Heating Buildings and Business Operations with Biomass Fuel (E3044)

Heating Buildings and Business Operations with Biomass Fuel (E3044)

Significant cost savings may be possible when locally produced biomass is used to heat large buildings, farm operations, and heat-intensive commercial ventures such as food processing, greenhouses, and fuel alcohol and biodiesel production.

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Abstract

Significant cost savings may be possible when locally produced biomass is used to heat large buildings, farm operations, and heat-intensive commercial ventures such as food processing, greenhouses, and fuel alcohol and biodiesel production. Biomass fuels in this context are solid plant-derived products, including grain, pelleted plant material and wood chips, which are alternatives to conventional fossil fuels. This guide introduces the steps involved in planning the installation of a biomass-fueled heating system that will safely and perhaps economically meet the needs of operators of larger scale heating systems in agricultural, institutional and commercial applications. Critical factors influencing heating system performance include fuel quality, furnace design and exhaust gas venting options to reduce airborne pollution.

Introduction

Common biomass heating fuels include cordwood, coal, corn kernels, cherry pits, wood chips and wood pellets made from compressed sawdust. Less common biomass fuels include switchgrass, salix (short-rotation forestry willow), sugar beet pulp, ethanol distiller’s grain, oilseed meal from biodiesel processing, wastepaper, livestock manure and cropland residues such as corn stover.

Pros of biomass-fueled systems

• The processing and transportation of locally produced biomass fuel, and the installation, operation and servicing of biomass-fueled heating systems provide new employment opportunities.

• Use of locally produced biomass fuel reduces the need to import costly fossil fuel. Biomass fuel dollars remain at home, strengthening the local economy.

• Biomass fuel may be more economical than propane, electricity, fuel oil and, in some cases, natural gas. The cost benefit of biomass heating fuel may be significant in large-scale furnaces for commercial or institutional applications.

• Current fossil fuel prices are volatile. This makes it difficult for managers of large-scale heating systems to predict energy costs from one year to the next.

• Properly engineered and maintained furnaces burn biomass cleanly and safely with minimal airborne pollution.

• Ash produced from biomass-fueled furnaces can be used as a plant nutrient source for growing more biomass.

• The productivity potential of Michigan soils and climate creates great potential for the production of solid fuel feedstocks within relatively short distances.

Cons of biomass-fueled systems

• Biomass-fueled heating systems tend to have higher capital and maintenance costs than fossil-fueled systems.

• To ensure safe furnace operation with minimal pollution, vigilance is needed to ensure that biomass producers provide clean, properly sized and contaminant-free fuel. The heating system operator must maintain a rigorous fuel quality control program.

Cons of biomass-fueled systems

• Biomass-fueled heating systems tend to have higher capital and maintenance costs than fossil-fueled systems.

• To ensure safe furnace operation with minimal pollution, vigilance is needed to ensure that biomass producers provide clean, properly sized and contaminant-free fuel. The heating system operator must maintain a rigorous fuel quality control program.

• Biomass-fueled heating systems occupy more floor space than fossil-fueled systems. Additional space is needed to store biomass fuel.

• Some biomass fuels may not be pricecompetitive with natural gas and other fossil fuels.

• Neighboring citizens and/or governmental units may have a negative perception of biomass-fueled furnaces, despite the fact that these furnaces, when properly maintained, are environmentally friendly. A strategy to inform the local public and build local support should be developed prior to furnace installation.

• Biomass pellet fuel is more expensive than raw, unprocessed biomass (e.g., chipped wood waste) because of the costs associated with pellet manufacturing. Some large-scale furnaces are capable of burning both pelleted and nonpelleted biomass fuel. Residential-scale furnaces are generally not equipped to burn raw, unprocessed biomass such as wood chips. Some residential furnaces can burn wood pellets only. Other residential units are capable of using several fuels, including wood pellets, corn kernels, cherry pits and other aggregated solid fuels.

• Because biomass fuels generate less heat per unit of volume than fossil fuels, transportation distances between producers and heating systems are critical for biomass systems to be economically viable.

• Dangerous airborne emissions are possible without the proper furnace design. Some outdoor wood boilers have shown a need for exhaust cleaning and engineering improvements.

• The Michigan Department of Environmental Quality (MDEQ) requires an air permit for all non-residential biomassfueled heating systems. Air permits are not currently needed for residential installations. The cost of these permits may be substantial.

Operations Issues

Energy price comparison

Heating fuel prices are expressed in various and sometimes confusing units. The following example provides a convenient method of comparing energy prices from different sources. Let’s say we wish to calculate the annual fuel cost for a 2,000-square-foot residence in Michigan. The residence will require, on average, a total of 85.3 million British thermal units (Btu) of heat energy throughout a typical Michigan winter. The annual heating bill, H, is calculated as such: H=85.3A/BC

In this example, A = Michigan average retail energy price per pricing unit, B = heat content of fuel in million BTU per pricing unit and C = furnace efficiency factor. As shown in Table 1, the bill varies greatly, with coal as the least expensive option, followed by wood chips, natural gas, oak wood, corn kernels and wood pellets. Heating oil, propane and electricity are the most costly alternatives. Coal is not advised as a heating fuel unless the furnace is designed to eliminate hazardous airborne emissions (sulfur, mercury) that commonly result from burning coal.

Wood furnace efficiency varies from one furnace design to the next. In Table 1, a low efficiency of 0.5 is estimated for fireplaces and woodstoves that are not equipped with advanced heat recovery systems such as a catalytic converter or a dual-stage combustor. Without this equipment, much of the energy available in wood fuel is wasted in the form of heat and smoke exiting the chimney.

Prices for corn, fuel oil, propane, natural gas, electricity and wood pellets were obtained from the U.S. Department of Energy (http://www.eia.doe.gov/neic/ experts/heatcalc.xls). The price of wood chips was obtained by a private communication with a Michigan supplier in November 2007.

Examples of biomass installations

Figure 1 illustrates the components of an institutional-scale solid-fuel combustion system designed to burn biomass. The system is made up of five primary components: the fuel storage bin, the fuel handling system, the combustion system, a computerized control system and a secondary fossil-fueled boiler. The facility is designed to accept semitrucks that deliver wood chips to the storage bin. The fuel handling system consists of belt conveyors, safety shutoff switches and covered troughs powered to move the chips from the storage bin into a metering bin. The metering bin is unloaded by an auger system that automatically regulates the flow of fuel into the combustion system. A computerized control system regulates the main components of the heating system, including the augers, belt conveyors, blowers, exhaust gas monitors, emergency shutdown system and fire suppression devices.

A secondary fossil-fueled boiler is located adjacent to a biomass-fueled boiler. As a general rule, biomass-fueled furnaces are capable of high thermal efficiencies only when operated at maximum power. As a result, institutional-scale biomass-fueled systems are often shut down during low heat demand periods, and the secondary fossil-fueled boiler automatically turns on.

Maker (2004) published an extensive guide to help facility planners considering the installation of wood-chip-fueled heating systems for institutional or commercial applications. That reference contains an excellent discussion of financial strategies underlying the planning and construction of institutional-scale biomass heating units.

Another useful reference is the industry journal Bioenergy International, which provides engineering and economic details of the rapid growth of the European biomass heating fuel industry (http://www.bioenergyinternational.com). Focusing mainly on the manufacture of fuel pellets from wood and other biomass materials, the journal chronicles the growing number of European installations of large-scale boiler systems for electric power production, municipal district heating, institutional and commercial heating, and so on. When examining these and other information sources, be sure you have a clear understanding of any subsidy programs (e.g., government incentives), which may not be available for your system.

Definitions

The British thermal unit (Btu) is defined as the energy to raise the temperature of 1 pound of water by 1 degree Fahrenheit. As a rough approximation, one Btu is the energy produced by burning a single wooden match. Burning a single corn kernel (15.5 percent moisture) produces approximately 5.24 Btu of energy. Earlier it was mentioned that heating an average 2,000-square-foot residence throughout a typical Michigan winter requires 85.3 million Btu. To produce this much energy would require burning approximately 85.3 million matches or 85,300,000 / 5.24 = 16,278,626 corn kernels (12,540 pounds or 224 bushels).

Heat of combustion is defined as the amount of combustion energy available per pound of fuel. For example, the heat of combustion of 15.5 percent moisture corn kernels is 6,808 Btu per pound or 5.24 Btu per corn kernel (approximately 1,300 kernels per pound). Propane has roughly twice the heat of combustion: 15,000 Btu per pound of fuel.

An important consideration for operators of large-scale heating systems is the overall furnace efficiency. This is the percentage of available power that is transferred from the fuel to the surroundings as useful power. The two main components of overall efficiency are thermal efficiency and combustion efficiency.

Low thermal efficiency occurs when too much hot flue gas exits a chimney into the atmosphere. As the exit gas temperature increases, the power loss increases and the thermal efficiency decreases. The higher the flow rate of the exit gas, the greater the power loss and the lower the thermal efficiency. Furnaces with high thermal efficiency are designed to transfer maximum power from the flue gas to the surroundings.

In contrast, combustion efficiency refers to how completely a furnace burns fuel. It is important to note that complete combustion fully transforms biomass fuel into the flue gases carbon dioxide (CO2) and steam (H20), both of which are transparent to the naked eye. In other words, high combustion efficiency is indicated when a chimney emits colorless gas. Low combustion efficiency occurs under oxygen-depleted conditions, and the fuel does not fully transform into CO2 and steam. Instead, the fuel decomposes into carbonaceous ash and organic gases, which are smoky looking. Such incomplete combustion fails to extract the maximum power available in the fuel. This problem is corrected by raising the oxygen concentration of the furnace supply air to so-called stoichiometric conditions so that the fuel fully combusts into CO2 and steam. Such complete combustion produces maximum power per unit of weight of fuel consumed.

Electric furnaces are typically 100 percent efficient. Each Btu of electric energy supplied to an electric furnace typically converts to a Btu of useful building heat energy. However, the initial generation of electricity at a commercial coal or natural gas-fueled electric power plant is only about 40 percent efficient. Therefore, using electricity for building heat is like burning coal or natural gas in a 40 percent efficient furnace.

Furnaces that are fueled with propane, natural gas or biomass are usually limited to 80 to 85 percent overall efficiency because a certain amount of latent heat is needed to ensure that steam in the flue gas is removed from the exhaust stack. The only way to overcome this is through the use of a moisture-condensing system to extract additional heat from the steam. This adds significant cost with only a small gain in efficiency (5 to 9 percent). A well-designed biomass furnace will extract the most heat possible from the flue gas without causing water condensation in the exhaust stack. Let us return to the hypothetical example above, where a 100 percent efficient furnace burns 636 corn kernels per minute, producing 200,000 Btu per hour of useful power. It turns out that a more realistic overall efficiency would be up to 85 percent for a typical corn-burning furnace. Such a furnace burning 636 corn kernels per minute would produce a total of 0.85 x 200,000 = 170,000 Btu per hour of useful power.

Currently, there is no reliable method of estimating the overall efficiency of a biomass-fueled furnace by simply visually inspecting the furnace design. Accurate determination of overall furnace efficiency requires a trained professional to perform calculations based on measurements of exhaust gas flow rate and temperature. Before purchasing a biomass-fueled furnace, it is important to ask the manufacturer to verify its overall efficiency.

 

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