Improving the Cofiring Process of Wood Pellet and Refuse Derived

It is also pelletized in order to offer all the advantages of this pretreatment and thereby make its adaptation to the boiler-stove plant easier. This...
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Energy & Fuels 2008, 22, 2121–2128

2121

Improving the Cofiring Process of Wood Pellet and Refuse Derived Fuel in a Small-Scale Boiler Plant D. Patiño, J. Moran,* J. Porteiro, J. Collazo, E. Granada, and J. L. Miguez ETS Ingenieros Industriales, UniVersidad de Vigo, Lagoas-Marcosende, s/n 36200 Vigo, PonteVedra, Spain ReceiVed February 8, 2008. ReVised Manuscript ReceiVed March 4, 2008

The aim of the present work is to study the viability of a cofiring system in a small-scale biomass plant using a mixture of wood pellet and refuse derived fuel (RDF pellet). RDF comes from municipal solid waste, and a valorization process via combustion is a useful way of solving the problem of the accumulation of nonrecyclable wastes. In this paper, the characterization of both fuels is carried out. In addition, a new procedure for measuring emissions, based on artificial dilution, is presented, allowing for the measurement of high peaks of certain species, such as CO, forthcoming in the cofiring process. Improving the overall process is necessary to comply with EU rules on biomass emissions. Accordingly, two approaches are studied; the use of a secondary air supply and the recirculation of gases. The results obtained indicate a better performance, thereby allowing for the application of the process to a real system (nonexperimental one).

1. Introduction Although biomass was traditionally one of the more important energy sources, its employment was overtaken by the convenience of fossil fuels. However, clean and renewable energies have begun to recover their lost standing, helped by growing environmental awareness and the development of sustainable energy policies.1,2 The definition of biomass includes all those materials of organic origin whose generation is recent, as well as the biodegradable part of waste and even decommissioned rocket fuels.3 Several current literature references and databases4,5 mention the problem of eliminating waste. The high standard of living in developed countries and the increase in the world’s population (especially in densely populated areas) have aggravated this problem. One of the best ways to eliminate the nonrecyclable fraction is the valorization process in which energy is released, albeit with certain drawbacks in the form of emissions. Among all the thermochemical processes that use biomass (gasification, liquefaction, etc.), one of the most common in the household sector is combustion in small boilers. In Europe, the share of energy production in the domestic heating sector in 2004 was around 18% of the total,6 which is a significant slice of overall fuel consumption. The substitution of fossil fuels in applications of this kind is one of the major goals in the * Corresponding author. E-mail: [email protected]. Fax: +34 986818624. (1) Kyoto Protocol to the United Nations framework ConVention on Climate Change; United Nations; New York, 1998. (2) Green paper. A European Strategy for Sustainable, CompetitiVe and Secure Energy; Commission of the European Communities: Brussels and Luxembourg, 2006. (3) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17–46. (4) Municipal waste generation; CSI 016, European Environmental Agency, Nov 2005. http://www.eea.europa.eu/themes. (5) Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2005; United States Environmental Protection Agency: Washington, D.C., 2006. (6) EN16 Final Energy consumption by sector; European Environmental Agency, 2007. http://themes.eea.europa.eu/Sectors_and_activities/energy/ indicators.

field of biofuels in the short-medium term.7 Considering all existing combustion technologies, those which are more appropriate for their simplicity and cost are fixed bed devices. The aim of the present work is to study the viability of a cofiring system with wood biomass and refuse derived fuel (RDF) in a fixed bed boiler. The partial and occasional substitution of the usual fuel by a cheaper RDF pellet in a commercial boiler could reduce the operational expenses for the owner and help to reduce the problem of eliminating waste. References to cofiring coal and municipal solid waste (MSW) in several different boilers8,9 vouch for the viability of similar systems, but its behavior on a small scale has not been thoroughly studied. 2. Experimental Details 2.1. Wood Pellet. Wood pellet is the most widely used fuel in biomass stoves and boilers. Using a pelletizing process resolves some of the most typical problems of biomass fuels: transport and storing costs are minimized, handling is improved, and the volumetric calorific value is increased. The pellet used in this work is a commercial one and it is manufactured from forest wood. Its composition can be seen in Table 1. The high content of volatiles (75%) in the biomass will determine the fireplace design because a large volume is required to develop the flame completely. The high oxygen content (40%) of lignocellulosic fuels will also reduce the need for combustion air. Low ash content (0.7%) provides a clean and stable combustion that is assisted by the use of lignin as the only agglutinant and a reasonably low content in sulfur and chlorine that will prevent the formation of harmful emissions. Although the pellets’ mean lengths vary from 8 to 12 [mm] and their diameters are constant, as with other solid fuels, they have a wide length distribution due to their manufacturing and handling processes. The standard simplified hypothesis of identical particles will give rise to significant errors. That is the main reason why a granulometric characterization is proposed. Several distributions (7) Nussbaumer, T. Energy Fuels 2003, 17, 6. (8) Changqing, D.; Jin, B.; Zhong, Z.; Lan, J. Energy ConVers. Manage. 2002, 43, 2189–2199. (9) Frankenhaeuser, M.; Hiltunen, M.; Manninen, H.; Palonen, J.; Ruuskanen, J.; Vartiainen, T. Chemosphere 1994, 29, 9–11, 2057–2066.

10.1021/ef800093c CCC: $40.75  2008 American Chemical Society Published on Web 04/26/2008

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Table 1. Properties of Wood Pellet Proximate Analysis [% wb] moisture volatiles fixed carbon ash (550 °C)

8.5 74.68 16.2 0.62

Ultimate Analysis [% db] carbon hydrogen oxygen nitrogen sulfur chlorine ash

51.6 5.3 42.10 0.22 m) m ) exp MT µ

k⁄3

[()]

(1)

Where, µ and k are the adjustment coefficients obtained experimentally. The final distribution is presented in Table 2 which is taken from the work of Porteiro11 and the best-fit curve is shown in Figure 1, here also compared with the RDF pellet curve. For wood pellet, k ) 2.185 and µ is the characteristic mass that is used to calculated λ ) 11.77 [mm] ≈ 1.96dp which represents the characteristic distribution size. The difficulty in handling the smallest particles (dust) was overcome by creating an F0 family, which is not included in the analysis. Y [-] is the mass fraction of (10) Gbor, P. K.; Jia, C. Q. Chem. Eng. Sci. 2004, 59, 1979–1987. (11) Porteiro J. Desarrollo de un modelo estático y dinámico de combustión de partículas de biomasa en lecho fijo y contraste experimental. Aplicación a una caldera de baja potencia. Ph.D. Thesis, University of Vigo, Spain, 2005.

every group; it represents the probability of finding, in a random sample, one pellet with the characteristics of the group. The axis l/d shows the length to diameter ratio. Despite all these problems, the pelletization process is the most convenient pretreatment for solid biomass.12 The high bulk density and smaller size of pellets favor the automation of the feeding system, which is very important in small-scale boilers as their efficiency improves with an increase in power and automation level, which enhances control during transitory periods.13 2.2. RDF Pellet. The other fuel studied in this work is a noncommercial one, made of municipal solid waste (MSW), and is in an experimental phase. It is also pelletized in order to offer all the advantages of this pretreatment and thereby make its adaptation to the boiler-stove plant easier. This refuse derived fuel (RDF) is obtained following the scheme in Figure 2. The first step is the selection from MSW, where noncombustible materials (glass, wire, etc.) are removed. In the next step, the sample is dried and ground until the compacting phase is reached and plastic is used as agglutinant. A small addition of water is necessary to increase compaction. The more significant properties of this fuel are its higher ash content (14%) and ultimate analysis, which is quite similar to that of wood pellets, except for the fact that the chlorine concentration is approximately 3%. Table 3 shows the fundamental properties of RDF. Regarding granulometry, RDF has a higher dust content. Table 4 illustrates the granulometric analysis fitted to a Rosin-Rammler distribution. The experimental adjustment coefficients are µ ) 1.53 [g] with λ ) 14.65 [mm] ≈ 1.127dP and k ) 6.02 (continuous line in Figure 1). Mean diameter is 13 [mm], and F4 is by far the largest group. These particles, with such a low length to diameter ratio and with such a high ash content, will behave very differently in the burner bed. Although a large range of correlations is available to predict the high heating value (HHV) of the fuel and since reliable experimental data have not yet been obtained for RDF, eq 2, proposed by Channiwala,14 has been considered suitable. Likewise, the lower heating value (LHV) is achieved by eq 3, eliminating the moisture content of the fuel and the potential formation of water from the hydrogen. Both equations employ the elements in percentages. HHV ) 349.1C + 1178.3H + 100.5S - 103.4O - 15.1N 21.1A LHV ) HHV - [5.828(9H + H2O)]4.186

[ kgkJ ]

[ kgkJ ] (2) (3)

(12) Miguez, J. L.; Porteiro, J.; Moran, J. C.; Granada, E.; Lopez Gonzales, L. M. Description of a pilot lignocellulosic pellets stove plant. 6th European Conference on Industrial Furnaces and Boilers INFUB, Estoril, Portugal, April 2–5, 2002. (13) Obernberger, I.; Thek, G. Recent developments concerning pellet combustion technologies. A review of Austrian developments. Pellet 2006; Jönköping, Sweden, May 30–June 1, 2006. (14) Channiwala, S. A.; Parikh, P. P. Fuel 2002, 81, 1051–1063.

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Figure 2. Manufacturing process of RDF pellet. Table 3. Properties of RDF Pellet Proximate Analysis [% wb] moisture volatiles fix carbon ash (550 °C)

13.22 65.39 7.37 14.02

Ultimate Analysis [% db] carbon hydrogen oxygen nitrogen sulfur chlorine ash

43.71 5.32 34.00 0.50 1.8

0.325 0.975 3.9 9.75 16.25 21.45 26

0.026 0.064 0.196 0.426 0.134 0.078 0.049

2.3. Small-Scale Boiler. The experimental part of the work has been carried out in a small-scale pellet boiler-stove plant (∼20 kW) that has been thoroughly described in previous works.12,15,16 The combustion technology employed is fixed bed, where primary air is fed from the bottom side of the burner, whereas the fuel is fed from the upper side (drop chute) in a countercurrent system (overfed

system). Efficiency and emissions in this kind of boiler are largely determined by the automation level (control and regulation).13,17 All data acquisition and control in this plant is centralized in a computer (SCADA), and the feed system has a special design15 that improves the control of the cofiring process (Figure 3). Pellets are fed by a conveyor belt from a two-compartment hopper that prevents the fuels from mixing and allows for a precise and continuous control of the feeding ratio of both fuels. Several improvements have been made to this plant to increase the number of tests that can be made. The plant offers the possibility of preheating the primary air with flue gases before they enter the chimney. Furthermore, part of the combustion air can be provided through a secondary supply, located laterally above the combustion bed (Figure 4). A fraction of the flue gases can also be recirculated through the same pipes. 2.4. Methodology. Tests in this type of boiler are somewhat laborious and time-consuming, due to the large number of variables and the difficulty in overall control. Conclusions reached in previous papers16,18 have identified the following as the main process (15) Granada, E.; Lareo, G.; Miguez, J. L.; Moran, J.; Porteiro, J.; Ortiz, L. Biomass Bioenergy 2006, 30, 238–246. (16) Moran, J. C.; Granada, E.; Porteiro, J.; Miguez, J. L. Biomass Bioenergy 2004, 27, 577–583. (17) Fiedler, F. Renewable Sustainable Energy ReV. 2004, 8, 201–221. (18) Wiinikka, H.; Gebart, R. Energy Fuels 2004, 18, 4.

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Figure 4. Photograph and diagram of the fireplace.

variables: excess air ratio (n), total mass fuel rate (mP), and ratio of mixed fuels (in this case %RDF). Moreover, as and when required, a secondary air supply (%Sec) or recirculation (%Rec) should be used. Another key variable affecting boiler efficiency is water temperature, which must be maintained as stable as possible to render the different results comparable. To define all the possible values of the variables, the system’s operating limits must be set. Preliminary tests were carried out accordingly. Values of mP will vary from 0.5 [g · s-1], which corresponds to the lower limit admitted by the feeding systems, to 1.7 [g · s-1], determined by the maximum capacity of the plant’s energy dissipation. In high power mixtures, the maximum %RDF fed is 30% because its higher ash content jeopardizes system stability. Regarding the excess air ratio, limit values were chosen according to previous tests: 1.4 and 1.8. No combustion is stable below this lower limit, while the upper level is enough to guarantee a good process. Neither secondary nor recirculation should exceed 30%, which is the optimum percentage found in earlier studies.19 The main parameters delimited by the rules of combustion in small-scale biomass boilers are usually efficiency and NOx and CO emissions. Boiler efficiency is defined as the ratio between the energy used and the energy supplied (eq 4), and combustion efficiency, as the ratio between the total energy released and the energy supplied (eq 5), although part of the released energy is dissipated by the flue gases and an adiabatic process is supposed due to the boiler insulation so heat transfer to the room was neglected. ηboiler )

˙ H2OCe∆T1 m n

∑ m˙

(4)

PiLHVi

i)1

ηcomb )

˙ FGCP(T)∆T2) ˙ H2OCe∆T1) + (m (m n

∑ m˙

(5)

PiLHVi

i)1

2.5. Emission Measuring and Correction. Regarding emission measurements, one of the most important characteristics in granular fuel feeding systems is their irregularity. The mass of an average single wood pellet particle is around 0.3–0.4 [g] and the power range in small boilers can vary from 10 to 100 [kW], which requires a net mass flow rate from 0.6 to 6 [g · s-1]. In such cases, where individual mass is not insignificant with respect to total mass, the feeding process is inherently intermittent, whatever feeding system is used, although the fan-fed air supply is continuous. Consequently, (19) Moran, J. C.; Miguez, J. L.; Granada, E.; Porteiro, J.; Lopez Gonzalez, L. M. Energy Sources, Part A 2006, 28, 1135–1148.

although average mass and air supply are constant and predefined in the test, variable instantaneous excess air ratio is achieved. This ratio will be higher or lower than average depending on the mass recently dropped. This dynamic behavior, so specific to fixed beds, will generate sporadic emission peaks, as can be seen in Figure 5. This pattern illustrates the misleading and insufficient information that the average values provide. The aforementioned problem, together with the poor combustion process observed in previous experiments, generates emission peaks, which are so high (over 30 000 ppm at 6% O2) that measurement with a standard analyzer is often very difficult due to CO sensor saturation. The standard way to overcome this problem is to produce an artificial dilution that reduces the total emission concentration and prevents saturation of the sensor. The solution adopted is shown in Figure 6. The analyzer extracts a sample of flue gas with an internal vacuum pump and eliminates particles and moisture with filters. In a system that is not perfectly sealed, the sample will necessarily be diluted with ambient air, thereby minimizing the total concentration of pollutants and consequently allowing for their measurement. Thus, the theoretical combustion equation must be solved to eliminate this dilution and obtain the real concentration. The combustible part of biomass fuels can be expressed generally as CHYOZNX, with a minor presence of other species. Equation 6 is the simplified reaction of the combustion of a mixture of two fuels with excess air ratio (n), dilution (D), and main pollutant formation. aCHYOZNX + bCHY′OZ′NX′ + ns(O2 + 3.76N2) + D(O2 + 3.76N2) f cCO2 + dCO + eH2O + fN2 + gNOx + hO2 (6) Expression 6 has seven unknowns (D, c, d, e, f, g, and h) and four possible species balances. In order to solve the problem, a constant dilution hypothesis must be adopted. Although certain oscillations in the process are unavoidable, measurements are carried out once the stable operation of the plant has been achieved, so averaged data are considered to be the valid ones. It is necessary to bear in mind that the averaged values of “a” and “b” are already known. Having taken all these factors into account, with the aid of eqs 7-9, and with the concentrations provided by the gas analyzer with dilution, the system of equations can be solved. [CO] )

d c+d+f+g+h

(7)

[NOx] )

g c+d+f+g+h

(8)

[O2] )

h c+d+f+g+h

(9)

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Figure 5. CO and NOx [ppm] instantaneous emissions. Conditions of the test were as follows: mP) 0.5 [g · s-1], n ) 1.4, %RDF ) 20% with primary air only. [O2_ref] ) 6%.

Figure 6. Diagram of flue gas analyzer operation with dilution.

Once average dilution has been resolved, the instantaneous equations can be solved. The new unknowns are now a* and b*. The ratio between them (eq 10) can be considered here as a constant, as the feeding system we developed allows us to adopt this hypothesis.20 Consequently, a system with seven unknowns and seven equations is again obtained (eq 11). b (10) a a*CHYOZNX + τa*CHY′OZ′NX′ + ns(O2 + 3.76N2) + D(O2 + 3.76N2) f c*CO2 + d*CO + e*H2O + f*N2 + g*NOx + h*O2 (11) τ)

In this paper, all concentrations refer to an oxygen content of 6% in the flue gases (dry basis). Numerical values were averaged over long periods of time, while the best-fit surfaces were obtained statistically from representative tests.

3. Results and Discussion 3.1. Feasibility of RDF. The first results are shown in Figure 7. Graph a represents the fall in boiler efficiency by increasing the RDF pellet feeding rate when only primary air is supplied, which can be defined as the reference situation. The higher the %RDF (Figure 7a) or the higher the net mass rate, the worse the performance of the boiler. In addition, increasing %RDF and/or decreasing n leads to a rise in unburned emissions, expressed by CO, as shown in Figure 7b. As RDF is a waste, efficiency should not be considered the key factor in the process. Instead, the higher concentration of CO emissions jeopardizes the system’s viability. In order to overcome these drawbacks, it is necessary to minimize the final CO emissions, otherwise the use of RDF in

commercial plants will not be possible due to regulatory noncompliance. With a view to limiting such emissions, the maximum %RDF in the mixture was reduced to 10%. The next steps evaluated were the use of secondary air and flue gas recirculation. These solutions have been tested before in this plant with other pellets, and the results obtained were very encouraging. On the basis of this prior research, carried out only with wood pellet, the suggested value for the upper limit of both conditions was 30%. A sample of these results is illustrated in Figure 8a and b. An increase in boiler efficiency is clearly seen when secondary air is used. Moreover, emissions are reduced drastically in relation to the reference situation (Figure 8b). Although CO concentration is still rather high when compared to using only wood pellets, it is acceptable and the viability of the process is no longer compromised. In addition, as shown in Figure 8b, the use of both solutions (recirculation and secondary) has a very similar impact. The possibility of completing the oxidation of the unburned gases by using flue gas recirculation, or the generation of different combustion areas by secondary air supply, may have a similar influence on efficiency and emissions. A typical response in this kind of boiler is an improvement in combustion (efficiency and emissions) when the air excess ratio is increased and the fuel mass flow rate is decreased.16 In general, it can be stated that the small volume of the combustion chamber limits the complete development of the flame before quenching occurs at the water tubes. The higher the volume of the flame, the higher the emission of unburned gases. Besides, at low feeding rates, the residence time of the gases is increased, thereby making better use of the energy.

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Figure 7. (a) Boiler’s efficiency in the reference case. (b) CO emissions in the reference case for mP ) 0.5 [g · s-1].

Figure 8. (a) Boiler efficiency with secondary air. (b) CO emission comparison between the reference case and secondary/recirculation.

3.2. Emissions. As expected, SO2 concentration in the flue gases was almost negligible, as the sulfur content in both fuels is low according to the ultimate analysis (see Tables 1 and 3). When working with primary air, NOx emissions are scattered. Values are not excessively high (100–300 ppm at 6% O2) but tend to increase with the %RDF. As the main NOx formation mechanism in biomass combustion is fuel NO,11,21 the higher nitrogen content of the RDF pellet explains this trend. Figure 9a and b shows the comparison between the two abovementioned configurations. Both results are very similar, but it seems that the base level and peak level are lower when recirculation is used. These emissions decrease when the excess air ratio is reduced, possibly because the thermal NOx formation is inhibited. Figure 9c and d compare CO emissions. With the new configurations, CO emissions for the usual excess air ratio (1.6–1.8) and with 10% of RDF are lower than 5000 [ppm 6% O2], which is below the limit for small-scale plants in several (20) Lareo, G. Estudio de la co-combustión en calderas de fitomasa de baja potencia con pelet y residuos forestales típicos de Galicia. Ph.D. Thesis, University of Vigo, Spain, 2004. (21) Salzmann, R.; Nussbaumer, T. Energy Fuels 2001, 15, 3.

European regulations.22 It is worth mentioning that system response improves slightly when recirculation is used. There is a tradeoff between NOx and CO formation related to the excess air ratio, which has been reported in other papers.16,23 One of the most serious problems found during the experimental phase of this study involves the high ash content of RDF. Ashes hinder the contact between fuel and oxidizer, thereby increasing the excess air ratio and the residence time required. Furthermore, in tests with RDF, considerable sintering throttles the bed24 (Figure 10), thereby increasing the negative effect of the ashes. Over 1100 °C, high boiling point oxides, which have not been volatilized in earlier phases of drying, pyrolysis, and char oxidation, are fused and form agglomerates.25,26 We observed that particles bond with one another and consider(22) Oravainen, H. Testing methods and emission requirements for small boilers (