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Energy & Fuels 2007, 21, 3676–3683
Hydrothermal Catalytic Gasification of Municipal Solid Waste Jude A. Onwudili and Paul T. Williams* Energy and Resources Research Institute, The UniVersity of Leeds, Leeds, LS2 9JT, UK ReceiVed June 21, 2007. ReVised Manuscript ReceiVed July 27, 2007
The influence of sodium hydroxide, water density and reaction temperature on the hydrothermal gasification of a compositionally complex municipal solid waste material in the form of refuse derived fuel (RDF) has been studied. RDF and sodium hydroxide corresponding to NaOH/C mass ratios of 0.2 to 1.2 were reacted at 375 °C using initial water densities of 0.04, 0.10, and 0.15 g · cm-3 in a 500 mL stainless steel batch reactor. The effect of reaction temperature was studied between 300 and 375 °C. The reactions produced a hydrogenrich gaseous product. Other gases included CO, CO2, and small amounts of C1–C4 hydrocarbons. The results showed that hydrogen gas production increased dramatically with increasing NaOH/C mass ratio up to a ratio of 0.8 but decreased slightly when the ratio was increased to 1.2. CO2 and CO concentrations decreased with increasing NaOH/C mass ratio, indicating that NaOH catalyzed the gasification via removal of CO2 as carbonate. Hydrogen production was also enhanced with increasing water density up to the reactor design limit. The results further suggest that the predominant reactions taking place during this process can be represented as 2C(s) + 2NaOH(aq) + 3H2O(g) f Na2CO3(aq) + 4H2(g) + CO2(g). Comparable gasification results were obtained for glucose, cellulose, and starch under a set of identical reaction conditions.
Introduction Environmental concerns are currently being expressed more than ever before regarding the adverse effects of the traditional waste disposal methods such as landfill, composting, and open and closed incineration technologies.1 These waste disposal methods are generally often characterized by secondary pollution problems. Landfills are associated with the emission of greenhouse gases and the formation of leachates with the potential for groundwater pollution.2 Land composting can generate greenhouse gases in addition to releasing chemicals into the environment through runoffs and erosion. Incineration can produce and release hazardous flue gases into the atmosphere. While various legislative mitigating procedures have been and/ or are being introduced to make existing facilities more environmentally friendly, current trends support the efforts to extract as much benefit from wastes as possible for environmental, social, and economic reasons. Consequently, waste itself can become a sustainable and renewable raw material for industrial processes. Hydrogen is currently being canvassed as one of the environmentally “cleanest” energy vectors. Its combustion produces pure water as a by-product. Moreover, hydrogen energy can be efficiently distributed via, for example, fuel cell technology. However, production of hydrogen gas requires innovative technologies such as electrolysis of water, hydrocarbon reforming, and steam gasification of carbonaceous substances like coal, biomass, and waste materials. Steam gasification of carbonaceous materials involves two main reactions, namely, (a) reaction between water and carbon and (b) water-gas shift reaction between water and carbon monoxide * Corresponding author. Phone: 1133432504. Fax: 1132440572. E-mail:
[email protected]. (1) Hester, R. E.; Harrison, R. M. EnVironmental and Health Impact of Solid Waste Management ActiVities; The Royal Society of Chemistry: Cambridge, UK, 2002. (2) Williams, P. T. Waste Treatment and Disposal, 2nd ed.; John Wiley and Sons: Chichester, UK, 2005.
to produce hydrogen and carbon dioxide. The first reaction is highly endothermic and requires high temperature usually in the range 700–1200 °C, such that the energy requirements can make these technologies rather expensive and unsustainable. Hydrothermal gasification provides a lower temperature organic waste treatment process for hydrogen production from carbonaceous materials with high moisture content.3,4 This process gives an additional potential advantage of avoiding the costly dewatering of very wet wastes. In the energetic subcritical and supercritical water medium, carbonaceous biomass and waste materials can produce carbon monoxide which simultaneously reacts with water to produce hydrogen and carbon dioxide via the water-gas shift mechanism within the same reaction space. These reactions can be promoted principally by the use of alkali and alkali-earth metal catalysts.5–10 Kruse et al.5 report that potassium carbonate influenced the gas yield of a solution of glucose by more than a 6-fold increase. Schmeider et al.9 also found that KOH and potassium carbonate effectively gasified carbohydrates, aromatic compounds, glycine, and real biomass at 600 °C and 250 bar to produce a hydrogen-rich gas. Lin et al.6,7 reported that calcium oxide and calcium hydroxide catalyzed biomass gasification under hydrothermal conditions; the catalyst served to purify the (3) Williams, P. T.; Onwudili, J. A. Energy Fuels 2006, 20, 1259–1265. (4) Williams, P. T.; Onwudili, J. A. Ind. Eng. Chem. Res. 2006, 44, 8739–8749. (5) Kruse, A.; Maniam, P.; Spieler, F. Ind. Eng. Chem. Res. 2007, 46, 87–96. (6) Lin, S.; Harada, M.; Suzuki, Y.; Hatano, H. Energy ConVers. Manage. 2005, 24, 869–880. (7) Lin, S. Y.; Suzuki, Y.; Hatano, H; Harada, M. Energy ConVers. Manage. 2002, 43, 1283–1290. (8) Onsager, O.-T.; Brownrigg, M. S. A.; Lodeng, R. Int. J. Hydrogen Energy 1996, 21, 883–885. (9) Schmieder, H.; Albein, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. J. Supercrit. Fluids 2000, 17, 145– 153. (10) Watanabe, M.; Inomata, H.; Osada, M.; Sato, T.; Adschiri, T.; Arai, K. Fuel 2003, 82, 545–552.
10.1021/ef700348n CCC: $37.00 2007 American Chemical Society Published on Web 09/25/2007
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effluent gas by CO2 capture via the formation of calcium carbonate. They also reported that the calcium based catalysts removed sulfur, chlorine, and fluoride ions. Further, they proposed the HyPr–RING system for hydrogen production from biomass through hydrothermal gasification. Watanabe et al.10 found that sodium hydroxide catalytically influenced the gasification of cellulose, glucose, and lignin in supercritical water with or without partial oxidation. Other authors have also reported the use of sodium hydroxide for effective gasification of coal and biomass materials. Onsager et al.8 were among the earliest authors to report the influence of alkali metals on the production of hydrogen using water and CO. They concluded that between 70 and 140 °C and within a pressure range of 0.2–40 bar, KOH and CO produced intermediate metal formate which reacted with water to produce hydrogen, as shown below: K2CO3 + H2O f KHCO3 + KOH
(i)
KOH + CO f KOOCH
(ii)
KOOCH + H2O f KHCO3 + H2
(iii)
According to the above equations, the most important reaction is the generation of the potassium formate. Hence, in alkalicatalyzed hydrothermal biomass gasification, the early production of CO from biomass or other carbonaceous materials holds the key to hydrogen production. In this paper, we report the systematic catalytic activity of sodium hydroxide on the hydrothermal gasification of refuse derived fuel (RDF) produced from municipal solid waste (MSW). Using such a complex material as RDF, which is a mixture of paper, cardboard, wood, plastics, sewage, and inorganic materials, hydrothermal gasification as a potential route for hydrogen production from the predominant carbonaceous portion of municipal solid waste was thereby investigated. Experimental Section Municipal Solid Waste. For ease of experimentation and to improve sample homogeneity, the municipal solid waste used for experiments was in the form of refuse derived fuel. Refuse derived fuel (RDF) was obtained from a UK municipal waste treatment plant. RDF represents a fraction of the municipal solid waste stream where the recyclable components, such as glass and metals, have been removed and the sample dried to produce a low moisture content of less than 8 wt %. The major steps involved in producing RDF pellets are preliminary liberation where bags of waste are mechanically opened and size screening, magnetic separation and coarse shredding, a refining separation stage, and finally a series of processes to control the physical characteristics of the fuel for ease of combustion. The characteristics of the RDF are shown in Table 1.2 It is important to note here that the RDF was obtained in dried form in deference to the process of the municipal treatment plant; however, its wet form is most appropriate and applicable for hydrothermal gasification, such that avoidance of dewatering becomes a veritable cost-saving step. Municipal solid waste can reach high moisture contents of over 50 wt %, particularly for loads collected with high contents of vegetable matter and food wastes which have moisture contents of 70 wt %.2 Rainfall during waste collection and processing can also raise moisture content. The RDF was crushed to a 0.25 mm size and homogenized for 1 h before use. About 10 g (corresponding to 7.77 g of volatile matter and fixed carbon) of the RDF was used in each experiment. Hydrothermal Reactor. The experimental work was carried out in a batch stainless steel reactor obtained from Parr Inc. USA which is shown in Figure 1 and has been described in detail earlier.4,11 It briefly consists of a 500 mL reaction bomb, fitted with a pressure gauge (11) Onwudili, J. A.; Williams, P. T. Int. J. Energy Res. 2005, 30, 523– 533.
Table 1. Proximate and Ultimate Compositions of UK Refuse Derived Fuel (RDF) analysis
results
gross calorific value (MJ/kg) proximate analysis (wt %) volatile matter fixed carbon ash moisture ultimate analysis (wt %) carbon hydrogen oxygen nitrogen chlorine fluorine sulfur bulk density (kg/m3)
18.7 67.5 10.2 15 7.3 55 6.9 35.9 0.6 0.9 0.01 0.1 600
and designed for a maximum temperature of 500 °C and a pressure of 34.5 MPa. The reactor was heated with a 3 kW ceramic heater and was fitted with a cooling loop through which cooling water can be used to quench the reaction and cool the reactor to room temperature. Experiments were carried out between 300 and 375 °C. Sodium hydroxide (99% purity pellets) was obtained from SigmaAldrich, UK. The amount of NaOH used ranged from 0.84 to 5.12 g. The initial water density used for experimentation was based on the volume of water added to the fixed volume of the reactor. A water volume between 25 and 75 mL was added to the reactor, thereby producing an initial water density between 0.04 and 0.15 g/cm3. To compare the gasification results with biomass model compounds, 10 g each of glucose, cellulose, and starch was also gasified under a similar set of hydrothermal conditions. A Hanna Microprocessor pH meter, model pH 211, was used to read the pH of liquid effluent solutions. Analysis of the Gaseous Effluent. Once the reactor has cooled to around or below 25 °C, the exact temperature and pressure readings were respectively taken before sampling. The gaseous effluent was sampled for the analyses of permanent gases and hydrocarbon (C1–C4) gases as described earlier.4 The permanent gases were analyzed via two packed column gas chromatographs, one for permanent gases and the other for hydrocarbon gases. A Varian CP-3380 gas chromatograph with two packed columns and two thermal conductivity detectors (GC/TCD) was used to analyze permanent gases. Hydrogen, oxygen, and carbon monoxide, methane, and nitrogen were analyzed on a 2 m length by 2 mm diameter column, packed with a 60–80 mesh molecular sieve. Argon was used as the carrier gas. Carbon dioxide was analyzed on a 2 m length by 2 mm diameter column with Haysep 80–100 mesh packing material. The gas chromatograph oven was held isothermally at 30 °C for the analysis. The detector oven was operated at 120 °C with the filament temperature at 160 °C. Hydrocarbon gases from C1 to C4 were analyzed using a second Varian C-3380 gas chromatograph with a flame ionization detector (GC/FID). The column used was 2 m long by 2 mm diameter and packed with 80–100 mesh Hysesp. Nitrogen was used as carrier gas. The temperature was programmed to start at 60 °C for 3 min, ramped at 10 °C/min to 100 °C, held for 3 min, finally ramped to 120 °C at 20 °C/min and held for 9 min at 120 °C. The injector was held at 150 °C, while the detector temperature was 200 °C. With the final temperature and pressure readings of the reactor after cooling (before sampling), the number of moles (n) and mass (in grams) of each gas produced during the reactions was calculated using the general gas equation. Analysis of the Liquid Effluent. After gas analysis, the liquid effluent was transferred into a beaker. No tars were observed for the RDF experiments, so that deionized water was used to ensure quantitative transfers of each liquid effluent to a known final volume. The liquid effluents were analyzed for total solids by gravimetry. The samples were expected (Table 1) to contain high ash content and were first centrifuged and then allowed to settle and the supernatant liquid decanted for filtration using water suction
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Figure 1. Schematic diagram of the hydrothermal reactor.
(vacuum), on a 47 mm Whatman Filter paper grade 1. The filter paper had been previously dried in an oven at 105 °C for 1 h and weighed. The filter paper and solids were carefully transferred onto a preweighed porcelain crucible, dried in an oven at 105 °C for 5–6 h, cooled in a desiccator, and weighed to a constant weight. In the case of glucose, cellulose, and starch, tars and not solid residues were found at the bottom of the reactor. These were quantitatively transferred onto crucibles, dried, and weighed. Qualitative tests with hydrochloric acid confirmed the presence of Na2CO3 and water-soluble products in the liquid effluents. It is important to report here that the filtrates were quite homogeneous and dark in color, but when the acid was added, liberation of CO2 was observed while the filtrates became clear with dark tiny particles suspended in the liquid. This is possibly due to the acid dehydration of the water-soluble products. Liquid effluents from lower temperature experiments (300–350 °C) showed much more of a presence of suspended particles than effluents obtained at 375 °C. This suggested that the two possible HCl reactions below were taking place.
Table 2. Elemental Composition of Solid Residues in Terms of Carbon, Hydrogen, and Nitrogen sample type RDF RDF RDF RDF RDF RDF RDF RDF RDF RDF
375 375 375 375 375 375 375 300 330 350
0.4 0.4 0.6 0.4 0.6 0.8 0.6 0.6 0.6
0.15 0.04 0.10 0.10 0.15 0.15 0.15 0.15 0.15 0.15
72.1 56.4 55.9 48.3 52.8 49.3 35.9 50.3 50.0 48.9
4.30 7.10 6.55 5.85 6.25 4.50 3.80 7.55 7.40 6.70
0.60 0.60 0.60 0.50 0.55 0.45 0.30 0.60 0.60 0.40
glucosea starcha cellulosea
375 375 375
0.6 0.6 0.6
0.15 0.15 0.15
65.6 70.2 62.5
6.60 7.50 7.30
ND ND ND
a
When a similar volume of the acid was added to a suspension of fresh RDF in a solution of NaOH, no evolution of gas or carbonization was observed. The final volume of the filtrate was noted, and further gravimetric analysis to determine the amount of water-soluble products and Na2CO3 in the filtered effluent was undertaken. The initial pH of the liquid effluents ranged between 10.4 and 11.7. To a 25 mL aliquot of the filtrate, a 0.2 M HCl solution was added gently from a burette. Evolution of carbon dioxide and formation of dark tiny particles were observed. At about a pH of 9, the liquid became clearer and then one drop of methyl orange was added, followed by drop-wise addition of the acid until the first indication of red color, at which point the pH was approximately 7, when CO2 evolution stopped. The resulting solution (suspension) was filtered through a 47 mm Whatman filter paper. The filter paper was washed two to three times with deionized water. The clear liquid obtained was transferred to a porcelain crucible and placed on a water bath to evaporate the water to dryness. Afterwards, the crucible was dried in an oven at 105 °C for 1 h, placed in a desiccator, and weighed to a constant weight to obtain the weight of NaCl formed. A second 25 mL aliquot of the same sample was treated similarly with 0.2 M HCl, but the resulting solution was not filtered. The difference in weight between the first and second crucibles was taken as the weight of carbonized water-soluble products. The original
NaOH/ water temperature carbon density carbon hydrogen nitrogen (°C) ration (g cm-3) (wt %) (wt %) (wt %)
Tars.
amount of Na2CO3 was calculated based on the weight of NaCl obtained based on eq 4. Analyses were carried out in duplicates, and results were well within the 95% confidence level. This method was validated with a standard solution of 0.2 M sodium carbonate and 0.2 M hydrochloric acid using methyl orange indicator. Elemental Analysis of Solid Residues. The elemental composition of the solid residues from the RDF, tars from glucose, cellulose, and starch, and water-soluble products was determined with respect to hydrogen, nitrogen, and most importantly carbon using a CHN analyzer. The instrument was a Carlo Erba Flash EA 1112 compact analyzer for automated determination of CHN in the samples. Sample sizes between 3.0 and 4.5 mg corresponding to the linear working range of the instrument were used. The elemental carbon analysis of the residue was important in calculating the carbon mass balances for the gasification experiments.
Results and Discussion Table 2 shows the results of elemental analyses of the solid products for some of the experiments in relation to sample type, reaction temperature, NaOH/C mass ratio, and initial water (loading) density. Generally, there was a gradual increase in the amount of RDF residues with a corresponding decrease in their carbon contents with increasing NaOH/C mass ratio. The results were used to compute the carbon content of solid residues and tars for carbon balances. The product mass balances of some
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Table 3. Mass Balances for Products Obtained during Some of the Gasification Experiments sample type
C1–C4 mass temperature NaOH/carbon water density solid residue (g) gases (g) hydrogen (g) CO (g) CO2 (g) Na2CO3 (g) WSPa (g) balance (%) (°C) ratio (g cm-3)
RDF RDF RDF RDF RDF RDF RDF RDF RDF RDF
375 375 375 375 375 375 375 300 330 350
0.4 0.4 0.6 0.4 0.6 0.8 0.6 0.6 0.6
0.15 0.04 0.10 0.10 0.15 0.15 0.15 0.15 0.15 0.15
5.68 4.36 4.29 4.01 4.05 4.81 6.91 5.36 4.77 4.51
1.64 0.09 0.08 0.08 0.08 0.04 0.03
