.44

aProductivity converted from green tonnes to dry tonnes assuming 50% moisture content.

The productivity and cost of plantation harvesting systems are quite variable. In industrialized or high-labor cost countries, estimated harvesting costs generally range from about $18 to $35/dry tonne for felling, skidding, and chipping.45 The low-end costs tend to presume the availability of prototype plantation harvesters while the high-end costs tend to be based on the use or modification of conventional forestry equipment.46 A large component of total harvesting system cost (about 30 to 40%) is tree handling or skidding of bunched trees to a landing for chipping or loading.

In an effort to minimize handling operations, whole-tree energytm (WTEtm) technology is under development.47 In the WTEtm system, trees are severed, accumulated, and loaded directly onto specially designed trailers for transport. This concept differs from conventional approaches in following respects. First, whole-tree harvesting eliminates tree skidding or forwarding, a major cost of plantation harvesting systems. Second, trees are not chipped or processed in the field. Instead, feedstock size reduction is done at the conversion plant. Third, the minimization of tree handling steps reduces significantly, if not eliminates, feedstock handling losses. Fourth, whole-tree harvesting equipment is optimized for single-stem severance (not coppice growth) and is utilized year-round. These factors have potential to reduce greatly harvesting costs.48 However, a WTE system as now envisioned is likely to be limited to sites that are relatively flat and accessible. Soil compaction is also a potential concern.

In contrast to the industrialized countries, where harvesting and handling operations have been focused on the development of dedicated plantation harvesting machinery, developing countries are basing their harvesting systems on the availability of low-cost and underutilized labor. In Brazil, most felling is done with chainsaws. A typical harvesting operation has chainsaw operators cutting three rows of trees at a time, directionally felling the trees so that they line-up.49 The trees are then crowned and cut to length or left whole for moving to a landing area. Production rates for an experienced chain saw operator can reach 120 trees/day. This is about 60 m3 or 28 dry tonne (0.47 dry tonne per m3) per day (3.5 dry tonnes/hour). After felling and cutting to length, grapple loaders are used to forward the logs to a landing for loading onto trucks (tractor-trailers) for transport to the conversion facility.

For some applications, the availability of excess labor in rural areas, low wage rates, and scarcity of capital for equipment, maintenance and repairs, and fuel dictates that all harvesting operations (felling and forwarding) be done manually. For example, in Southwest China, Perlack et al. estimate that 75 workdays per hectare are required to fell, trim, carry, and stack logs at a roadside.50 Each hectare is assumed to yield 30 dry tonnes of wood energy at harvest excluding the smaller limbs and branches, which are left for nutrient recycling and fuelwood purposes. This production scenario implies a harvest rate of 0.4 dry tonnes/day. In the Philippines, Durst estimated that over 130 days would be required to cut, top, and stack one hectare using handtools.51 This translates into a production rate of about 0.6 dry tonnes/day. A similar rate is provided by Denton, (100 kg/hour or 0.5 dry tonnes/day assuming a 10 hour workday).52 In general, it is difficult to summarize production rates for manual harvesting operations because estimates are highly dependent upon local site conditions (topography, plantation density, tree size, forwarding distances, climate, season, etc.). However, relative to capital intensive harvesting systems, costs are generally a smaller percentage of total delivered feedstock costs.

Harvest and handling operations will result in losses in product yield (Fig. 3.1). The significance of these losses depend on local factors including the degree of mechanization used in harvest operations. Some studies have conservatively assumed that felling, forwarding, and chipping operations can result in a 5% loss in the total standing dry weight yield. In manual harvesting operations, losses can be higher depending on how tree crowns and smaller limbs are treated. It is more likely that these smaller pieces are left on-site for use as fuelwood (plantation by-product) and for nutrient recycling purposes.

When wood is harvested it normally contains about 50% moisture (wet basis). Moisture and other physical properties of wood should be taken into account when designing and operating wood-fired power systems. The presence of moisture in wood can affect combustion by absorbing heat during evaporation; increasing the time it takes for wood to burn, and reducing the temperature of the combustion gases. Operational experience suggests that there can be significant decreases in boiler efficiency when moisture content begins to exceed 50%. If feedstocks are allowed to air-dry to 30% moisture, there can be usable net heat gains. However, if the feedstock is allowed to absorb moisture during storage, a point can be reached where combustion can no longer be sustained. In this instance boiler blackouts can occur and auxiliary fuel will be needed to sustain combustion. Boilers specifically designed to handle high moisture content fuels do not have these problems, but they tend to be higher in cost.54 Moisture in wood feedstocks also creates problems in storing fuels for later use. These other problems include decomposition, self-heating, spontaneous combustion, and buildup of spores and moulds.55

Research on plantation-grown wood feedstocks in temperate climates has shown that harvest during the dormant growing season will be required to obtain good coppice regrowth. An added benefit is that leaves will be left on site to enhance nutrient recycling.56 Under such a scenario several months of storage is required to ensure a continuous supply of feedstock to the conversion facility.57 When feedstocks are stored for long periods decomposition losses can be high. Decomposition can be especially high under conditions of high humidity, rainfall, and evapotranspiration. How the feedstocks are stored (covered or uncovered) and in what physical form (chips or as bundles of whole-tree) also affects decomposition and the moisture content of the feedstock. Under less than ideal conditions, decomposition losses can easily reach 2% per month of storage. One option to avoid storage problems and decomposition losses is to harvest year-round. With year-round harvesting crop decomposition losses do not occur as trees are in effect stored on the stump. This practice may not promote good regrowth and new stands may have to be established. Moreover, there may be periods during year, especially in tropical climates, where the plantation may not be accessible or roads not passable (e.g., during moonsoon periods).

There are numerous factors that must be considered in designing fuel storage facilities. These include:

• the available storage area and its proximity to the boiler;
• the physical properties of the fuel (chips, chunks, whole-trees, etc.);
• additional fuel preparation requirements;
• back-up storage capacity and reliability of the fuel supply;
• seasonal weather conditions; and
• operational requirements of the boiler.

Fuel storage systems that are in use include both open and covered systems. For the open systems, the fuel is stored either directly on the ground or on a concrete pad. The covered systems can include plastic on slab (or on ground), open sheds, closed sheds, silos, and air bags. A typical operation is likely to include two types of storage facilities. An inactive area (open or covered on slab) where fuel is received and an active area (usually covered) that can store about 3 days supply. Front-end loaders and manual labor are most often used to move chipped wood feedstocks between inactive and active storage areas. The WTEtm technology uses a very large shed (air supported) to store and dry a 30-day supply of whole-trees using waste heat from the power plant.

Fuel-feed systems are used to transfer wood from the active storage area to the fuel hopper for metering and feeding into the boiler. This handling equipment can include various types of conveyors (belt, drag, screw, pneumatic) and elevators. Fuel hoppers are usually designed to avoid bridging and clogging (e.g., sloped walls), and they are covered to prevent sparks and smoke from escaping to the storage area. Screens and knife hogs or hammermills may also be used to remove unwanted material (e.g., rocks and debris) and to ensure appropriate particle sizes and uniformity. Uniform feedstock size helps to ensure more efficient fuel handling and combustion.

The characteristics and quality of biomass feedstocks greatly influences the design, choice, and performance of conversion technologies as well as the requirements for feedstock storage, fuel handling, and ash disposal. Biomass feedstocks that are variably sized and high in moisture or ash content can reduce boiler efficiencies, increase O&M costs, and lower capacity factors.

There is a variety of equipment on the market that can be used for sizing. This equipment ranges from small-scale chippers that are towed or trailered to a site to large-scale equipment. The large-scale equipment can be mobile (trailered or self-propelled) or stationary. This equipment is usually configured with grapples for fuel feeding. Whether small- or large-scale, chips can be blown directly into a van or trailer for transport or piled at the conversion facility for storage.

There also exists a number of options regarding fuel preparation and handling systems. The wood feedstocks can be chipped "green" immediately after harvesting. Because many conversion systems are designed to burn green or high moisture materials (<50%) and a variety of fuels, storing fuels to reduce the moisture content may not be necessary. Some conversion systems require that the feedstocks be dried significantly below 50% moisture. In this case, green chips would then have to be stored. Alternatively, felled trees can be stored in whole form (or chunks) for storage and drying. This practice would reduce decomposition losses. However, drier wood is more difficult to chip and to ensure uniform size.

Power Generation Options

The use of plantation-grown biomass can be used for power generation in three general applications, these include:

• stand-alone, grid-connected biomass-based power plants using plantation grown feedstocks or mixed with agricultural residues;
• cogeneration at agricultural (e.g., sugar mills and rice mills) and forestry processing facilities for on-site heat and power needs with export of excess power to the local distribution grid using a combination of mill wastes (bagasse, rice hull, wood waste) and plantation-grown biomass;
• and co-firing at fossil-fired electric generation facilities.

For each of these applications, the current conversion technology of choice is the steam-turbine cycle (Rankine cycle). The technology is relatively simple to operate and it can accept a wide variety of biomass fuels. However, at the scale appropriate for biomass, the technology is expensive and relatively inefficient when compared with fossil fuel power plants.58 As such, the technology is relegated to applications where there is a readily available supply of low-, zero-, or negative-cost feedstocks.

The low efficiency of biomass-fired power plants, relative to fossil-fuel plants, is due in part to the use of more moderate steam conditions. Biomass steam-turbine plants use lower pressures and temperatures because of the strong scale dependence of the unit capital cost ($/kW).59 Biomass plants can not be built at sufficiently large sizes to take advantage of scale economies (>300 MWe) because the cost of supplying fuel to the plant would be excessive. Woody biomass has an energy density of slightly less than 20 GJ/dry tonne. When freshly cut, wood contains about 50% moisture. The high moisture effectively reduces the energy content of the wood by half. Relative to coal, a tonne of dry wood has about one-third less heat value than a tonne of coal. The high costs associated with handling, transporting, and storing large quantities of biomass effectively will this negate any scale economies associated with building large conversion facilities.

The remainder of this section examines several combustion technologies for biomass power generation.60 The technologies can be classified into currently available technologies that are in operation at the present time or capable of being in operation with minimal developmental barriers, and emerging technologies that are expected to require overcoming some technical barriers, before commercialization can be considered.

Currently Available Technologies

Conventional steam turbine systems. Except for differences in fuel handling and preparation and emissions control, wood-fired steam turbine power systems use essentially the same technology as that found in coal-fired plants. However, the lower density and heating value of wood relative to coal means that biomass systems require more combustion and heat transfer surface area. The tradeoff between additional costs of fuel handling and extra boiler combustion area for wood, and simpler emissions controls relative to coal translates into approximately the same installed costs ($/kWe) for biomass systems.

In a conventional biomass-fired combustion steam turbine, wood is first prepared (sized and possibly dried) then burned in a boiler to produce pressurized steam. The steam is expanded in a turbine to generate electricity. For power production, a fully condensing turbine is used. If process heat is to be produced in addition to electricity, a condensing-extraction (or back-pressure) turbine is used instead. In this cogeneration mode, some steam after producing electricity is taken from the turbine for process heat.

Specific boiler types used to generate steam include pile burners (dutch oven), grate burners (stationary or traveling grate), suspension burners, and atmospheric fluidized-bed combustors (bubbling-bed or circulating-bed). These combustion methods will produce boiler efficiencies ranging from 65 to 75% with net plant efficiencies from 20 to 25%.

The most commonly used boilers for wood-fired systems are grate burners. The grates can be either stationary or traveling. Stollers are used to fuel wood into the boiler. For wood-fired systems, the spreader-stokes is most often used decause it can easily handle a wide variety of fuels. A second combustion design available for biomass are fluidized beds. In these designs wood (biomass) is injected into the combustion chamber through ports and burned in suspension. Air entering the boiler fluidizes a bed of hot, granular, inert material. The inert material heats the biomass quickly to ignition temperature, stores the thermal energy, and provides the appropriate residence time for full combustion.

Co-firing biomass feedstocks with coal. Co-firing plantation-grown biomass feedstocks with coal in existing utility steam boilers is a potentially useful process to reduce SO2 emissions. The addition of scrubbers to a coal-fired plant would sharply reduce SO2 emissions, but at a high cost. If only a moderate amount of SO2 emissions reduction is required, co-firing wood and coal can be a very cost effective alternative. Additions of new fuel handling equipment, modifications and improvements to boilers and electrostatic precipitators are required to co-fire with coal, but these changes would likely be less costly than installing scrubbers, switching to low sulfur coal, or purchasing emission allowances. Because of the moisture present in wood, some derating of the boiler may occur.

Emerging Technologies

The currently available biomass conversion technologies are relatively robust with minimal operating problems.61 However, a significant limitation of these technologies are low operating efficiencies that are due in part to the high moisture inherent in biomass feedstocks. One technology under development and testing that offers higher conversion efficiency is Whole Tree Energy (WTEtm) technology. WTEtm is an innovative steam turbine technology that uses an integral fuel drying process.62 Waste heat, preheated by the flue gas at 54C is used to dry the wood stacked in a large, air-inflated building for 30 days before it is conveyed to a boiler and burned. Allowing the waste heat to dry the wet whole tree fuel can result in furnace efficiency approaching 87% with a net plant efficiency comparable to that of a modern coal-fired plant (35%). WTEtm also reduces wood harvesting and handling costs as well as the need for equipment such as hammer mills, screens, and chippers that are used for reducing the size of the wood feedstock. In the Lake States region of the U.S., busbar electricity costs from WTEtm are projected to be about $0.043/kWh or about $0.015/kWh less than that of a coal-fired plant.63 WTEtm can be built in sizes as small as 25 MWe; however, it is more likely that the market for this technology will be utility-scaled systems. Although there exists technical potential to increase the conversion efficiency of WTEtm technology and other steam-turbine cycle systems, these developments are unlikely to be cost-effective.64

According to some experts the most promising technologies for wood-fired power generation lie in the development of gas turbine cycles.65 Gas turbines (or Brayton cycles) have already been developed for natural gas and clean liquid fuels. A key advantage of gas turbine technology is the potential for substantially reduced capital costs, which are relatively insensitive to scale, higher conversion efficiencies (upwards of 45%), and greater modularity. Adapting the technology for biomass (i.e., Biomass-gasifier/gas turbine -- BIG/GT) would require the use of a gasifier to thermochemically convert wood to a gas. The resultant gas would then be cooled and cleaned before being burned in a gas turbine. There are a number of technology choices for both the gasification and power generation portions of the BIG/GT cycle.

BIG/GT gasification. There are two principal gasification options for use with biomass or plantation-grown wood. These are the fixed-bed and fluidized-bed gasifiers. The most promising of the fixed-bed gasifier designs for biomass are updraft designs. In these gasifiers, wood is fed from the top of the gasifier undergoing drying, pyrolysis, and char gasification and combustion as it moves to the bottom of the gasifier. The gas is removed from the top side of the gasifier with air and steam injected into the bottom sides of the gasifier. Fixed-bed gasifiers are simple to operate and efficient when used with uniform and appropriately sized, high bulk density (e.g., wood chips) feedstocks. The most important technical issue associated with these gasifiers is hot-gas cleanup, primarily the removal of alkali compounds and particulates. Controlling the temperature of the gas and using cyclones for particulate removal can be used to provide a gas suitable for the turbines.

Relative to fixed-bed gasifiers, fluidized-bed gasifiers have greater throughput and can handle a greater variety of fuels including low-density materials such as agricultural wastes. This fuel flexibility characteristic may make fluidized-bed gasifiers the more appropriate choice for biomass applications.66 However, the resultant gas from fluidized beds is more problematic for gas cleaning. The exit gas has a temperature about 300oC higher than that for fixed-beds (500 to 600oC). The higher gas temperature requires gas cooling to condense the alkali compounds. Particulate control is also likely to be more problematic and require the use of advanced filtering techniques.

BIG/GT turbine cycles. There are a number of alternative turbine cycles available and under development that can use clean fuel gas. In a simple-cycle configuration, fuel gas is burned in air pressurized by a compressor. The hot combustion gases are then used to drive a turbine. The exhaust gases from the turbine can be discharged (open cycle) or used in a heat recovery steam generator to raise steam for industrial or agricultural processing needs (cogeneration cycle). Because waste heat is used, the overall efficiency of the cogeneration cycle is higher than that of the simple cycle.

Both the open cycle and cogeneration cycle can generate electricity with an efficiency of about 32% (less than that from a modern, large-scale steam-turbine cycle). There are a number of cycles under development and/or demonstration that have the potential to increase significantly the efficiency of the power generation side of the cycle. One of these is the BIG/GT combined cycle (BIG/GTCC). This cycle is similar to the simple cycle except that steam from the heat recovery steam generator is used to generate additional power in a conventional steam turbine.67 About one-third of the total power output of a BIG/GTCC would come from the steam-turbine side of the combined cycle.

Steam injected gas turbine systems (BIG/STIG) are another option under development. In the BIG/STIG system waste steam from the heat recovery steam generator that is not needed for process uses is injected back into the gas turbine combustor, and at additional points along the flow path the gas is reheated to turbine inlet temperature, and then passed through the turbine. The additional steam mass injected into the turbine increases the total power output of the system and does not consume power at the turbine's compressor. Full steam injection allows the total system efficiency to approach 40%. An advanced form of the STIG is the intercooled steam-injected gas turbine (ISTIG). This variation of the STIG can raise both thermal efficiency (from 47% to 50%) and shaft power output.

In addition to gas turbines, fuel cells are also under development. Fuel cells convert energy produced by electrochemical oxidation of a fuel like biomass into electricity. The operation of a fuel cell has gaseous fuels being fed continuously to the fuel cell's negative electrode (anode), while oxygen or air (oxidant) is fed continuously to the positive electrode (cathode). A continuous electrochemical reaction occurs which produces an electrochemical potential or voltage and electric current similar to that of a conventional lead-acid battery. Several fuel cell types are currently under development. However, the phosphoric acid fuel cell (PAFC) is closest to commercialization.68 The PAFC fuel cell operates at approximately 390F and has a cycle efficiency of as high as 45%. Advanced fuel cell designs could reach efficiencies of about 57% by 2020.69

Biocrude oils can also be produced from wood using a pyrolysis process similar to that used in gasification. The wood feedstocks are thermochemically converted to a sulfur free mixture of biocrude oil vapors, non-condensible gases, water vapor, and char particles. The biocrude oil vapors are condensed to form a higher viscosity mixture of organic compounds with a heat value of 8,000 Btu/lb. After the residual non-condensible gases exit the condenser, the gases are collected and burned as process heat for drying the feedstock or supplying heat to the reactor. Approximately 60% to 80% of the feedstock can be converted to biocrude. Biocrude can be used as a substitute for fuel oil for gas turbine cycles. Biocrude can also be co-fired with other fossil fuels. The pyrolysis process does not have to take place at the power generation site since the biocrude can be stored and transported.

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