Introduction
Biomass produces energy through the process of burning organic materials, such as wood fibers, composts, or charcoal. Energy is produced both from the heat produced during burning, as well as the gasses these materials produce when burned. Wood, for example, is composed of several chemical components that react differently when burned. In a wood fire approximately 80% of the solid wood or volatile matter converts to gas before it burns. This gas made up of carbon monoxide and hydrogen is commonly called producer gas or wood gas. If it is burned directly in a stove or furnace, its heat is transferred directly to the living space or to water where its heat can be distributed to buildings by means of hot water or steam.
Biomass Technology
Biomass technologies appropriate for Alaska fall into three categories:
|
Biomass systems
About 20% of wood is in the form of fixed carbon. Fixed carbon converts to charcoal (char) when heated. The charcoal does not convert to gas, but burns in direct contact with air. Charcoal burns at much higher temperatures than wood gas, so it is preferred as a cooking fuel around the world. With devices that make gas or oil, the char is recovered or burned to provide heat to make producer gas. In a stove or furnace the charcoal burns once the gases have evolved and air is available for direct combustion.
Wood biomass energy devices require a fuel handling system, a combustion vessel (furnace, boiler, or gasifier), ash removal, and general maintenance. In the simplest case of manual loading the equipment is needed to cut, gather, and store the wood; manpower is needed to load the unit regularly. As the gas and volatile components of producer gas cool, several components condense to form tars and oils. These oils can be converted to pyrolysis oil, also known as bio-oil, which can be used as a transportable liquid fuel. Technologies for making bio-oil are still in development.
In chip-fed systems, a chipper is added to the equipment list, a loader to handle woodchips, and a bin and feeder (moving floor and/or auger). Woodchips are more vulnerable to moisture than logs and require protection from weather. In many systems the woodchips require screening for oversized or undersized material, and they are subject to bridging, a resistance to flowing.
Pellet-fed systems require more ‘upstream’ processing to deliver pellets to the system, but less complex fuel handling; pellets can flow. Along with higher delivered cost, all forms of densified biomass have more predictable handling and combustion characteristics than stick-wood or woodchips.
Hydronic (hot water) systems add the costs of water or other fluid tanks, temperature and pressure control systems, insulation, and piping to end-users.
CHP gasification systems require a more complex temperature, pressure, and electrical control system; wood gas cleanup equipment; a generator, turbine or fuel cell; fail-safe switching; and a connection to the electrical grid and/or battery bank. Producer gas can be separated from the solids and burned as a fuel gas. Producer gas must be used close to the source because of its low heating value and low energy density. This gas has only 15% of the heating value of natural gas or propane, but it can be burned directly in boilers, as a fuel gas in engines, or externally to heat other heat transfer devices such as Organic Rankine Cycle (ORC) fluids or Stirling engines attached to generators. These devices are currently being developed for generating power at a small scale suitable for applications in Alaska.
ORC systems require heat and cooling to create a temperature differential for electric generation. Biomass-fired ORC systems are in development in the United States after the successful demonstration of a geothermal-fired system at Chena Hot Springs. For some time these systems have been in operation in Europe.
Heating Technology
Domestic heating devices and small boilers sometimes use gasification principles to burn the wood efficiently, but in these appliances all the wood is converted to heat. Many older-style outdoor wood boilers (OWBs) are inefficient burners. They cause significant air pollution from incomplete combustion and convert only 35% of the energy available in the wood to heat energy in water. Newer, more efficient boilers burn with a clean stack and convert more than 70% of the energy in the wood to heat in the form of hot water or steam.
Community-scale heat and power systems are usually based on boilers that convert the heat to hot water or steam for distribution. Larger boilers can produce steam at a high enough temperature and pressure to generate power in a steam engine or turbine. Steam engines and small turbines are usually very inefficient, so other fluids and generating devices such as ORC and Stirling engines are under development (see below for a list of manufacturers). In parts of Europe that have extensive district (community) heating systems, some plants are being modified to generate power using gasifiers with engines or furnaces with ORC and Stirling engines.
Large-scale power generators are usually based on wood boilers that can operate at sufficient steam temperature and pressure to make electricity efficiently. A small power boiler, 10 MW, would be enough to power a sizeable town. These systems are not suitable for small villages with moderate heat and power loads and limited technical expertise.
Biomass in Alaska
The high capital cost and projected operation and maintenance (O&M) costs of CHP systems will likely be feasible only in larger communities with high power demand, high diesel prices, and a way to use the substantial amount of heat from the system. As the technologies are refined and costs are reduced, smaller-scale applications may become feasible.
The sustainability of biomass supplies requires planning and coordination, and it will vary widely by area. Regional facilities that gather and process biomass could become a feasible option for upriver, forested communities to supply fuel to downriver communities. Mobile equipment could be shared by several villages in a region on a rotational basis. Road system communities could also benefit from medium-scale regional facilities.
Manufacturers
There are several options for heating, fewer for power generation. Listed below are some examples of high efficiency boilers and some new designs for small-scale power generation being used or considered for use in Alaska.
The following list includes developers who, at a minimum, have built a prototype device.
| Manufacturer | Device | Website | Location | Level of Development |
|---|---|---|---|---|
| Garn | High efficiency Hydronic Wood Fired Heaters | www.garn.com | Dot Lake, Tanana, Ionia, Homer (private individual) | Commercial 25 years, high fuel efficiency (75.4%), low emissions |
| Chiptec | Gasifier-boiler | www.chiptec.com | Craig | 130 installed, 1 - 30 MMBtu |
| Decton | Chip and sawdust boiler | www.decton.com | Dry Creek, Kenney Lake Regal Saw Mill | Industrial and community use |
| Crorey Renewable Resources | 25 kW gasifier | www.croreyrenewable.com | CCHRC Fairbanks | Prototype in development, not yet delivered |
| UTC Power | 200 kW Purecycle ORC | www.utcpower.com | Chena Hot Springs | Commercial demonstration with geothermal, biomass demo under development |
| Danish Stirling | 35 kW Stirling | www.stirling.dk | Denmark | Demo |
| AgriPower | 100+kW | www.agripower.com | New York | Demo |
Links and Resources
- Biomass Heating Systems Wiki: a link to the Wikipedia article covering biomass heating systems. A brief overview to how biomass technology can be utilized.
- Woody Biomass Utilization: University of California research on the utilization of woody biomass.
