Combustion, biomass methods

The specific design most appropriate for biomass, waste combustion, and energy recovery depends on the kiads, amounts, and characteristics of the feed the ultimate energy form desired, eg, heat, steam, electric the relationship of the system to other units ia the plant, iadependent or iategrated whether recycling or co-combustion is practiced the disposal method for residues and environmental factors. 

An alternative method of produciag hydrocarbon fuels from biomass uses oils that are produced ia certaia plant seeds, such as rape seed, sunflowers, or oil palms, or from aquatic plants (see Soybeans and other oilseeds). Certain aquatic plants produce oils that can be extracted and upgraded to produce diesel fuel. The primary processiag requirement is to isolate the hydrocarbon portion of the carbon chain that closely matches diesel fuel and modify its combustion characteristics by chemical processiag. 

A popular small-scale thermal biomass conversion method is combustion in grate furnaces. To meet the emission regulations for such a furnace, the operating conditions and design of the furnace have to be chosen carefully. Numerical models, known as computational fluid dynamics (CFD), can support the making of these choices, provided that accurate sub-models for the phenomena occurring in the oven are available. 

A new method to analyse the thermochemical conversion of biomass, in the context of packed-bed combustion, is modelled and verified. The sought quantities of the method are the mass flow and stoichiometry of conversion gas, as well as the air factors of the conversion system and the combustion system. 

This appendix presents a review of experimental work in the field of packed-bed combustion of biomass. It deals with the measurement methods used to analyse the thermochemical conversion of the biomass. This implies that thermochemical conversion studies of coal is outside the scope of this literature study. Wood stove research is not considered in this review either. Of special interest in this survey is the choice of sought physical quantities (target quantities) and measurands of interest in each study and how they are modelled and verified, and if uncertainty analysis is carried out. 

Below, the experimental work in the field of packed-bed combustion of biomass is presented in chronological order. The authors original terminology is used as much as possible. Each work is reviewed with respect to objective, apparatus, fuel, measurement methods, theory, procedure, and results and conclusions. 

The objective of AxelTs [11] experimental study is twofold (1) to develop methods to study the combustion process of a packed-bed of biomass (2) to study the effect of mass flow of air on the combustion process in different conditions with respect to fuel particle size, density, and shape. The results are planned to be applied to computer simulations of packed-bed combustion of wood fuels as well as design data for construction of PBC systems. 

Nine studies regarding the thermochemical conversion of packed-beds of biomass have been reviewed. The review is summarized in Table 7 below. The focus of the survey has been on the theories of the methods applied to measure ignition front rate, conversion rate (combustion rate, burning rate), conversion gas stoichiometry, and air factors. 

An alternative to the above described approaches is the radiocarbon method that allows a distinction of contemporary carbon (from biogenic emissions and combustion of biomass) and carbon from combustion of fossil fuels in particulate carbonaceous matter [15, 41,42]. In contrast to fossil fuels where the 14C isotope is completely depleted, CM emitted from WB shows a contemporary radiocarbon level. Radiocarbon measurements are often combined with measurements of complementary source specific tracers (macro-tracer) for additional information of source impacts [14, 43, 44]. 

Finally the so-called mono- and macro-tracer approaches can be applied for determining source contributions. These methods rely on the fact that a number of chemical compounds can be directly linked to biomass combustion emissions. For example, ambient concentrations of water-soluble potassium, certain PAHs, anhydrosugars and many other tracers have been used as indicators for the impact of biomass burning. When the fractions of one of these tracers in PM and carbonaceous aerosols emitted by wood burning are known (emissions ratios), the contribution of wood burning at a receptor site can be calculated based on the concentration of the considered tracer (mono tracer method). 

Table 1 lists a number of studies carried out in urban sites of the Eastern Mediterranean Basin that have used multivariate statistical methods to quantify the mass contribution of sources of particulate matter. In most of these studies, four or five major source categories have been detected. These categories include road/soil dust, traffic emissions, marine aerosol, fuel oil combustion emissions, biomass... 

Combustion is by far the oldest method of utilizing biomass and has traditionally involved the burning of wood, dung, and other materials. The other... 

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. 

To estimate crude protein content of biomass or other materials, the nitrogen content of the material is measured by Kjeldahl or combustion methods and multiplied by a conversion factor where ... 

Biomass combustion is the most common and historically oldest method of extracting energy from biomass (other than food) either directly, in the form of heat and light from fire, or indirectly through use of this heat to produce steam... 

Another fouling mechanism that can occur is corrosion of boiler tubing and erosion of refractories due to formation of acids and their buildup in the combustion units from conversion of sulfur and chlorine present in the fuel. Fortunately, the amounts of these elements in most biomass are nil to small. The addition of small amounts of limestone to the media in fluidized-bed units or the blending of limestone with the fuel in the case of moving-bed systems are effective methods of eliminating this problem. Other sorbents such as dolomite, kaolin, and custom blends of aluminum and magnesium compounds are also effective (Coe, 1993). 

Emissions from biomass-fueled boilers can be controlled by a variety of methods. The control systems needed depend mainly on the composition of the feedstock. First, good combustion control is essential to maximize combustion and to minimize emissions of unburned hydrocarbons and carbon monoxide. Efficient removal of particulate matter in the flue gases can be achieved by various combinations of cyclonic separation, electrostatic precipitation, agglomeration, and filtration. Removal of acid gas emissions can be achieved by flue gas scrubbing and treatment with lime. There are several approaches to the control of NO, emissions (Clearwater and Hill, 1991). Combustion control techniques include use of staged combustion, low excess... 

The first option is to remove the ammonia from the fuel gas before combustion. The advantage of ammonia removal at this stage is that the volume of the gas leaving the gasifier is much smaller than the flue gas volume. Conventional ammonia removal methods, involving condensation and absorption in water, may be used in biomass IGCC plants. These methods are associated with cooling and potential loss of sensible heat from the gas. Also the absorbed ammonia has to be disposed of. 

In addition the method can also be extended to estimate the likely combustion temperature for a particular fuel. In an ideal combustion system, heat released from biomass combustion is completely converted to gaseous products enthalpy (Eqns 4 to 6) which, in turn, determines the combustion temperature. Specific heat of each gas component is a function of temperature and can be derived as shown below 17). By combining the equations, combustion heat output (Q asproducts) can thus be described as a function of combustion temperature. 

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