Combustion biomass moisture

There are many interacting parameters and possible feedstock—process—product combinations, but all are not feasible from a practical standpoint eg, the separation of small amounts of metals present in biomass and the direct combustion of high moisture content algae are technically possible, but energetically unfavorable. 

The direct combustion of wood and other plant matter has been a primary energy source in the past. Any type of biomass can be burned to produce heat or steam to turn a generator or perform mechanical work. Direct combustion is used in large power plants that produce up to 400 megawatts. Most direct combustion systems can use any type of biomass as long as the moisture content is less than 60%. Wood and wood residues are commonly used along with a number of other agricultural residues. 

The biomass is fed overbed through multiple feed chutes using air jets to help distribute the fuel over the surface of the bed. Variable-speed screw conveyors are usually used to meter the fuel feed rate and control steam output. Feedstocks such as bark and waste wood are chipped to a top size of 25 mm (1 in) to ensure complete combustion. The bed usually consists of sand around 1 m (3 ft) deep. This serves to retain the fuel in the furnace, extending its in-furnace residence time and increasing combustion efficiency. It also provides a heat sink to help maintain bed temperature during periods of fluctuating fuel moisture content. 

Most biomass gasification systems utilize air or oxygen in partial oxidation or combustion processes. These processes suffer from low thermal efficiencies and low Btu gas because of the energy required to evaporate the moisture typically inherent in the biomass and the oxidation of a portion of the feedstock to produce this energy. 

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

The cost of transporting wood chips by truck and by pipeline as a water slurry was determined. In a practical application of field delivery by truck of biomass to a pipeline inlet, the pipeline will only be economical at large capacity (>0.5 million dry t/yr for a one-way pipeline, and >1.25 million dry t/yr for a two-way pipeline that returns the carrier fluid to the pipeline inlet), and at medium to long distances (>75 km [one-way] and >470 km [two-way] at a capacity of 2 million dry t/yr). Mixed hardwood and softwood chips in western Canada rise in moisture level from about 50% to 67% when transported in water the loss in lower heating value (LHV) would preclude the use of water slurry pipelines for direct combustion applications. The same chips, when transported in a heavy gas oil, take up as much as 50% oil by weight and result in a fuel that is >30% oil on mass basis and is about two-thirds oil on a thermal basis. Uptake of water by straw during slurry transport is so extreme that it has effectively no LHV. Pipeline-delivered biomass could be used in processes that do not produce contained water as a vapor, such as supercritical water gasification. 

Absorption of water has serious implications for any process such as direct combustion that converts absorbed liquid water in the fuel into emitted water vapor in the flue gas, in that it reduces the lower heating value (LHV) of the biomass and requires more biomass per unit of heat released by combustion, an effect also noted by Yoshida et al. (10). Figure 5 shows the loss in LHV and the corresponding increase in biomass that must be delivered to a direct combustion-based biomass operation at 67% moisture level. Werther et al. (11) note some other problems with increasing moisture in the direct combustion of biomass reduced combustion temperature, delayed release of volatiles, poor ignition, and higher volumes of flue gas. These secondary impacts on efficiency and operability of a direct combustion unit are not considered in Fig. 5. 

Since, as shown in Fig. 5, changing the moisture level of wood chips from 50% to 67% increases the requirement for field biomass in direct combustion by 78% for a given output of heat and power, it is evident that water-based pipelining of wood chips cannot be economical for direct combustion, because the increase in field harvest cost associated with the higher biomass requirement is larger than any possible transportation cost saving. For straw, so much water is taken up that the LHV is effectively zero pipeline transport of straw to a direct combustion application would destroy the heating value of the fuel. 

Furthermore, water transport of mixed hardwood and softwood chips causes an increase in moisture level to 65% or greater, which so degrades the LHV of the biomass that it cannot be economical for any process, such as direct combustion, that produces water vapor from water contained in the biomass. The impact on straw is greater, in that moisture levels are so high that the LHV is negative. Pipeline transport of biomass water slurries can only be utilized when produced water is removed as a liquid, such as from supercritical water gasification. 

Wood and other biomass materials are hygroscopic and retain substantial amounts of moisture (15-60%). Hence, some of the thermal energy of the wood is lost in vaporizing this water to steam during combustion. In addition, wood has a lower density than... 

In grate-fired boilers, the biomass is fed in a thin layer, so it is evenly distributed over a sloped stationary, traveling, or vibrating grate. Improved control over the combustion process can be achieved with better carbon conversion. Fluidized-bed combustors are more complex systems, but offer much better control of combustion temperature, improved carbon conversion, and fuel flexibility. Using a boiler to produce both heat and electricity (co-generation) can improve the overall system efficiency to as much as 85 percent. Boiler efficiencies are affected by fuel moisture content, air-fuel ratio, excess air, combustion temperature, and biomass ash content. 

Empty Fruit Bunch. Apart from fiber and shell, EFB is another biomass that can be readily converted into energy. However, this material has only been utilized to a limited extent. This is because there is already enough energy available from fiber and shell. Also due to its physical nature and high moisture content (50-65%), the EFB has to be pretreated to reduce its buUdness and moisture content to below 50% in order to render it more easily combustible (37, 38). 

It is normally not necessary to reduce the water content of high-moisture-content or wet biomass feedstocks for microbial conversion processes. This contrasts with thermal conversion processes such as combustion. Dry biomass burns at higher temperatures and thermal efficiencies than wet biomass. For... 

Many of the large-scale biomass combustion systems for producing heat, hot water, or steam accept biomass fuels containing relatively large amounts of moisture and are operated without much apparent concern for the effects of moisture content of the fuel on the combustion process itself. One of the... 

TABLE 6.2 Typical Moisture Contents and Heating Values of Waste Biomass for Combustion in Fluid-Bed, Grate, and Suspension Firing Units... 

When biomass is combusted under normal conditions, a flame is produced as visible radiation, provided oxidation occurs at a sufficient rate. By use of thermodynamic data, the theoretical temperature at which the products of combustion form under adiabatic, reversible conditions can be calculated. The theoretical flame temperature for the combustion of wood of various moisture... 

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