Article pubs.acs.org/EF
Performance of a Residential Pellet Combustor Operating on Raw and Torrefied Spruce and Spruce-Derived Residues Roger A. Khalil,*,† Quang-Vu Bach,‡ Øyvind Skreiberg,† and Khanh-Quang Tran‡ †
Department of Thermal Energy, SINTEF Energy Research, NO-7465 Trondheim, Norway Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway
‡
ABSTRACT: The heterogeneous nature of solid biomass fuels makes their combustion a substantial challenge compared to the more traditional fuel types, such as fossil fuels and natural gas. Many studies found in the literature attempt at identifying enhancements in fuel properties of biomass after a thermal pretreatment step, such as torrefaction, but only few investigate specifically the combustion behavior of these fuels. In this study, pellet combustion of raw and torrefied spruce and spruce tree tops and branches (T&B) has been investigated with regard to the emissions of gaseous pollutants and particulate matter (PM). The combustion was performed in a residential pellet stove, where a total of six different feedstocks, with and without pretreatment, were tested. The wide range of the feedstock properties was shown to go beyond the design limitations of the pellet stove. This could be seen as combustion instability for the T&B torrefied at 275 °C. Technology adjustments might be needed in terms of combustion air distribution and chamber design for these fuels. Mild torrefaction, in general, reduced the emissions of CO, unburned hydrocarbons, and the organics in particles smaller than 1 μm. Combustion at a low load (low thermal input) resulted as expected in increased emissions of organic compounds, which was again reduced substantially for the mildly torrefied feedstocks. In comparison to raw spruce at low load, a reduction by a factor of 3 from the organic share of the PM1.0 particles is obtained. For the same experiments, CO in the flue gas is reduced by 150%. For T&B, similar trends were obtained for organic particles; however, torrefaction resulted in an increase in the total PM1.0 emissions. The decrease in the organic share was more than offset by a substantial increase in the inorganic share of the PM1.0 emissions. For this reason, torrefaction might not be a viable pretreatment solution for feedstocks with high ash content for use in stoves for residential heating, without combustion technology adjustments.
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INTRODUCTION In the cold winter days in Norway, many rely on wood stoves for residential heating. As in the rest of Europe, the emission of particulate matter (PM) is a great concern in Norway because of their negative effects on health and the environment.1 Also, emissions of black carbon (BC) have been recognized to have a significant effect on global warming.2 A recent study showed that emission factors of wood stoves vary in the range of 1100− 7200 mg/MJ for CO and 38−350 mg/MJ for total particulate emissions.3 Pellet stoves exhibit much lower emission factors compared to wood stoves, because of their continuous operation principle.4−7 Meyer2 has shown that the emission factors (reported in mg/MJ) for BC and particle-bound organic matter (POM) are approximately 20 times higher in wood stoves compared to pellet stoves. For the total organic carbon (TOC), the ratio is approximately 35 times higher, and for carbon monoxide (CO), the ratio is approximately 20 times higher. Only the inorganic fraction of the particles was higher for pellet stoves compared to wood stoves (5 times higher). Hence, it is safe to conclude that it is possible to substantially reduce the particulate emissions from residential heating by replacing wood stoves with modern pellet stoves. Unfortunately, because of a high investment cost of pellet stoves and a high fuel price compared to wood stoves, these appliances are yet to be embraced by consumers in Norway. The pellet stoves would unquestionably have an advantage if they were able to compete against wood stoves using biomass pellets made from © 2013 American Chemical Society
low-grade fuels that would be cheaper to acquire. A typical fuel that could be used is forest residues, such as tree tops and branches (T&B), a potential pellet raw material that can be found in large excess in Norway. The drawbacks of using T&B lies in the higher ash and nitrogen content, which will potentially result in increased emissions of particles, nitric and nitrous compounds, to an extent determined by the pretreatment method and conditions chosen for the combustion technology. Sippula et al. found that an increased fuel ash content contributed to an increase in the PM1.0 mass concentration.4 Pellets in their study were made of separate stem and bark materials of five different wood species. In particular, the concentrations of potassium, sodium, chlorine, and sulfur seem to correlate well to the PM1.0 mass concentration. Another factor contributing to increased emissions is stove operation at a low thermal input. This is a factor that needs to be seriously addressed because modern houses with reduced heating requirements force users to reduce the thermal input. The increase in emissions of gases and particles as a function of the decreased thermal input has been documented in several studies and for different fuel types8 or types of heating units, such as domestic pellet boilers9,10 and a masonry heater.11 Received: April 4, 2013 Revised: June 21, 2013 Published: June 21, 2013 4760
dx.doi.org/10.1021/ef400595f | Energy Fuels 2013, 27, 4760−4769
Energy & Fuels
Article
Table 1. Torrefaction Conditions for All of the Fuelsa spruce
spruce (225 °C)
spruce (275 °C)
T&B
T&B (225 °C)
T&B (275 °C)
SWC
SWC 200 5 225 30
SWC 225 5 275 30
ST&B
ST&B 200 5 225 30
ST&B 225 5 275 30
feedstock type preheating temperature (°C) preheating time (min) torrefaction temperature (°C) residence time (min) a
SWC, spruce woodchips; ST&B, spruce tops and branches.
Table 2. Proximate Analysis (Weight Percentage, Dry Basis), Heating Values, and Bulk Density proximate analysis volatile matter (wt %) fixed carbon (wt %) ash (wt %) moisture content (wt %, wet basis) effective heating value (MJ/kg)a bulk density (kg/m3) energy density (GJ/m3)a a
spruce
spruce (225 °C)
spruce (275 °C)
T&B
T&B (225 °C)
T&B (275 °C)
85.1 14.4 0.5 11.9 15.7 620 9.8
76.0 23.3 0.7 8.2 17.7 581 10.3
72.9 26.4 0.8 5.8 19.7 517 10.2
77.3 20.3 2.4 11.9 16.1 535 8.6
65.8 31.0 3.2 9.2 19.3 525 10.1
60.3 36.0 3.7 6.3 22.4 504 11.3
Wet ash free.
recently reviewed.22 Most of the torrefaction studies thus far have concentrated on understanding the reaction mechanisms23−25 and the major product distributions for several biomass types.26−28 The combustion characteristics of torrefied fuels have not been studied in much detail, although an early publication claims a significantly higher combustion rate in addition to less smoke generation because of the decreased content of oxygen in the fuel.29 The differential thermogravimetry (DTG) curves from the combustion of raw and torrefied reed canarygrass are reported by Bridgeman et al.30 The same study also reports that the combustion of willow in a methane air flame resulted in a longer duration of char burnout for the torrefied material. On the other hand, the ignition times for the torrefied fuels were greatly reduced, which according to the authors, translates into improved combustion properties. To the authors’ knowledge, there has been no small-scale pellet combustion studies giving details on emission factors or combustion stability for torrefied and raw feedstocks.
The main question that we try to answer in this study is whether the thermal pretreatment method torrefaction has the potential to (1) reduce the emission factors of spruce and T&B from spruce in pellet stoves, in particular for stoves operating at low load, and (2) improve the combustion stability. Torrefaction enhances the solid fuel quality by making most of its properties more suitable for thermal conversion processes. Torrefaction can be referred to as a mild pyrolysis process because it, in principle, is a slow pyrolysis process, where the fuel is heated in an inert atmosphere to a typical temperature between 200 and 300 °C. The lower temperature range compared to pyrolysis allows the fuel to retain most of its energy content (typically 90%) while reducing its dry weight to typically 70% of its original dry mass.12 The volume of the solid particles is marginally affected by torrefaction, resulting in a decrease in the volumetric energy density for the torrefied fuels. This is why torrefaction is almost always discussed in combination with a grinding and compacting stage to increase the energy density. However, recent studies have shown that there are some challenges to overcome in pelletizing torrefied materials because of the loss of natural binding agents.13 Torrefaction leads to an impressive list of fuel improvement properties; among these is increased hydrophobicity, which makes the fuel water-repellant and more resistive to biodegradability.14 Torrefied fuel can therefore be stored for longer periods without quality degradation. The mild temperature treatment that the fuel undergoes affects mostly the hemicellulose fraction of the biomass by breaking the bonds they provide to the strong fiber structure of trees. This gives a vast improvement in the grindability of the fuel. In a milling stage, the torrefied fuel is ground to a smaller particle size distribution and an improved sphericity and requires much less energy compared to milling the raw biomass.15,16 Grindability improvement yields a fuel that is quite suitable for co-firing with coal using powder burners because the particle entrainment properties along with its elemental composition is close to coal. The improved fuel properties are also advantageous in other thermal processes, such as gasification.17,18 The torrefaction/ gasification combination is also being considered as a part of processes for biofuel production,19−21 a concept that has been
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MATERIALS AND METHODS
Sample Preparation. Two biomass fractions from the same spruce tree were used as starting materials in this study, woodchips containing some bark and tree tops and branches (T&B). Both feedstocks were first ground and afterward compressed to produce pellets of 6 mm in diameter without adding binders. The two pellet types were also torrefied at two different temperatures, 225 and 275 °C, producing in total six types of solid fuels that were used for the combustion tests. The torrefaction process was performed in an ownbuilt reactor composed of different horizontal screw conveyors positioned on top of one another. The pellets are stored in a bin at the top and first go through a feeding screw, followed by separate conveyors for drying, heating, torrefaction, and cooling, before they fall into a product container. The conveyors are temperature-controlled using electrical heating elements, with the exception of the watercooled cooling conveyor. All parts of the reactor (including the storage bin and the product container) were purged with separate streams of N2 of 8 NL/min each to ensure inert conditions during operation. The pellets are transported in between the conveyors using pneumatically controlled sliding valves. The torrefaction conditions used to produce the studied fuels are shown in Table 1. Table 2 shows the proximate analysis for all samples, including the effective heating value and the bulk density of the pellets. All of the 4761
dx.doi.org/10.1021/ef400595f | Energy Fuels 2013, 27, 4760−4769
Energy & Fuels
Article
Table 3. Ultimate Analysis (Weight Percentage, Dry Basis) C (wt %) H (wt %) O (wt %) N (wt %) S (wt %) Cl (mg/kg, dry basis) a
spruce
spruce (225 °C)
spruce (275 °C)
T&B
T&B (225 °C)
T&B (275 °C)
47.90 6.31 45.29
