Most discussions of energy use in paper production and papermaking are confusing because of a failure to define exactly what is meant by energy. It is important to keep track of the form in which energy is used. The major inputs to a paper mill are wood, purchased energy in the form of electricity and fossil fuels, and water. Some components of the wood are used as a raw material and some as by-product fuel. What the industry generally reports is the process energy use at the mill, which is the sum of the purchased energy and the process by-product energy (part of the energy originally in the wood) that is used. (more…)

Air-blown circulating fluidized bed gasifiers are of interest because they produce a good quality, low calorific value (LCV) gas (4–6 MJ/Nm 3 ) and possess a very high carbon conversion efficiency while allowing high capacity, good tolerance to variations in fuel quality, and reliable operation. The high and homogeneously distributed temperatures and the use of particular bed materials, such as dolomite, favor tar cracking. Successful tar cracking can also be achieved using secondary circulating fluidized bed reactors. Also, successful tests on catalytic tar cracking have been performed, for example, by introducing nickel compounds into the gasifier. Sulfur control is made easier because of the significant reduction that can be achieved by adding limestone or dolomite to the gasifier bed. (more…)

Biomass Storage

Biomass storage is required to ensure the continuous operation of the facility. To limit the space required for storage at the plant site, biomass must be stored in relatively high piles. Two main problems associated with fuel storage are decomposition and selfheating. Self-heating increases the rate of decomposition and fire risk, and it encourages the growth of thermophilic fungi whose spores can cause a respiratory condition in humans similar to farmers lung. Some small virgin biomass losses may occur at the storage stage, but they are likely to be negligible. For intermediary storage of the fuel between the pretreatment (e.g., drying and sizing) and gasification stage, storage silos may be used. (more…)

A combination of legislation and technology has helped clean up many of the world’s coal-burning plants. Both developed and developing countries have adopted increasingly stringent environmental regulations to govern emissions from coal-fired power plants. In the United States, all coal-fired power plants built after 1978 must be equipped with postcombustion cleanup devices to capture pollutants before they escape into the atmosphere. Cyclones, baghouses, and electrostatic precipitators filter out nearly 99% of the particulates. Flue gas scrubbers use a slurry of crushed limestone and water to absorb sulfur oxides from flue gas. The limestone reacts with the sulfur dioxide to form calcium sulfate, which may be used to produce wallboard. Staged combustion and low-NOx burners are used to burn coal to minimize NOx formation. Another strategy, selective catalytic reduction, reacts ammonia with NOx over a catalyst to produce nonpolluting nitrogen and water vapor.

Conventional coal-fired power plants capture pollutants from the flue gas after it leaves the boiler. Circulating fluidized bed (CFB) combustors capture most of the pollutants before they leave the furnace. Crushed coal particles and limestone circulate inside the CFB combustor, suspended by an upward flow of hot air. Sulfur oxides released during combustion are absorbed by the limestone, forming calcium sulfate, which drops to the bottom of the boiler. The CFB combustor operates at a lower temperature (14001F) compared to pulverized coal (PC) boilers (27001F), which also helps reduce the formation of NO x .

Precombustion coal cleaning is another strategy to reduce sulfur emissions by cleaning the coal before it arrives at the power plant. Sulfur in coal is present as pyrite (FeS2 ), which is physically bound to the coal as tiny mineral inclusions, and as ‘‘organic sulfur,’’ which is chemically bound to the carbon and other atoms in coal. Pyrite is removed in a coal preparation plant, where coal is crushed into particles less than 2 inches in size and is washed in a variety of devices that perform gravity-based separations. Clean coal floats to the surface, whereas pyrite and other mineral impurities sink. Additional cleaning may be performed with flotation cells, which separate coal dust from its impurities based on differences in surface properties. Precombustion removal of organic sulfur can be accomplished only by chemical cleaning. So far, coal combustion emissions and chemical cleaning has proved to be too costly, thus flue gas scrubbers are often required to achieve near-complete removal of sulfur pollutants.

The tightening of environmental regulations is likely to continue throughout the world. In the United States, for example, by December 2008, it is anticipated that coal-fired power plants will have to comply with maximum emission levels for mercury. Emissions of mercury and other trace metals, such as selenium, are under increasing scrutiny of coal combustion emissions because of suspected adverse effects on public health.

Coal is sometimes combusted with waste material as a combined waste reduction/electricity production strategy. The disposal of waste from agriculture and forestry (biomass), municipalities, and hospitals becomes costly when landfill space is limited. Some wastes, particularly biomass feedstock, are combustible, but their low energy density (compared with coal) limits their use as an electricity production fuel. Blending coal with these fuels provides an economical method to produce electric power, reduce waste, and decrease coal plant emissions. Most wood wastes, compared to coal, contain less fuel nitrogen and burn at lower temperatures. These characteristics lead to lower NO x formation. In addition, wood contains minimal sulfur ( o 0.1% by weight) and thus reduces the load on scrubbers and decreases scrubber waste biomass.

Numerous electric utilities have demonstrated that 1–8% of woody drying biomass can be blended with coal with no operational problems. Higher blends may also be used, but require burner and feed intake modifications as well as a separate feed system for the waste fuel. Cofiring in fluidized bed boilers may avoid some of these drawbacks, but the economics of co-firing are not yet sufficiently attractive to make it a widespread practice.

The moisture content of the feedstock affects the gas composition and the energy balance of the process since gasification is an endothermic process. Water vapor, however, is an essential component of gasification reactions. Therefore, there is a trade-off between the extent of fuel drying and the quality of product gas. Drying of the feedstock to a moisture content of approximately 15% is commonly adopted. Fuel drying is likely to be the most energy intensive activity in the biomass gasification process. Important contributions can be made to the energy balance by using flue gases or steam to dry the biomass. The heat used for drying does not have to be high temperature, and a low temperature level is actually desired because it will prevent the evaporation of undesirable organic components. (more…)

Gasification is a thermo chemical process that has been exploited for more than a century for converting solid feedstocks to gaseous energy carriers. The first gasifier patent was issued in England at the end of the 18th century and producer gas from coal gasification was mainly used as lighting fuel throughout the 19th century. At the turn of the 20th century, the main use of producer gas, obtained essentially from coal, switched to electricity generation and automotive applications via internal combustion engines. The use of producer gas was gradually supplanted by the use of higher energy density liquid fuels and as a result confined to areas with expensive or unreliable supplies of petroleum fuels. (more…)

Thermo chemical processing of biomass yields gaseous, liquid, and solid products and offers a means of producing useful gaseous and/or liquid fuels. Biomass gasification is a total degradation process consisting of a sequence of thermal and thermo chemical processes that converts practically all the carbon in the biomass to gaseous form, leaving an inert residue. The gas produced consists of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4), and nitrogen (N2) (if air is used as the oxidizing agent) and contains impurities, such as small char particles, ash, tars, and oils. The solid residue will consist of ash (composed principally of the oxides of Ca, K, Na, Mg, and Si) and possibly carbon or char. (more…)

Estimation of the future technical potential of biomass as an energy source is dependent on assumptions with respect to land availability and productivity as well as conversion technologies. With the emergence of energy crops as the major source of biomass fuel, land use conflicts, especially in relation to food production, may arise. However, with efficient agricultural practices, plantations and crops could supply a large proportion of energy needs, with residues playing a smaller role without compromising food production or further intensifying agricultural practices. (more…)

The quantification of the actual reduction in green house gases sourcess emissions resulting from the substitution of fossil fuels with energy from waste biomass requires a complete lifecycle assessment (LCA). A systematic framework for estimating the net Green House Gases emissions from bioenergy systems and comparing them against the fossil fuel reference system that it would replace has been developed. The major considerations of the life cycle assessment approach to quantifying the greenhouse impacts of bioenergy are as follows: (more…)

The gradual change in the energy consumption pattern of the United States from 1860 to 1990. In the mid-1800s, biomass, principally woody biomass, supplied over 90% of U.S. energy and fuel needs, after which energy biomass consumption began to decrease as fossil fuels became the preferred energy resources. For many years, a safe illuminant had been sought as a less expensive substitute for whale oils. (more…)