|A model chip pile. The inner pile (dark brown) is low in oxygen; the outer pile (light brown) is in equilibrium with atmospheric oxygen. Liquid water (H2Ol) evaporates in the warm center of the pile, diffuses as water vapor (H2Og) and recondenses in the cool outer area of the pile.|
Thomas Kimmerer, Ph.D., Senior Scientist, Moore Ventures
Editor's Note: This article is based on a paper presented at COAL-GEN 2011 in Columbus, Ohio.
Co-firing of woody biomass with coal is regarded as a cost‐effective way for coal‐fired power plants to reduce greenhouse gas emissions. Additional benefits of woody biomass use include reduced SO2 and Hg emissions and reduced NOX emissions under some circumstances.
Compared with coal, biomass has a lower energy density, lower bulk density and highly variable moisture content, ranging from nearly zero for wood from mills to more than 200 percent (on a dry weight basis) or 70 percent (on a wet weight basis). The heat content of woody biomass is less variable than the heat content of coal, averaging 8,860 BTU/lb for all temperate wood species.
The supply chain for woody biomass is substantially different from that for coal, and provides opportunities for increasing the quality of biomass as a fuel. Woody biomass supply chains are highly dispersed, with fuel often coming from multiple sources and with highly varying particle size and moisture content. Fuels for co-firing rarely come from single sources, creating complex supply landscapes. The supply landscape for a large co-firing or repowering project may consist of wood from:
- Forest residues, such as tree tops and branches (collectively known as slash) and defective trees;
- Mill residues from primary and secondary wood industries;
- Urban residues, including yard waste and street trees;
- Wood from right‐of‐way clearance along transmission and distribution lines, gas pipelines, roads and railroads;
- Disaster wood, including urban and forest trees damaged by ice, wind, insects and disease.
Such a complex supply landscape poses challenges to provisioning any biomass project, particularly in variation in particle size distribution, contamination with soil and metal, and variation in moisture content. This can result in lack of predictability in biomass fuel quality arriving at the plant gate, with the consequence of the need for further processing to alter particle size distribution and reduce moisture content.
As an alternative to dealing with supply variability at the plant, it may be more economical to reduce variability at points along the supply chain where costs are minimal.
Adding or Reducing Value
Each step in the supply chain provides opportunities to upgrade fuel quality, and offers risks that reduce quality.
Unlike coal, biomass is subject to deterioration by microorganisms. As soon as a tree is cut, bacteria and fungi begin the decay process. Left in the woods, a log will completely decay in a short period of time. Development of supply chains for woody biomass must take into account the potential to lose fuel quality over time.
Biomass fuel is easily contaminated by soil, rock fragments and other debris. Such debris is often picked up during removal from the forest, but can also be added during fuel handling steps. Debris reduces fuel quality, and can also produce dangerous conditions in chipping and grinding steps. Avoiding the addition of debris improves fuel quality and reduces down‐stream processing costs. For example, clean wood can be comminuted with a chipper rather than a grinder, reducing energy input by about 30 percent. Chippers are more sensitive to debris than grinders, and even a small rock can take a chipper out of service.
Deciding when, where and how to remove moisture from wood is a critical aspect of supply chain development. A common supply chain configuration is to grind green wood and deliver it to the biomass facility. This is a low‐input approach that does not provide the best fuel quality at the gate.
The Chip Pile
Fuel is commonly stored at biomass facilities in the form of chip piles, typically using outdoor open storage. European experience with chip piles shows that the green chip pile is prone to loss of quality, stabilization of moisture at high values, and the potential for spontaneous ignition. There has been substantial chip pile research conducted in Sweden and Finland since the 1980s.
The photo on pagbe 67 shows a model green chip pile. The center of the pile is anaerobic because microbial activity depletes oxygen, and oxygen diffusion from the atmosphere is slow. As microbial activity increases, the temperature of the pile rises. While this might be expected to drive off water, that is not the case. Liquid water in the pile center evaporates to water vapor, which then diffuses out of the warm center of the pile and recondenses to liquid water on the pile edges. The moisture content of the pile as a whole remains unchanged over time. There is considerable variability in this process, with some studies showing moisture loss, some showing moisture stability and some showing an increase in moisture.
In addition to the high and stable moisture content of chip piles, anaerobic decomposition produces volatile organic compounds, including methane (CH4), and may produce N2O. CH4 and N2O are potent greenhouse gases that should be accounted for in carbon lifecycle analysis of biomass projects. Anaerobic decomposition also creates odor and runoff water problems. These problems can be partly mitigated by maintaining numerous small piles, or by stirring the pile, but these are expensive solutions. Many existing coal plants that can potentially co-fire biomass are space‐constrained, with little opportunity for expanded fuel yards with small piles.
Another approach to chip storage is to reduce moisture content so that the ambient relative humidity inside the chip pile is less than 20 percent. This maintains aerobic conditions within the pile, and avoids the high heat production and volatiles production characteristic of wet chip piles. However, this approach is not feasible without artificial drying and covered storage. Both are quite expensive.
The Cost of Moisture
In addition to the complexities of managing wet chip piles, high moisture poses challenges throughout the supply chain. If not properly managed, high moisture in wood adds cost and reduces fuel value from the forest to the boiler. At 50 percent moisture content (on a green basis), a chip van load of wood contains only 216 MMBTU compared with 400 MMBTU at 8 percent moisture (Figure 1). A receiving facility should be contracting for delivered fuel on an MMBTU basis, rather than a green ton basis. However, even with a contract on a MMBTU basis, costs are incurred for the transportation of water to the plant, and these costs are necessarily included in the contract price.
In the plant, high moisture also causes problems with fuel handling, blending with coal and with combustion. During combustion, the rapid flash of liquid water to water vapor substantially de-rates the boiler.
Taken together, the cost of moisture imposes burdens throughout the supply chain. Rather than living with the problems of high moisture, it is more practical to look for low‐ or no‐cost methods to dry biomass as early in the supply chain as possible. By drying wood before comminution, cost of transportation is reduced, chip pile management is improved, fuel quality loss is reduced and boiler de-rating is minimized.
Transpiration is free
Although moisture moves poorly through chip piles, the same is not true of intact tree stems.
Plants are amazingly efficient machines for moving water, and depend on efficient movement of water from soil to leaves throughout their lives. This efficiency continues even after a tree is cut. A cut tree stem, regardless of size, will lose moisture to the atmosphere until it reaches equilibrium.
While a chip pile may never reach equilibrium with the atmosphere, residue from logging operations or harvested energy‐crop trees will quickly reach equilibrium. Consider a concentrated pile of residue and low‐grade logs at a commercial logging site (see photo on page 68). As air moves through the pile by diffusion and mass flow, fluid water in the wood evaporates in equilibrium with the water vapor concentration of the atmosphere. The rate of drying is a complicated process with many independent variables.
These variables interact in a complex but predictable way, and lead to the concept of Equilibrium Moisture Content (EMC), which has been used in the wood industries for many years.
EMC varies throughout North America, but within a narrow range. For all North American sites, EMC ranges from 4.6 percent to 20.2 percent. At a single location, EMC may vary very little with seasons or quite substantially (Table 1).
Note that although there is considerable variation in EMC, nearly all locations in North America will permit wood to dry to below 20 percent MC in a short time. Designing a supply chain in which residue is allowed to equilibrate before comminution will result in a low‐moisture fuel suitable for combustion without further drying. It will also result in a supply chain that is focused on delivering the greatest amount of energy to the biomass plant at the least cost, and will solve many of the inside‐the‐gate problems of biomass.
Piles, Bales and Bundles
Following a commercial logging operation, and removal of merchantable sawlogs and pulplogs, a considerable amount of residue, approximately 40 percent of the entire harvested biomass, is left on the ground as dispersed residue, or in residue piles. Residue piles are usually left at the side of a logging road. Dispersed residue has to be picked up by machines such as forwarders and moved to the roadside. The residue can be left until it reaches an appropriate moisture content, at which time it can be comminuted on site and taken to the biomass plant.
|Drying of a wood residue pile by equilibration with bulk air. Variables include tempearture, wind velocity, relative humidity, insolation, time, precipitation and pile pore space.|
An alternative to dealing with dispersed residue or residue piles that may allow easier handling is the use of baling or bundling equipment. Although there are only a few manufacturers of balers or bundlers suitable for woody plants, several manufacturers are actively developing efficient, low‐impact machinery. Baling and bundling allows uncomminuted residue to be picked up and transported to storage sites for subsequent processing. This has the advantage of removing fuelwood at the same time, or soon after, the main logging operation. Stored properly, bales and bundles will dry to equilibrium.
|Logging residue shown on the left as dispersed residue, and on the right as concentrated residue piles.|
In Europe, it is now becoming increasingly common to cover concentrated residue piles, bales and bundles with specially adapted paper covers that allow transpiration, reduce snow and rain impacts, and allow more efficient drying. One product, the Walki Biomass Cover, can be placed over a pile and then comminuted directly with the biomass, avoiding the extra cost of removing the cover.
With increasing attention being paid to woody energy crops, such as poplar, as biomass feedstock, it is important to note that the same principles apply: trees that are chipped during harvest have a high moisture content. Bundling, baling, or stacking harvested trees will allow them to dry to a more suitable form for use as fuel.
There has been considerable attention paid to the low density of biomass fuel and the need for densification. The manufacture of pellets for export from North America to Europe is becoming an important market for the forest products industry. A 400,000 ton-per-year pellet mill has recently opened in coastal Georgia. However, the economics of biomass consumption are dramatically different in Europe, where there is high demand for low‐GHG energy production. Pellets are also attractive to the home heating and small boiler market because of their convenience in handling. For North American power producers, the added convenience of pellet has so far not justified the high cost.
Torrefaction, the roasting of wood at low temperature and low oxygen, produces a product that is similar in physical properties to coal, and is low in moisture, hydrophobic and easy to handle. Torrified wood is still largely at demonstration scale. With costs up to four times the cost of ground wood delivered to a plant, terrified wood has not yet been economically justified in production scale operations. The economics may improve as the technology matures.
Densification is often seen as a panacea to the relatively low energy density of wood (although it should be noted that the energy density of wood is within the range of some coals, such as Powder River Basin subbituminous coal). However, this is not necessarily justified. A truckload of fuel has two limits: limitation on the volume of the truck and limitations on permitted weight of the truck. Figure 2 shows the limitations of volume and weight, and suggests that a limit of 400 MMBTU per truck (for 40 ton gross vehicle wt.) is reached with a variety of materials, including dry ground wood, pellets and torrefied wood.
Biomass is a low‐value feedstock that is intrinsically high in moisture and low in energy density. While many current biomass projects use biomass in this low‐value form, it is apparent from the above discussion that there exists many opportunities for producers of woody biomass to upgrade the value of the product they produce at minimal cost. Supply chains built on green wood are inherently inefficient, reduce revenue for biomass producers and increase cost for biomass consumers.
It is incumbent upon biomass consumers to work with their suppliers to develop high‐efficiency, high value supply chains. Many participants in biomass supply chains lack the sophistication of operations or access to capital to acquire expensive equipment, preventing them from entering the market. Biomass consumers, including utilities wishing to co-fire biomass, will need to work with their suppliers and provide financial support in order to create high‐value supply chains.
The bottom line is that the quality of biomass delivered to a plant for co-firing is determined before the feedstock arrives at the plant gate. Ensuring that the feedstock is of the highest quality at the time of delivery will reduce costs and improve the efficiency of biomass co-firing.
Power Engineerng Issue Archives
View Power Generation Articles on PennEnergy.com