One option to fight global warming is to convert our use of fossil energy into renewables such as biomass energy. However, the forest preservation and the quality of the ambient air are also two major issues. Therefore, the use of biomass waste without any supplementary emissions could represent a part of the solution. In this study, two fuels were considered for a 200 kW moving grate boiler. A multicyclone and a bag filter were fitted on the boiler. The first fuel consisted of classical wood chips whereas the second was a mixture of wood chips with sewage sludge. This second fuel presented a high ashes mass ratio compared to wood chips. The aim was to verify the possibility to burn this kind of fuel without any modification of the installation. The first relevant result is that the conventional pollutants, i.e., CO and NOx, remained under the emissions limits even with the sewage sludge combustion. The Total Suspended Particles emissions at the exhaust were always under 5.4 mg·Nm−3 dry based corrected at 6% of O2, which is low with respect to the standard limitation. The majority of the ashes remained on the combustion room. However, with both fuels, about 5% of ashes mass remained in the heat exchanger. Nevertheless, the heat exchanger was more clogged with the second fuel, which produced five time more ashes. This may lead to a yield loss. Thus, sewage sludge can be used in a wood boiler without any issue if an automatic exchanger sweep is fitted on the installation.
CombustionenergybiomassashesTSPIntroduction
The necessity to reduce greenhouse emissions and to increase sustainable energy production has led to the promotion of renewable energy, such as biomass energy, which is the first renewable energy in the world: representing 9% of the world total primary energy supply, in 2018 [1]. In France, biomass energy is the main source of renewable energy: it represents more than 55% of final energy production in 2020 and therefore contributes significantly to reduce fossil fuels consumption [2]. Wood combustion energy represents the majority of the biomass energy, with 35.8% (114 TWh) in primary renewable energy production in 2019 in France [3]. Currently, wood combustion is used to produce heat and/or electricity in plants with capacities ranging from a few kWth to more than 500 MWth [4].
In this context, wood combustion represents an important source of energy. Nevertheless, the use of this source of energy needs to be realized in accordance with the forest preservation policy and the quality of the ambient air standards. At the European level, directives 2004/107/EC and 2008/50/EC set sanitary standards which involves the obligation to monitor air quality, to inform the population about air quality and to implement action plans [5,6]. Therefore, expanding the wood resource could be very challenging since not only natural wood is used but also wood waste, coated and treated wood and even wood mixed with other waste will be increasingly used [7], which may induce a degradation of the quality of the ambient air.
It has been proved that natural wood combustion does not seem to cause emissions problems when it is burned alone [8]. Therefore, special attention has been paid to the combustion of wood waste such as wood collected from waste disposal sites, industrial by-products and those emerging from construction and demolition activities [4,9–11]. These types of fuels have a very different composition and could contain high levels of pollutants, which both promotes harmful emissions and leads to the production of combustion waste like: ash and pollutants in the flue gas. During combustion processes, pollutants formation occurs due to two reasons: either the incomplete combustion leading to high emissions of unburnt pollutants such as CO, VOC, soot,… and the fuel constituents such as N, K, Cl,… from which pollutant such as NOx and particles are formed [12]. Moreno et al. presented a study on the characterization of gaseous emissions and ash from the combustion of waste furniture in comparison with emission from solid wood combustion [11]. The results of this study showed that NO emissions are higher in the combustion of furniture wood waste mixed with 10% of polyurethane foam and in the combustion of furniture wood waste in comparison with emissions from solid wood. This was explained by the higher nitrogen content in the furniture waste used [11].
Testing new biomass combustion and studying its effects and its products comparing with the usual biomass used; namely “wood”, will surely help expand biomass resources used in biomass boiler.
In this context, the aim of the present paper is to check the feasibility to burn biomass waste mixtures without any modification of the installation while keeping pollutants emissions at the lowest possible level.
For this purpose, the results of the ashes deposition and gaseous and particulate pollutants emissions of two different biomass fuels combustions will be compared and studied.
Materials and MethodsBoiler
The tests were conducted in an automatic 200 kW boiler (50 kg per hour) with moving grate firing and separate primary and secondary air supply (Fig. 1). The removal of waste gas particles is carried with a multicyclone and a fabric filter. The flue gas composition was measured thanks to two analyzers: TESTO 350XL and HORIBA PG350.
Sectional photo view of the LERMAB pilot boiler (200 kW, 50 kg/h) with primary air inlet (green arrows) and secondary air inlet (orange arrows)
Fuels
Two different fuels were used in the 200 kW LERMAB pilot boiler. The first test was conducted with natural forest chips as a fuel. The second test was carried out with a mixture of natural forest chips and sewage sludge (Table 1).
Amount of the 2 fuels
Natural forest chips
Natural forest chips + sewage sludge
Mass of burnt dry fuel (kg)
706
619*
Note: *619 kg including 396 kg forest wood chips and 223 kg of sewerage sludge.
The mixture wood chips and sludge was prepared by making alternative layers with each fuel and then mixing it all until it becomes as homogeneous as possible. The Table 2 presents the proximate/ultimate analysis results for both used fuels.
Proximate and ultimate analysis of used fuels
Natural forest chips
Natural forest chips + sewage sludge
Moisture(% r.b.)1
27
35
Average ash (% r.b.)1
1
6
HHV (high heating value) (kJ/kg)
19650
17694
Net Calorific Value (wet basis) (kJ/kg)
12744
10593
Mass fractions
C2
0.51
0.47
H2
0.06
0.06
O2
0.43
0.47
N2
0.002
0.004
S2
0.0001
0.0006
Cl2
<0.00007
0.0003
Note: 1: calculated on a raw basis.
2: calculated on a dry basis.
The Oxygen mass fraction is calculated by subtracting the measured elements to complete the total mass fraction.
Experimental Parameters
The experiments have been performed with an optimized primary and secondary air supply in a way to maintain the adequate levels of O2 and CO2. The air factor was calculated according to the mass of the burnt fuels and those parameters (Table 3).
Combustion parameters
Air excess
Average combustion temperature (°C)
O2 (% dry)
CO2 (% dry)
Natural forest chips test
1.9
737
9.4
9.7
Natural forest chips + sewage sludge test
1.9
789
8.3
8.9
Sampling and Analysis
After each tests, a complete cleaning of the boiler was carried out and all the ashes obtained were collected for later analysis. The ashes were deposited on different parts of the boiler, which do not have the same aspect and color. This point leads to suppose that these ashes do not have the same composition and/or size distribution. This is why each type of ash were collected apart (Fig. 2).
Diagram of the LERMAB pilot boiler and ash sampling points from 1 to 7. 1: ashes from the grate and the inner walls of the combustion chamber, 2: ash pan, 3: combustion chamber door and surrounding walls, 4: before exchanger, 5: exchanger medium, 6: cyclone, 7: bag filter
During these tests. the concentration in O2, CO, CO2 and NOx of the smoke was measured. Elementary analyzes (C, N, Cl, S, metals, i.e., Cr, Cu,…) were carried on the ashes as well particles size distribution.
Results and DiscussionAshes Deposition
After collecting the deposited ashes from the different parts of the boiler after each combustion test, the results show that the majority of the ashes remain on the combustion room (sampling points 1, 2 and 3 on the Fig. 2). After the combustion of the mixture of wood chips and sewage, most of the ashes were collected at the combustion chamber, on the grate and the surrounding walls and in the ash pan. Whereas, after the combustion test of wood chips, in addition to the combustion chamber, the grate and the ash pan, there is a quiet a lot of ash collected in the cyclone. Ashes from different sampling point are collected separately and weighted.
Table 4 presents ashes weight (per kg) collected from each sampling point as well as the mass ratios.
Comparison between weight of the ashes collected after each test
Sampling point
Natural forest chips test
Natural forest chips + sewage sludge test
Ash mass (kg)
Mass ratio (%)
Ash mass (kg)
Mass ratio (%)
1
1.04
12.81
16.81
41.49
2
1.76
21.55
3.51
8.76
3
3.10
38.18
17.98
44.86
4
0.17
2.09
1.10
2.74
5
0.23
2.83
0.32
0.80
6
1.38
17.00
0.22
0.55
7
0.45
5.54
0.14
0.35
Total
8.13
100
40.08
100
The second fuel containing the sewage sludge led to five times more ash than natural chips wood. Results show that the majority of the ash remain in the combustion chamber; 95% of ashes were collected from the combustion chamber (sampling point numbers 1, 2 and 3) during the second fuel test while with the natural forest chips, 73% of ash were collected from these same 3 points. This can be explained by the high ash content of the second fuel comparing to the first one.
With both fuels, about 4% to 5% mass of the ashes are trapped in the heat exchanger (sampling point numbers 4 and 5). Nevertheless, as the second fuel produced 5 time more ashes, the heat exchanger is more clogged, which induce a yield loss. Whereas for ashes collected from the last two sampling points (sampling point numbers 7 and 8 referring to cyclone and bag filter, respectively), 23% of total collected ashes is measured after the first fuel test, while only 1% is measured after the second fuel test. This result show that the combustion of natural wood chips leads to a higher level of fly ashes than the combustion of the mix woodchips/sewage sludge.
Pollutants Emissions
In order to limit emissions of pollutants into the air from combustion plants, the European standard limits the emissions levels of CO, NOx, SOx and Total Suspended Particles (TSP), which are particular for each type of plants depending on their power output (capacity). For those with a capacity of less than 5 MWth. the emissions limits were [13]:
250 mg.Nm−3 for CO corrected at 6% of O2;
200 mgSO2.Nm−3 for SOx corrected at 6% of O2;
500 mgNO2.Nm−3 for NOx corrected at 6% of O2;
50 mg.Nm−3 of TSP corrected at 6% of O2.
Table 5 presents combustion emissions results measured during both tests. For both fuels, the levels of emitted pollutants were below the standards limitation. NOx emissions were lower during the combustion of natural forest chips than forest chips with sludge and was about 280 and 399 mgNO2.Nm−3, respectively. The same observation was concluded for SOx emissions with only 1.2 mgSO2.Nm−3 emitted during the first test against 97.1 mgSO2.Nm−3 measured during the second test. However, the CO emissions were lower with the second fuel (164 mg.Nm−3) then the first one (255 mg.Nm−3) which slightly overshot the allowed limit. Similar results were reported in bibliography, while the combustion of virgin wood, wood pellet and beech induced only low NOx emissions, other biomass fuels such as MDF, particle board and plywood caused higher emissions when burned in the same combustion systems (fluidized sand bed reactor and automatic waste combustor of 50 kW) [7].
Combustion emissions results for both test
Pollutants (emissions corrected @6% of O2)
First test: Natural forest chips
Second test: Natural forest chips + sewage sludge test
O2 (%)
9.4
8.0
CO (mg.Nm−3)
255
164
NOx (mgEqNO2. Nm−3)
280
399
SOx (mgEqSO2.Nm−3)
1.2
97.1
TSP (mg.Nm−3 dry based)
5.4
5.5
Concerning the TSP, both tests emissions remain under the standard limitation; 5.4 mg.Nm−3 during the first test and 5.5 mg.Nm−3 during the second.
All the emission values presented in Table 5 were corrected at 6% of O2 in order to be compared with the standard.
Values for both fuels are similar in the same order of magnitude excepted for SO2. Emissions of SO2 are very high for the waste biomass fuel. If it contains six times more sulfur than natural wood chips, values of SO2 in the exhaust is close to be 100 higher than for natural chips woods. The combustion of the mixture natural forest chips and sewage sludge seems to have good emissions results since all pollutants present low emission regarding to the standard limitations.
Conclusions
The new renewable energy target of the European Commission is an increase of 27% of its use and a reduction of 40% in greenhouse gas emissions by 2030, compared to 1990 [14]. Consequently, the use of renewable resources, in particular biomass, has an important increase in the last decade.
In order to expand the resource of efficiently and environment friendly fuels used in energy production, two combustion tests were conducted: one with the classical biomass “natural forest chips” and the second with a mixture of natural forest chips and sewerage sludge. These fuels were combusted in a biomass moving grate automatic boiler. The emissions performances of CO, CO2, NOx and TSP were studied. They were measured below the regulation limits for both fuels meaning that the biomass waste could be combusted for energy production in such facilities below 5 MW without modification of the system. Smoke cleaning devices are also performant even with the waste biomass.
Despite the fact that the mass of ash collected is much larger with the mixture wood chips and sewage, there was not much difference in the deposition of the ashes into the different parts of the boiler between the two tests.
If SO2 is below the regulation limit, its emissions should be controlled as NOx by suitably adjusting the ratio of sludge in the mixed fuel in order to deal with standard limits.
Further tests could be considered with the same biomass and different mixing ratios. Moreover, future studies could focus on other different types of biomass such as wood waste and the resulting pollutants emissions. In order to help expanding more the biomass resource and thus make a further step towards “waste-to-fuel” purpose, other pollutants can be added to the classic tracked ones such as Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs).
NomenclatureNOx
Nitrogen Oxides (mgEqNO2.Nm−3)
SOx
Sulphur Oxides (mgEqSO2.Nm−3)
TSP
Total Suspended Particles (mg.Nm−3 dry based)
Authors thank the French National Agency for Ecological Transition for their financial support.
Funding Statement: This study was realized thanks to the financial support of ADEME and the financial and technical support of LERMAB, especially the ERBE platform. LERMAB is supported by a grant over seen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” Program (ANR-11-LABX-0002-01. Lab of Excellence ARBRE) and is part of ICEEL.
Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.
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