Production of Renewable Natural Gas from Waste Biomass
Sachin Kumar, S. Suresh & S. Arisutha
Journal of The Institution of Engineers (India): Series E Chemical and Textile Engineering ISSN 2250-2483 Volume 94 Number 1 J. Inst. Eng. India Ser. E (2013) 94:55-59 DOI 10.1007/s40034-013-0021-x
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Author's personal copy J. Inst. Eng. India Ser. E (March–August 2013) 94(1):55–59 DOI 10.1007/s40034-013-0021-x
BRIEF COMMUNICATION
Production of Renewable Natural Gas from Waste Biomass Sachin Kumar • S. Suresh • S. Arisutha
Received: 14 April 2013 / Accepted: 18 November 2013 / Published online: 5 December 2013 The Institution of Engineers (India) 2013
Abstract Biomass energy is expected to make a major contribution to the replacement of fossil fuels. Methane produced from biomass is referred to as bio-methane, green gas, bio-substitute natural gas or renewable natural gas (RNG) when it is used as a transport fuel. Research on upgrading of the cleaned producer gas to RNG is still ongoing. The present study deals with the conversion of woody biomass into fuels, RNG using gasifier. The various effects of parameters like temperature, pressure, and tar formation on conversion were also studied. The complete carbon conversion was observed at 480 C and tar yield was significantly less. When biomass was gasified with and without catalyst at about 28 s residence time, *75 % (w/w) and 88 % (w/w) carbon conversion for without and with catalyst was observed. The interest in RNG is growing; several initiatives to demonstrate the thermal–chemical conversion of biomass into methane and/or RNG are under development. Keywords Biomass Gasification Renewable natural gas Gas cleaning Gas conditioning Gas treatment
S. Kumar Viresco Energy LLC, Riverside, CA 92507, USA e-mail:
[email protected] S. Suresh (&) Department of Chemical Engineering, Maulana National Institute of Technology, Bhopal, India e-mail:
[email protected] S. Arisutha Energy Centre, Maulana National Institute of Technology, Bhopal, India
Introduction World-wide natural gas consumption was approximately 100 EJ or 2,750 billion cubic meters in 2005 [1, 2]. Biomass is considered a CO2 neutral fuel, as the amount of CO2 released on burning biomass equals the uptake of CO2 from the atmosphere during growth of the biomass. The substitution of natural gas by a renewable equivalent is an interesting option to reduce the use of fossil fuels and the accompanying greenhouse gas emissions, as well as from the point of view of security of supply. Methane produced from biomass is referred to as bio-methane, green gas, bio-substitute natural gas or renewable natural gas (RNG) when it is used as a transport fuel. The composition of RNG is similar to conventional natural gas, making replacement of natural gas by RNG straight forward. Biomass energy is expected to make a major contribution to the replacement of fossil fuels. The future world-wide available amount of biomass for energy is estimated to be 200–500 EJ per year, based on an evaluation of availability studies [2]. Wood and other forms of biomass are some of the main renewable energy resources available and provide the only source of renewable liquid, gaseous and solid fuels [3]. Wood and biomass can be used in a variety of ways to provide energy: • •
•
By direct combustion to provide heat for use in heating, steam production and hence electricity generation; By gasification to provide a fuel gas for combustion for heat, or in an engine or turbine for electricity generation; By fast pyrolysis to provide a liquid fuel that can substitute for fuel oil in any static heating or electricity generation application. The liquid can also be used to produce a range of speciality and commodity chemicals.
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Fuels like hydrogen (H2), methane (CH4), FT diesel and methanol produced from biomass have the potential to become a CO2 negative fuel, because part of the biomass carbon is separated as CO2 in a concentrated stream during the production process. This might be an attractive option for reducing the level of greenhouse gasses in the atmosphere [4]. The conventional processes of biomass gasification are problematic because of tar and char formation during the gasification process even at very high temperature. In order to get the higher energy efficiency, when the process is carried out at lower temperature (\850 C) more tar and char are produced. The catalytic gasification of biomass seems to be promising to reduce tar amount in the product gas even at low temperature, however, the traditional Ni based or dolomite catalysts hardly reduce the tar content in the product gas. These catalysts are suddenly deactivated in the in-bed reaction system due to deposition of carbon on the surface [5]. Highly efficient catalyst and a suitable reactor are necessary to overcome the problems. The specifications for bio natural gas are given in Table 1. Preparation of Cr-Promoted Zirconia The Cr-promoted zirconia was prepared by dry impregnation of zirconium oxide with aqueous solutions of chromium containing the required amount of Cr concentrations between 1 and 6 % for the chromium supported on zirconia. After drying at 120 C K for 6 h, the catalyst was calcined at 480 C for 3 h under an air atmosphere. The final catalyst was pressed, crushed and sieved to 100–220 lm particle size. In each run 20 g of catalyst was used and pretreated by a Table 1 The specifications for renewable natural gas Components
Units
Values
Gross calorific value (GCV)
MJ m-3
31.6–38.7
Wobbe-index (WI)
MJ m-3
43.4–44.4
Maximum liquid hydrocarbons
-3
mg m below– 3 C@any P
Solid hydrocarbons
5.0 Technically free
Aromatic hydrocarbons
mol%
0.1 (or even 0.025)
Water dew point Total sulphur content
C@70 bar mg m-3
-8 \20
H2S ? COS
mg m-3
\5
-3
\6
Sulphur (mercaptans)
mg m
CO2
mol%
\3
O2
mol%
\0.0005 (or even nil)
Hg
mg m-3
\0.015
Cl
mg m-3
Nil
F
mg m-3
Nil
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hydrogen flow at 380 C for 0.5 h. In this study, we prepared zirconia based catalysts using incipient wetness and impregnation method with zirconium oxychloride octahydrate as the precursor. The physico-chemical characterization of the in-house prepared catalyst material was performed using standard procedures. The Brunauer– Emmett–Teller (BET) surface areas of samples were measured using an automatic Micromeritics Chemisorb 2720, Pulse Chemisorption System. SEM analysis of Cr Promoted Zirconia before and after biomass gasification were carried out by using LEO 435 VP Scanning electron microscope. Experimental Set Up A continuous feeding fluidized-bed reactor was used for biomass gasification. However, the reactor dimension and feeding system have been modified for continuous-feeding gasification system. The schematic diagram of biomass gasifier is shown in Fig. 1. Gasifier which can utilize biomass is well developed, but further possible development needs to utilize a variety of feedstocks which include municipal solid waste. Experimentation continues on different feedstock processing techniques but technical barriers remain. In terms of product gas quality, there remains scope for further improvements in gasifier designs. Advances on this end could reduce the number of stages and complexities in downstream gas cleaning. The bench scale set-up consists of a stainless steel reactor with a temperature controller, a screw-feeder to supply the desired amount of feedstock. The molar ratio of steam to carbon in the feedstock H2O:C ranged from 0 to 18; and the gas residence time ranged from 1.20 to 1.60 s. The position of the Cat-bedTM was chosen from the temperature profile over the reactor, which was determined with the gasifier and methanation reactor temperature. Catalyst particle size (in gasifier): 250–300 lm; catalyst particle size (in reactor): 50–120 lm. Gas flow at 680 ml min-1 (gasifier) and 483 ml min-1 (reactor). Starting point in the bench scale line-up process for an integrated RNG system was the lab-scale system for Fischer–Tropsch synthesis from biomass gasification gas. Wet biomass (wood-chips, water hyacinth, banana trees, cattails, green algae, kelp, etc.) after pretreatment is used as feedstock. In pretreatment, drying of incoming biomass is essential in order to secure steady operation of the gasifier and a high quality product gas which is the requirement of gasification process. Biomass is gasified and the raw product gas passes a high temperature gas filter operated at 350 C to remove essentially all the solids and then through a cat-bedTM (proprietary material-patent pending), one pre-stage in the gasifier and second post-stage in the methanation reactor and both catalyst bed contained chromium promoted zirconia material with different metal concentration and sieve size at 380 C.
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Raw Product Gas
Gaurd Bead Catalyst Bed
The samples were analyzed by gas chromatography using a Perkin–Elmer Autosystem equipped with a HayeSep DB Packed column (60–80 mesh; 6 m 9 1/82 9 2 mm) and the following conditions: detector at 300 C, injector at 275 C, and carrier flow, 15 ml min-1 (He). The column temperature was set to increase from 40 to 200 C at 15 C min-1 rate. 0.5 L of the sample volume was injected for each GC-FID run. The sensibility (limit detection) is 2 ppm.
Results and Discussions Biomass Slurry
Heat Transfer System
Steam as Fluidising Agent
Fig. 1 Schematic diagram of bench-scale biomass gasifier
All the tars and a maximum amount of the benzene, toluene and xylene are removed in the scrubbing unit (SU). The gas leaving SU at a temperature of 80–120 C is further cooled and cleaned from NH3, HCl, and other inorganic impurities in a water scrubber at room temperature. Water is condensed from the clean gas and subsequently the gas is compressed to the desired pressure (32–65 bar). The compressed gases are passed through a filter to remove the H2S and a guard bed to remove all remaining trace impurities. To allow intermediate compression, the product gas needs to be free of condensable compounds like residual tars and especially water. After compression the gas is almost completely dry, whereas in the methanation steam needs to be added to prevent coking and soot formation. Figure 2 shows flow diagram for bench-scale laboratory set-up for biomass gasification and conversion to RNG. The Carbon conversion was evaluated as follows: . P MCO þ MCO2 þ n MCn;hydro C Conversion ¼ ðTotal feed carbon molÞ 100; where M is molar concentration.
The BET surface area of Cr-promoted zirconia was found to be 436.17 m2 g-1. The physical and chemical properties of renewable natural gas and tested biomass fuel were shown in Tables 1 and 2. Figure 3a, b shows pictorial view of laboratory preparation catalyst material before and after application for methanation process. The morphology of Cr-promoted zirconia before and after biomass gasification in methanation reactor has been examined by SEM analysis shown in Fig. 3c, d. SEM has been a primary tool for characterizing the surface morphology and fundamental physical properties of the catalyst. It is useful for determining the particle shape, porosity and appropriate size distribution of the catalyst. Scanning electron micrographs of Cr-promoted zirconia surface (Fig. 3a, b) show that the peel particles pores are highly heterogeneous. Changed the surface morphology of Crpromoted samples shown by the SEM micrographs and it clearly shows that chromated with high temperature calcination resulted in the surface erosion, the increase of the surface roughness and some particle disintegration of Crpromoted zirconia (Fig. 3c, d). The composition of biomass fuel is given in Table 2. Effect of Temperature Complete carbon conversion was observed at 480 C; however, as temperature dropped, carbon gasification efficiency decreased drastically. When the reaction temperature was below 410 C, resulting in incomplete gasification conversion, the liquid effluent became yellowish and there was a thin layer of a dark brown, oil-like tar. The tar yield at 480 C is significantly small. On Tar Formation The total amount of tar produced in the gasifier without the use of catalytic bed material is relatively high and varies a lot. Increasing the temperature affects slightly the total amount of tars in the gas heterocyclic components, like phenol, pyridine and cresol decrease in concentration with
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Fig. 2 Experimental BenchScale Line-up Process Flow Diagram
Methanantion Reactor Cat-Bed TM
Gas Clean-up Gas Conditioning Cat-Bed TM
Tar (Trace)
Renewable Natural Gas (RNG)
Scrubber Biomass Slurry Gaurd Bed Steam Gasifier Solids
Table 2 Composition of tested biomass fuels Biomass fuel (wt% dry)
Cedar wood
Grass
C
47.7
40.0
H
4.2
3.2
O
36.8
34
N
2.2
2.5
S
0.06
0.1
Cl
0.04
0.6
Ash
0.8
8.2
H2O
8.2
11.4
Higher heating value (HHV) (MJ kg-1 dry)
20.10
17.26
Sieve size (mm)
0.7–2.0
1.3
increasing temperature. Heavy poly-aromatic hydrocarbons increase in concentration with increasing temperature. The heterocyclic tar components are the least stable and therefore readily broken down. The heavy poly-aromatic hydrocarbons are formed from lighter tars (i.e. via polymerization). This behaviour is also observed in bubbling fluidized and circulating fluidized gasifier [6]. On Fuel Conversion The carbon conversion in the gasification varies between 78 and 88 %. The unconverted fuel (char) is send to the combustor where it is completely combusted and produces the heat for the gasification reactor. The amount of char going to the combustor determines the temperature in the gasifier, so the fuel conversion in the gasification reactor determines the temperature in the combustor and the gasifier. Effect of Reactant Concentration When biomass was gasified without the catalyst at about 28 s residence time at 580 C approx 75 % (w/w) carbon conversion was observed. The liquid sample was clear. As
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the feedstock flow rate increased, the carbon conversion decreased. The presence of catalyst resulted in complete conversion of biomass feedstock (*88 %w/w) WHSV of 32.2 (g h-1) g-1. The liquid effluent was clear. Gas yields of H2, CO, CH4, and CO2 increased significantly with the addition of catalyst. The composition of biomass fuel is given in Table 2. Effect of Pressure When the pressure increased from 25 to 35 MPa, the overall carbon gasification efficiency remained almost the same. However, as pressure increased, the yield of methane increased. The effects of the most important operation variables associated with a fluidized bed gasifier on the product distribution are observed at the gasifier exit [7].
Conclusions and Remarks Replacement of fossil natural gas by RNG is an attractive option to reduce CO2 emissions and even to make them CO2 negative if CO2 sequestration is included. The cost for RNG will be higher than the cost for fossil natural gas at this moment, but the difference is relatively small. Results from the lab-scale gasifiers are promising. The process is tested extensively and results are up to expectations. The technology is ready for up-scaling. Progress is made in increasing the lifespan of the catalyst for complete conversion of tar produced to RNG. The upgraded gas produced by gasification of waste biomass is in principle suitable for use as RNG to replace fossil natural gas. The most important deviations from conventional natural gas is the small amount of H2 in the gas. The heating value of RNG is somewhat lower than the heating value of most natural gas standards, because the gas contains no hydrocarbons like ethane. Financial incentives are required to make the technology viable. Because of the urgent need of reducing CO2 emissions it is to be expected that
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Fig. 3 a, b Shows pictorial presentation of laboratory preparation catalyst material before and after application in methanation. c, d SEM of Crpromoted zirconia before and after biomass gasification in methanation reactor
governments will continue the incentives that were introduced to promote sustainable energy.
3. 4.
References 5. 1. BP Statistical Review of World Energy, in Quantifying energy (U.k., 2006), pp. 1–48 2. V. Dornburg, A. Faaij, P. Verweij, H. Langeveld, G. Van de Van, F. Wester, H. Van Keulen, K. van Diepen, M. Meeusen, M. Banse, J. Ros, D. van Vuukeulen, G. J. van den Born, M. van Oorschot, F. Smout, J. van Vliet, H. Aiking, M. Londo, H. Mozaffarian, K. Smekens, Biomass Assessment: Global Biomass Potentials and their Links to Food, Water, Biodiversity, Energy Demand and
6.
7.
Economy, Main Report (Climate Change Scientific Assessment and Policy Analysis) Policy 1–108, (2008) A.V. Bridgwater, D. Meier, D. Radlein, An overview of fast pyrolysis of biomass. Org. Geochem. 30, 1479–1493 (1999) C. De Meijden, H. Veringa, B. Vreugdenhi, B. van der Drift, R. Zwart R, R. Smit, Production of Bio-methane from Woody Biomass. (Energy Research Centre of the Netherlands (ECN). Petten) L. Garcia, R. French, S. Czernik, E. Chornet, Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition. Appl. Catal. A 201, 225–239 (2000) S.V.B. Van Paasen, J.H.A. Kiel, Tar formation in a fluidised-bed gasifier: impact of fuel properties and operating conditions. ECN, ECN-C-04-056 (2006) A. Sanz, J. Corella, Modeling circulating fluidized bed biomass gasifiers. Results from a pseudo-rigorous 1-dimensional model for stationary state. Fuel Process Technol. 87, 247–258 (2006)
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