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Measurement of fuel quality aspects of carpet tile waste – preliminary investigation

Report by AFBI for Carpet Tile Solutions Ltd.

September 5th 2013

Report by
Dr Gary Lyons & Dr Colin McRoberts
Agri-Food and Biosciences Institute for N. Ireland,
Newforge Lane,
Belfast
BT9 5PX
UK.
Tel: +44 90 255245/255462
Email: gary.lyons@afbini.gov.uk / colin.mcroberts@afbini.gov.uk

 

Conclusions

• Calorific values of carpet tile waste varied, with values exceeding that reported for woody biomass and in some cases approaching that of coal.

• Ash content was high resulting in large combustion residues.

• Combustion profiles were influenced by synthetic fibre type and bitumen content with thermal decomposition between 200 – 650 ºC.

• Combustion rate and char burnout temperature were highly variable.

• High volatile and very low fixed carbon content indicated that carpet tile waste had a much lower heating value than fossil fuels.

• Gas phase volatiles produced on heating were associated with the synthetic fibre and bitumen components. The release of sulphur could affect Sox emissions.

• Heavy metals were detected in samples and further investigation would be required to study emission and discharge levels.

• It was not possible to mix carpet tile waste with biomass to examine co-combustion properties.

• Carpet tile waste samples could be combusted to provide heat or energy provided that emissions legislation is complied with.

Carpet tile waste samples.

The following work program was used to study carpet tile waste samples:

1. Gross Calorific Value (CV).
2. Ash content.
3. Combustion properties.
4. Proximate Analysis for volatile and fixed carbon content.
5. Gas phase volatile compounds.
6. Heavy metal content.
7. Carpet tile waste mixed with Biomass

Samples were delivered from the factory as waste cut pieces and for the purpose of the study, two sample types were identified due to differences in synthetic fibre composition and characteristics of the backing material. The samples were further subdivided based on weave and backing characteristics and were labeled as Nylon A, Nylon B, Polypropylene Tufted, Polypropylene A and Polypropylene B (see figure 1 for sample images). Samples of bitumen and chalk (backing materials) were also obtained. All analyses were undertaken in triplicate.

Experimental.

1. Calorific Value (CV).
Samples were punched out of carpet tile pieces using a No. 12 circular borer and analyzed without further preparation by total combustion in oxygen in a Bomb Calorimeter to obtain gross calorific value.

2. Ash Content.
Bored samples (see 1) were heated in a muffle furnace at 600 ºC overnight to determine ash content.

3. Combustion properties.
Combustion properties of carpet tile samples were investigated using Thermogravimteric Analysis (TGA). Samples were punched out of tile pieces with a No. 2 circular borer and placed in alumina crucibles, chalk and bitumen samples were analysed separately. Fibre samples were cut from the tile using a sharp blade for analysis. Samples were heated in a furnace from room temperature to 800 ºC with an air flow rate of 50 mL min-1 and a heating rate of 20 ºC min-1. Major decomposition temperatures were measured for each sample

4. Proximate Analysis.
Volatile and fixed carbon contents were measured using the same technique as (3) above, however samples were heated in nitrogen with an isothermal hold for 10 minutes at 105 ºC, followed by dynamic heating up to 950 ºC at a heating rate of 10 ºC min-1. Nitrogen was passed through the furnace at a flow rate of 50 mL min-1. At 950 ºC the gas was switched to air (50 mL min-1) and samples were combusted for 30 minutes at this temperature. Thermogravimetric weight losses were measured to give proximate results for samples.

5. Gas phase volatiles.
Pyrolysis-Gas Chromatography Mass Spectrometry (Py-GC/MS) was carried out using a CDS 5200 series linked to an Agilent Technologies 5975 GC/MS system. Samples were pyrolysed at 600°C at a ramp rate of 20°C ms−1 for 20 s. The pyrolysis products were separated on an Agilent Technologies HP5-MS (60 m × 0.25 mm × 0.25 μm) capillary column. The column was initially held at 40°C for 2 min and then ramped at 5°C min−1 to 300°C and held at this temperature for 5 min with a constant flow rate of 1.8 mL min−1. The MS was operated in the full scan mode scanning the mass range 30–700 amu.

6. Heavy Metals.
The ash supplied was re-ashed at 450⁰C, microwave digested and reconstituted in 10% nitric acid/internal standard solution and assayed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES).

Results & Discussion

Calorific Value & Ash content.
CV was highly variable between the five types of sample analysed and results are presented in Table 1. Nylon A had the lowest CV (20.9 MJ/Kg) and polypropylene A the highest (34.8 MJ/Kg). Tufted polypropylene, polypropylene B and nylon B CV's ranged from 23 – 29 MJ/Kg respectively. Ash content varied from 48.3 – 61.2 % (Table 1). Nylon B and polypropylene A had lowest ash content and tufted nylon was higher than the others. There was good correlation between CV and ash content with samples containing highest ash (nylon A and tufted polypropylene) displaying lowest CV and the opposite was true for nylon B and polypropylene A. This was as expected in that a sample with higher ash content (i.e. a higher non-combustable fraction) will release less energy when combusted. The lower CV's for the carpet waste samples would be higher than those reported for woody biomass fuels (15-18 MJ/Kg) and the CV for polypropylene A was similar to values reported for coal. However, due to ash content of 48 – 61 %, all of the waste samples would generate greater combustion residues than biomass if they were used as commercial fuels for heat or energy generation.

Combustion properties.
Combustion profiles (derivative thermograms) of fibres from polypropylene, tufted polypropylene and nylon samples are shown in Figure 2. Onset of thermal decomposition was lowest for tufted polypropylene at 200 °C, followed by polypropylene at 220 °C and nylon began to decompose at a much higher temperature (310 °C). Peak combustion temperature (where greatest rate of weight loss occurred) was lowest for tufted polypropylene (335 °C), followed by polypropylene (370 °C) and nylon was significantly higher (440 °C). There was also a visible char formed between 480 – 580 °C for nylon which was not present in the other fibres, suggesting that nylon combustion was a two step process. Endset of combustion occurred between 550 – 650 °C for all samples

The combustion profile for the bitumen used in the backing is shown in Figure 3. Thermal decomposition was a multistep process, with onset of combustion at 220 °C and endset at 640 °C, and a small combustion residue of less than 5%. Major combustion events were noted at 330 °C, 445 °C and 550 °C. The multistep decomposition is not surprising as bitumen is made up of 500 – 700 chemical components, composed mainly of polycyclic aromatic hydrocarbons (asphaltenes, polar aromatics, non-polar aromatics, saturates) and a number of toxic elements. Figure 4 shows the combustion profile for the chalk component used in tile manufacture. This is an inorganic material derived from limestone and thermal decomposition of 40% of the material occurred in a single step from 550 – 770 °C, leaving a high residue content of more than 50% after combustion. This component contributes to the high ash content of the carpet tile waste.

Figure 5 shows the combustion profiles of all of the carpet tile components overlayed on one thermogram. The large combustion peaks are associated with the fibres which decomposed from 200 – 600 °C and the bitumen which decomposed over the range 220 – 640 °C. The chalk partially decomposed between 550 – 770 °C and the two backing material samples underwent thermal decomposition over the whole temperature range (these samples contained all of the components found in the tiles).The backing materials show decomposition patterns which are a composite of the bitumen and chalk, whereas the fibres have degradation profiles which are distinct from the backing materials. Figure 6 shows the actual combustion profiles for the intact pieces of carpet tile waste. There were common features between the sample types, all underwent devolatilisation between 220 - 500 °C, however there were differences in the peak temperature and rate of combustion. All samples formed a char between 475 - 500 °C and char burnout temperature varied. Thermal decomposition of the chalk component in the tiles was elevated with onset at 600 °C and weight loss still being recorded beyond 800 °C. Table 2 shows the combustion properties of the different tile sample types. Nylon B and tufted polypropylene had lowest combustion onset temperature (210 °C), the polypropylene samples were higher (230 – 240 °C) and nylon A began to decompose at 280 °C. Nylon A displayed lower combustion rate (1.1 mg/min) and peak combustion temperature than nylon B, tufted polypropylene and polypropylene A (3.4 – 3.5 mg/min & 432 – 450 °C respectively). Polypropylene B had a similar peak temperature (445 °C) to other samples, but displayed a much higher combustion rate (5.6 mg/min). The different thermal events highlighted in Figures 2 – 5 for individual components, have merged and overlapped in Figure 6 for the tile waste samples and an increase in thermal stability was noted. This was probably due to less efficient heat transfer in intact samples compared to finely powdered usually analysed by this method, although general differences in the thermal characteristics of the different samples were easily observable. The combustion peaks at 300 °C, those between 350 – 475 °C and the char peaks between 475 - 500 °C were due to combined synthetic fibre and bitumen thermal decomposition.

Proximate Analysis
Proximate analysis results displayed in Table 3 revealed that all samples had high volatile (VC) and very low fixed carbon (FC) contents. Tufted polypropylene had lowest VC and FC values (62.3 and 0.35% respectively) whilst polypropylene A had highest VC (70.8%) and nylon B had highest FC (3.53%). The VC of the carpet tile samples was comparable with that seen in biomass (70-80%), but much higher than that of bituminous coal (25%). The FC of the samples was negligible compared to biomass (15-20%) and also coal (60-80%). Ash residue was lower than the quoted ash contents in Table 1 because for proximate analysis, samples were heated to 950 °C and at this temperature a significant amount of the chalk fraction would be volatilized with the conversion of calcium carbonate (CaCO3) to calcium oxide (CaO) and carbon dioxide (CO2). The ash contents in Table 1 were measured at 600 °C. These differences between the values for ash content is an important consideration when determining the likely ash residue from a combustion system. At temperatures higher than 750⁰C it is likely the ash content will be in the range found in table 3 (24.8-37.4%) rather than the values found by conventional ash determination (48.3-61.2%). Using coal as an example of a fuel type which produces good heating value when combusted, it is easy to see that the high VC and low FC of the carpet tile wastes would produce lower heating values. The VC consists of gaseous species which evolve before combustion of fixed carbon occurs and is involved in ignition of materials. High VC is less desirable as furnace operating temperatures have to be lowered below ash melting temperatures and this will cause fouling and slagging problems. FC is the solid fuel left in the furnace after the VC has been distilled off. It consists mostly of carbon but also contains hydrogen, oxygen, sulphur and nitrogen not driven off with the VC. FC gives a rough estimate of the heating value of a material.

Gas Phase Volatiles
Three types of carpet tiles were examined polypropylene, nylon and tufted polypropylene. The tiles were separated into fiber, backing and adhesive with the exception of polypropylene tiles where it was not possible to obtain the adhesive layer. These separated products were subjected to Pyrolysis-Gas Chromatography/Mass Spectrometry to identify the volatiles released. In addition bitumen and filler material used in the backing were also analysed. The products released from the fibers (Figure 7) were in agreement with that found in the scientific literature. For the polypropylene and tufted polypropylene fibres the major product was 2,4-dimethyl-1-heptene with a range of hydrocarbons also released. For the nylon fibre the major product was caprolactam which is itself the precursor of nylon. The products released from the polypropylene and nylon backings were very similar to each other with styrene the dominant product (Figure 8). The tufted polypropylene backing had the characteristics of these products but also those of the fibre itself (2,4-dimethyl-1-heptene) which suggests that it was harder to separate.

Adhesive samples had peaks from both the respective fibre and backing material. The tufted polypropylene adhesive also produced caprolactam which suggests that nylon was present. The bitumen sample contained a series of alkane/alkene products but also measurable amounts of sulphur which may impact upon SOx emissions dependent upon incorporation levels of bitumen into the carpet tile (Figure 9). No compounds were observable from the filler material which is as would be expected for pyrolysis of calcium carbonate at 600⁰C.

Heavy Metals
The amounts of arsenic, cadmium and lead in the ash from the carpet tiles was measured (Table 4). Lead showed the largest variation between the samples ranging from 5.6-34.3 mg/kg in the ash. Arsenic and cadmium levels ranged between 0.2-0.6 mg/kg and 0.08-0.33 mg/kg respectively.

Carpet tile waste mixed with Biomass
It was not possible to mix carpet tile waste with biomass because the experimental equipment which was used to measure the combustion and proximate properties of the samples can only handle milligram quantities of fine materials. This type of experimentation needs to be undertaken with larger pieces of tile waste either mixed with for example wood chips or biomass pellets in a biomass combustion unit or boiler. We do not have access to such a combustion unit and so could not complete this part of the investigation.

Conclusion
Carpet tile waste samples could be combusted to provide heat or energy provided that emissions legislation is complied with. Calorific values were greater than biomass, however, high ash content would result in large combustion residues. The combustion of the tile samples was influenced by the synthetic fibre composition and bitumen content and at higher temperatures chalk decomposition. The high volatile and very low fixed carbon content indicated low heating value compared to fossil fuels and gas phase volatiles emitted were consistent with thermal degradation of nylon, polypropylene and bitumen. The presence of sulphur in the bitumen could have an impact on Sox emissions. Heavy metals detected in the carpet tile waste samples warrant further investigation for emission and discharge levels.

A paper by Lemieux et al. 2004 Emissions study of co-firing waste carpet in a rotary kiln provides information relevant to combustion of carpet waste. At continuous feeding up to 30% of the total energy input no transient puffs and little change was observed in the incomplete combustion products carbon monoxide (CO), total hydrocarbons (THCs) and polyaromatic hydrocarbons (PAHs). NO (nitric oxide) emissions were raised due to the nitrogen present in the nylon fibre. The conversion of this feedstock nitrogen to NO increased with increased mixing with air. This conversion is likely to be controlled by the feeding method and preparation such as carpet size.

Acknowledgements

The following AFBI staff were involved in this work:
Mr. Eugene Carmichael
Dr Gary Lyons
Dr. Colin McRoberts
Ms Ruth McCormack

 

Figure 1.
Images of the five different carpet tile waste samples analysed in the study. There are differences in the synthetic fibre content, weave structure and backing structure which help to identify differences in the samples.

Figure 2.
Combustion profiles (derivative thermograms) for carpet tile fibres heated from room temperature to 800 °C.

Figure 3.
Combustion profile (derivative thermogram) for bitumen backing heated from room temperature to 800 °C.

Figure 4.
Combustion profile (derivative thermogram) for chalk component heated from room temperature to 800 °C.

Figure 5.
Combustion profiles (derivative thermograms) overlayed for the fibre components and backing materials, heated from room temperature to 800 °C.

Figure 6.
Combustion profiles (derivative thermograms) overlayed for the five sample types analysed, heated from room temperature to 800 °C.

Figure 7.
Pyrogram of the fibre removed from three types of carpet tile.

Figure 8.
Pyrogram of the backing removed from three types of carpet tile.

Figure 9.
Pyrogram of bitumen

Table 1.
Gross CV and Ash content for carpet tile waste samples (average of 3 replicates).

Table 2.
Combustion properties of carpet tile waste samples.

Table 3.
Proximate Analysis of carpet tile waste samples showing the amount of volatile and fixed carbon present (average of 3 replicates).

Table 4.
Arsenic, cadmium and lead content in ash from carpet tile waste samples (results in mg/kg ash).