Encyclopedia of Renewable Energy. James G. Speight
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– Land Requirements
If biomass is to play a major role in the energy supply of the world, energy farming is required. However, it is uncertain whether fallow land will be available to produce biomass for energy instead of food. The main stimulus in the European Union and the United States for introducing non-food crops in agriculture is the surplus production of food. Worldwide food demand is expected to increase considerably, but the amount and type of food that will be required depend on many factors. Population growth, as well as increasing incomes, may result in shifts to more protein-rich diets; both lead to increased demand for food crops. Loss of agricultural land caused by erosion, limited water resources and the increased demand for land for urbanization, infrastructure, and also preservation of nature all limit the possibilities for energy crops.
The availability of land for energy farming, however, does not depend only on the availability of agricultural land. Degraded land, for instance, which is not really suitable for food crops can represent a large potential for energy farming. At the same time, energy crops may help restore these damaged soils. Yields, however, will generally be lower than from good quality land.
Land that is needed to grow energy crops competes with land used for food and wood production unless surplus land is available. Furthermore, the demand for biomass (especially woody biomass) as a renewable energy source has increased continuously in recent years, especially for the direct provision of heat and power generation. However, the biomass potential is limited due to technical constraints and ecological restrictions, as well as the sustainability principles of land management, leading to a predicted supply shortfall.
To avoid competition with food and feed production land, there is the need to establish marginal sites with low soil quality, but sufficient water availability. Tree species capable of stump sprouting, such as poplar, willow, and black locust, can attain high biomass increments on cropland with low soil quality, but sufficient water supply due to their low nutrient requirements and deep-reaching roots. For example, sites for biomass production can be established on cropland with a low depth-to-groundwater table that poplars or willows with deep-reaching roots can utilize, but is inaccessible to annual cultures. The annual crop yield is typically low on croplands with low soil quality. Therefore, these lands can be seen as agricultural marginal sites, where the combination of yields and prices barely covers the cost of production.
There is the need to conduct an assessment of the potential for land use based on (i) hydrological and climatic conditions, (ii) restrictions by various boundaries, from the technical, ethical, and ecological standpoints, and (iii) energy potential, considering the effect of global climate change.
See also: Biomass.
Biomass – Liquefaction
Biomass liquefaction is the conversion of solid (cellulosic) biomass materials to oil. During the mid-to-late 1980s, commercial interest of the thermochemical conversion of biomass focused on liquefaction. Unlike the initial gasification studies, the preliminary liquefaction studies utilized woody biomass, or lignocellulose material, as the feedstock. Woody biomass was considered superior to that of corn stover due to its potentially lower cost and greater availability.
Pyrolysis is the process of heating biomass in the lack of oxygen or in the presence of a limited amount of oxygen. The pyrolysis oil can be used directly as a fuel or as an intermediate for production of chemicals.
Fast pyrolysis is a thermal decomposition process operating at moderate temperatures (450 to 600°C, 840 to 1,110°F), with high heat transfer rates to the biomass particles and a short residence time. Under these conditions, organic vapors, gases, and char are produced. A short residence time is required to obtain the maximum yield of the liquid.
The vapors are condensed to produce pyrolysis oil (often referred to as bio-oil). Yields of liquid products as high as 79% of the initial dry weight of the biomass can be achieved. Generally, the process produces little or no waste and either the pyrolysis gas or charcoal is used to heat the reactor and the other can be used to supplement the other in heating, dry the feedstock, the charcoal can be sold as a byproduct, or the pyrolysis gas can be used to fuel a gas engine.
Pyrolysis oil is greenhouse gas neutral, does not produce SOx (sulfur dioxide) emissions and produces approximately half of the NOx (nitrogen oxide) emissions compared to fossil fuels.
See also: Biofuels, Biomass, Biomass – Liquefaction, Biomass To Liquids, Torrefaction.
Biomass Power
Biomass power is carbon-neutral electricity generated from renewable organic waste that would otherwise be dumped in landfills, openly burned, or left as fodder for forest fires. When burned, the energy in biomass is released as heat.
See also: Biomass Energy, Biomass to Energy.
Biomass – Power Facility
Feedstock requirements for a biomass power facility are dependent upon the capacity of the facility and, to a lesser extent, the efficiency of a specific technology. Dramatic reductions in demand, on a normalized basis, are achievable with increased size of the facility. A 5-MW direct combustion (stoker) power plant has a much higher heat rate than a larger facility. Indeed the larger plant may be approximately 50% more efficient than the smaller installation. The major reason for the higher efficiencies at larger sizes is the increased temperature and pressure that can be economically accommodated in the big facilities to supply larger turbines.
Fuel characteristics greatly affect the combustion process and therefore the decision process for choosing combustion technologies. The most common problems associated with the direct combustion of wood are boiler slagging and fouling, erosion and corrosion, combustion instability, and particulate carryover.
Fuel characteristics that should be analyzed include heating value, moisture content, ash content, sodium and potassium quantities, particle size distribution, ash fusion temperature, and sulfur content. Physical fuel characteristics such as density and particle size affect combustion as well as material handling considerations. Changes in fuel density could cause combustion to occur in the wrong place in the boiler, upsetting the heat transfer scheme and therefore the boiler efficiency.
Biomass Properties
The properties of biomass that have a significant bearing on its thermal conversion are its relatively high moisture, oxygen, hydrogen, and volatile matter content, and low heating value. The high oxygen and hydrogen contents account for the high proportion of volatile matter and consequent high yields of gases and liquids on pyrolysis. A relatively high water yield results from the high oxygen concentration in biomass, and which consumes considerable hydrogen. Consequently, the advantages of the high H/C ratio associated with biomass are not reflected in the products to the extent that might be expected. In fact, pyrolysis gases can be deficient in pure hydrogen and pyrolysis liquids are highly oxygenated, viscous tars.
An additional and significant source of water vapor in biomass gases is the high moisture content of the source materials. In countercurrent flow schemes such as the Lurgi moving bed gasifier, this water is evolved in the relatively low temperature drying and pyrolysis zones and does not partake in gas phase or carbon-steam gasification reactions.
On the other hand, in fluidized bed systems, the moisture is evolved in the high- temperature well-mixed reaction zone and therefore does participate in the reactions. If the system is directly heated and air-blown, the additional heat required to evaporate the water will result in more nitrogen being introduced, and more carbon dioxide being produced, so reducing the calorific value of the product gas. As the gas from air-blown processes is, in any case, a low-calorific value product, this factor is probably of little consequence other than with very wet feedstock. In oxygen-blown systems, however, the additional pure oxygen required and higher carbon dioxide content of the medium calorific value off-gas