Fuel Indexes: A Novel Method for the Evaluation of Relevant ...

Energy & FuelsFor the evaluation of the ash-melting behavior, primarily thesintering temperature (start of ash melting) and the temperaturewindow between the sintering and melting temperaturesare of relevance. It has to be considered that the characteristicash-melting temperatures are valid for the total ash (fuel ash).In real-scale applications, the distribution of certain elements indifferent ash fractions may influence the melting temperaturesof the individual fractions.4. SELECTED RELEVANT FUEL INDEXES AND THEIRAPPLICATION4.1. N Content as an Indicator for the NO x EmissionPotential. In biomass combustion processes, NO x emissionsmainly result from the fuel N, while their formation from thecombustion air (prompt and thermal NO x formation) playsonly a minor role. 13−15 For the pre-evaluation of a new biomassfuel, it is important to know if problems with NO x emissionsexceeding the emission limits have to be expected, whichcannot be overcome by primary measures and make secondarymeasures for NO x emission control [e.g., selective non-catalyticreduction (SNCR)] affordable.On the basis of results of test runs at modern state-of-the-artgrate combustion plants (see section 2.3) equipped with airstagingtechnology, a correlation between the conversion rate offuel N to N in NO x emissions has been derived (see Figure 2),Figure 2. NO x emissions and fuel N converted to N in NO xdependent upon the N content. Explanation: both correlations arestatically significant (p < 0.05).which can be applied to estimate the NO x emissions formedduring the combustion of a certain fuel in grate combustionsystems. Because combustion plants investigated are equippedwith air-staging technology, the index is only valid for state-ofthe-artgrate-fired combustion units with air-staging technology.As seen in Figure 2, the NO x emissions increase with the Ncontent of the fuel. However, there is no linear correlationArticlebecause, as the second diagram shows, with an increasing Ncontent of the fuel, the conversion of fuel N to NO x decreases.The N concentrations in the biomass fuels can be categorizedinto (1) low-N fuels (200 mg N −1 m −3 (related to dry flue gas and 13 vol%O 2 ) have to be expected.4.2. Sum of K, Na, Zn, and Pb as an IndicatorRegarding Aerosol Emissions (Fine Particles Smallerthan 1 μm) and Deposit Buildup. This indicator can beused to evaluate if highly efficient dust precipitators [electrostaticprecipitators (ESPs) and baghouse filter] will be neededto keep the particulate matter (PM) emission limits for a plantusing a specific fuel. It is also an indicator for the depositbuildup on heat-exchanger surfaces. This index is related to theformation of aerosols (particles with a diameter smaller than 1μm = PM 1 ) during the combustion process and does notinclude coarse fly ashes.A part of the semi-volatile and volatile ash-forming elements,such as K, Na, S, Cl, Zn, and Pb, is released from the fuel to thegas phase during combustion. In the gas phase, these elementsundergo homogeneous gas-phase reactions, and later, becauseof supersaturation in the gas phase, these ash-forming vaporsstart to nucleate or condense on the surfaces of existingparticles or heat-exchanger tubes. 16,17Because for most biomass fuels K usually shows significantlyhigher concentrations than other aerosol-forming elements(e.g., Na, Zn, and Pb), the release of K is most relevant for theformation of aerosol emissions.It is reported in previous studies 18,19 that the main part of Kreleased to the gas phase consists of KOH and KCl in the entiretemperature range of 500−1150 °C. Smaller amounts of K arereleased as K 2 SO 4 and K 2 CO 3 in this temperature range. It isevident that there is a number of parameters influencing the Krelease.It has also been reported 20 that ash-forming elements, suchas Ca, Si, and P, may influence the K release to the gas phase toa certain degree. Therefore, well-defined mixtures of K, Ca, andSi (or P) species were heat-treated in a reactor at a constanttemperature (900 and 1000 °C). The main findings of theseexperiments are summarized as follows. The presence of waterin the gas flow was found to significantly enhance the K-releaserate in both the K−Ca−Si and K−Ca−P systems. The K−Ca−Si system shows higher release rates at 1000 °C than at 900 °C.Doubling of the molar Ca/Si ratio in K 2 CO 3 −CaO−SiO 2mixtures increased the K-release rate about 2 times. Thissuggests that it is more likely that SiO 2 reacts with CaO and Kis being released to the gas phase instead of being incorporatedinto silicate structures. For the K−Ca−P system, where K 2 CO 3was used as the K source, it has been observed that, with adecreasing molar Ca/P ratio, the K-release rate significantlydecreases. In the case of K−Ca−P mixtures, with KCl as the Ksource, the Ca/P ratio had no effect on the K-release rate. It hasbeen proven that the sublimation of KCl is less influenced byother elements, whereas the release of K 2 CO 3 is enhanced inthe presence of water vapor because of the reaction to KOH.Straw, Miscanthus, and maize residues are typical fuels withincreased Si and slightly increased P concentrations. Because ofstill existing uncertainties regarding the K release, theEdx.doi.org/10.1021/ef201282y | Energy Fuels XXXX, XXX, XXX−XXX