Article pubs.acs.org/EF
Low-Quality Fuels for Small-Scale Combustion Boilers: An Experimental Study Jorge C. Morán,* José. L. Míguez, Jacobo Porteiro, David Patiño, and Enrique Granada Vigo University, Escuela de Ingenieros Industriales, Lagoas-Marcosende, s/n, 36200 Vigo, Spain ABSTRACT: This work focuses on the combustion performance of cheap low-quality fuels as an alternative to existing pellets and chips for small-scale combustion. The market is keen on using locally available fuels, but a lack of information on their performance in boilers and legislation has prevented wider use. The approval of the regulation that implements Directive 2009/ 125/EC regarding ecodesign requirements for solid fuel boilers proposes even tighter requirements beyond 2018, as compared to class 5 in EN303-5:2012, and mainly gives more relative importance to the partial loads. Our study analyzes woody chip industry residues, agricultural residues, logging forest residues, agricultural pellet residues, and shells and stones from agricultural industry residues at nominal and especially at partial load. Two commercial boilers have been representatively chosen for the state-of-theart top-fed and bottom-fed boilers.
1. INTRODUCTION There is an increasing demand for pellets and growing interests in the use of alternative local low-quality fuels such as mulch, logging residues, agricultural residues, and energy crops for power plants and small biomass boilers.1−11 The EU has established sustainability objectives for the development of the biomass industry in order to avoid an overall negative environmental impact associated with the use of bioenergy.12−16 In 2012, the European Standard EU-303-5:1999 was revised to promote forceful competition in the small-scale boiler market.17,18 Moreover, Directive 2009/125/EC of the European Parliament for solid fuel small boilers (draft January 2014) proposed an even tighter amount of requirements for emissions of carbon monoxide (CO), OGC, and particles (PM), starting from January, 1, 2018.19 The use of suitable lowquality fuels in small-scale combustion boilers will be strongly affected by this Directive, as it affects not only the so-called “preferred fuel” but also all the “other suitable fuels” (recommended by manufacturers) in boilers with a rated heat output of 500 kW (also known as LOT15). It is quite difficult to have an overall prior idea of the suitability of a certain fuel for a specific boiler.20−23 In commercial small-scale biomass boilers, control variables (generally fans and/or screw augers velocities) have to be adjusted for a specific fuel in order to achieve the best combustion conditions.5,24,25 These variables establish values for excess air and the primary to secondary air excess split ratio. Austrian and German boilers are the most innovative in the low power range market. In the power range up to 100 kW (low power range), the two main type of feeding systems are top-fed burners and bottom-fed burners.26 In order to obtain as much information as possible, combustion assays and subsequent emission analyses are useful. Many studies regarding different low-quality fuels under different combustion conditions are benefical, but most of them have been focused on wood and/or agricultural pellets.27−31 Since the combustion of biomass is heavily influenced not only by its physical and chemical properties but also by the burning conditions,32−37 one key question is to © XXXX American Chemical Society
select the best combustion conditions. Thus, several experimental studies have been done recently that deal with combustion conditions, mostly only at nominal loads. The most important parameter to control gaseous emissions and efficiency is excessive air.38 An oxygen surplus is needed to ensure sufficient mixing and a complete burnout, but it also decreases the combustion efficiency; this is due to a lower heat loss in the flue gas, so process control and/or optimal design of air inlets is required. CO in the flue gas is a good indication of the completeness of combustion. The most precise process control is based on that combustion parameter, using the socalled lambda probe. However, the lambda probe control is not commonly used in small boilers.26 This is a problem when finding new optimum combustion conditions for alternatives to the so-called “preferred fuel” without a lambda probe or an equivalent system.39−42 Some studies even concluded that a lambda control was unable to adjust to an optimal level for a broad variety of nonwoody biomass fuels.27 Besides, for some authors,43−45 the primary to secondary air excess split ratio is the most important variable affecting emissions in small-scale combustion. Although not conclusive, these studies suggest an optimum primary to secondary air split ratio of about 25% for the nominal load.43 The influence of load in combustion is also of great importance. This will become a key question for future boilers in the frame of LOT15, where partial load conditions are crucial for the calculation of the performance variables. Studies at partial load in these type of boilers used mainly pellets and wood fuels and hardly any low-quality fuels.28,45,46 The general conclusion was that air−fuel ratios were significantly higher at partial load conditions than at nominal load conditions.47 Also, the effect of partial load is more important in CO emissions than in PM emissions, as under partial load, CO emissions increase substantially.44−49 Actually, one of the conclusions is Received: August 28, 2014 Revised: April 9, 2015
A
DOI: 10.1021/ef5019252 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Schematic of the boilers and their experimental setup. details of the boiler and its construction can be seen in previous works.41 Tests were carried out at 18 kW and 9 kW (50% value), instead of at 24 kW and 7.2 kW (30% partial load) to compare the results in similar conditions, as fuels have quite different high heating values. In some cases, an improved feeding system was used. This new feeding system consisted of a conveyor belt with a twin-compartment hopper which allows for a more precise and continuous control of the feeding ratio. The bottom-fed boiler was an improved commercial 60 kW multifired boiler. Fuels were introduced into the bed from the bottom by a screw mechanism and forced to ascend toward a circular grate where they later spread out. Primary air was introduced through the grate via several small holes arranged in a circular pattern. Secondary air was introduced through a ring located over the fuel bed which happens to inject air to generate swirl and intensify turbulence of the flame. A metal dome was located over the secondary air ring to prevent the flame from reaching the water-walls of the heat exchanger, cutting down carbon monoxide emissions. A sensor measured and delimited the height of the bed. The combustion plate had a conical shape that allowed the burning bed to move and be consumed before it reached the edge of the plate. The furnace walls were also refrigerated except for the frontal wall where a door was placed. Two ash worms transported the ash from the firebox to the built-on ash container. Fuels were burned at 100%, 50%, and 30% load, as this type of boiler has more potential to adapt with different types of biomasses than topfed boilers (pellet boiler). The biomasses were first burned in the topfed boiler to evaluate their suitability as an alternative to commercial pellets, and then in the bottom-fed boiler, to compare results with those obtained by burning chips. In order to substitute the function of a lambda probe control system, two different types of tests were practiced: a quick test to determine the best burning conditions and a second long-lasting test to study efficiency and gas emissions (CO and NOx in both boilers and PM10, HCl and HC in the bottom-fed boiler). The quick test determines the optimal lambda (interval) for each fuel by analyzing the measured dependence of CO emissions and lambda. This strategy is similar to those proposed by Petrocelli et al.57 for pellet stoves or by Quintero-Marquez et al.58 for a 180 kW grate automatic boiler. If a fuel is tested in a wide range of air excess ratios, we observe a prompt decrease in the instantaneous CO emissions when lambda increases and the fuel begins to burn “properly”. The use of these instantaneous plots can be successfully implemented to improve settings of the control systems, similar to what a lambda probe does.27 Later, each fuel was then burned into these determined best combustion conditions. This was to study efficiency and emissions in a longlasting way poststudy. The operating conditions were maintained constant during each test by means of PID controllers. Each
that reduced load operations of pellet boilers should be avoided in these type of boilers. However, this recommendation does not fit with LOT15. Besides experimental studies, Persson et al.50 developed different mathematical models for several types of boilers and stoves in order to test the theoretical behavior of biomass boilers. Klason and Bai51 studied the theoretical combustion process in a wood pellet furnace; meanwhile our research team52,53 performed a numerical simulation of a small-scale biomass boiler. Byproducts from the food industry such as olive stones and nut shells are gaining importance in the residential sector in Southern Europe.54,55 Garcia et al. created a large database of well-defined Austrian and Spanish biomass resources.56 Some studies [i.e., DOMOHEAT, MixBiopellets, etc.] analyzed the economic market and identified some potentially profitable low-quality fuels for these type of boilers. It is therefore significant to compare these locally available biomass fuels in a single combustion unit in order to facilitate the development of fuel-flexibility pellet combustion units.38 Although the use of these fuels is already widespread in the small-scale residential sector, very little is known about their emissions. The purpose of our work is to compare the suitability of 12 low-quality Mediterranean biomass fuels (identified as profitable in the DOMOHEAT project), with 4 market proven pellets and chips in one top-fed and one bottom-fed boiler. Our contribution focuses mainly on the combustion performance of such fuels under the best possible combustion conditions, most of them never reported to be used in these small boilers, regarding the frame of the EN-303-5:2012 and the forthcoming Directive 2009/125/EC. Hence, special attention has been given to the analysis of emissions at partial load and to the influence of the process control when missing a lambda probe.
2. EXPERIMENTAL SECTION For our research, we chose a commercial pellet and a chip boiler (Figure 1), which we considered to be state-of-the-art for small combustion units (up to 150 kW): a 24 kW pellet boiler (top-fed burner) and multifire chip boiler with 60 kW output capacity (bottomfed burner). Primary air crosses the pellet bed of the top-fed boiler from beneath and through. A door at the front provided access to the interior of the boiler. The composition and temperature of the gases emitted were measured before they entered the chimney. Further B
DOI: 10.1021/ef5019252 Energy Fuels XXXX, XXX, XXX−XXX
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7.3 386.34
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20,750 19,428 20.8 2.9 0.1 0.01 13.2 86.6 0.2 1.53 1.42 0.35
