biomass/RDF and air/steam ratios on the gas composition was carried out. ... One of the key aspect of the plant layout is the gas cleaning. ... Fuel ----> x1C + x2CO + x3CO2 + x4 H2 + x5CH4 + x6H2O + x7C6H6O + x8C7H8 +x9C7H8O2.
MODELLING OF THE BIOMASS/RDF GASIFICATION PROCESS IN AN UPDRAFT REACTOR L. CONTUZZI, N. CERONE, D. BARISANO, G. BRACCIO ENEA, Dipartimento Tecnologie per l'Energia, Fonti Rinnovabili e Risparmio Energetico, Sezione Biomasse – Rotondella (MT) 75026, Italy
SUMMARY: To evaluate the expected performances of a gasification plant based on the updraft reactor, simulation were performed with ChemCad software tool. Literature and experimental data were included in the model. This complete, though simplified, model recapitulated the overall gasification process in a pilot plant fed by 50 kg/h of biomass and RDF, which combines an updraft gasifier and a gas cleaning section composed by a biodiesel scrubber and two coalescer filters in series. The flexibility of the model allowed to evaluate how the gas composition changes by varying the gasification medium. To this end a sensitivity study of the biomass/RDF and air/steam ratios on the gas composition was carried out. At the highest biomass/RDF ratio, by using air as a gasification medium, the expected composition of CO and H2 was respectively 24.7 and 14.3 % vol at the exit of the gasifier. However, by using a steam/air mix, the expected composition of CO and H2 was 20 and 21 % vol, respectively. The results of this preliminary study were used to design the pilot plant. 1. INTRODUCTION The thermochemical conversion of biomass is one of the most credible candidate for the future biorefinery. Among the available technologies, gasification offers several advantages such as flexibility in the feedstock choice, reactor typology and the final products: heat, electricity, mechanical power and biofuels (e.g. Fischer Tropsch liquids, methanol etc). Furthermore, the gasification warrants attention because it can help in solving disposal problems and reducing environmental pollution from renewable resources. Although the product gas contains the desired amounts of carbon monoxide and hydrogen, also other gases (carbon dioxide, nitrogen, water), higher hydrocarbons, small char particles and ash are found in the outlet stream. In an updraft gasifier, the biomass is fed from the top while the gasification agent is introduced from the bottom, so the whole gas flow is upwards. The biomass particles travel downwards progressively trough zones of drying, pyrolysis, reduction and combustion. In the combustion zone the heat necessary for the endothermic reactions is produced. The quality of the produced gas is characterised by a high tar content and high thermal efficiencies due to the low exit temperatures of the produced gas based on an excellent energy exchange in the fixed bed. The aim of the work was to evaluate how the gas composition changed by varying the gasification medium using a complete and flexible, though simplified, model able to describe the overall gasification process. To this end a sensitivity study of the biomass/RDF and Air/steam
ratios on the gas composition was carried out.
2. PROCESS LAYOUT At the Enea-Trisaia Research Centre an updraft gasification plant of 150-200 kWth input is under construction. This plant will use an air/steam mix as gasification medium and will be used in experimental tests in order to explore the operating conditions at which biomass/ RDF blends can be gasified to generate a hydrogen rich syngas. One of the key aspect of the plant layout is the gas cleaning. In this plant to assure a high cleaning efficiency, at the exit of the gasifier the syngas is treated in a wet gas cleaning section equipped with an oil scrubber and a train of two coalescer filters (Figure 1). In this work, we report the results obtained by modelling the gasification process and the gascleaning section.
Figure 1. Plant scheme
3. MODEL DEVELOPMENT As a fixed bed, in an updraft gasifier four main reaction zones can be identified (i.e. oxidation, reduction, pyrolysis and drying) (T.B. Reed, 1981). Combustion takes places in the bottom, where a mixture of air and steam are injected, supplying heat for the remaining processes which are stratified along the reactor axis (C. Branca & al, 2005). As a consequence of the countercurrent configuration and the process stratification, each zone of the gasifier can be modeled as a separate reactor in which solid phases of different nature and a limited numbers of chemical reactions take place. However, this is only a rough schematization and, depending on feedstock characteristics and operation conditions, the different zones may overlap (C. Di Blasi, 2004). A general scheme of the updraft gasifier is reported in Figure 2.
Figure 2. Scheme of updraft gasifier.
Several experimens in the field of gasification have been carried out so far on feedstock such as coal an biomass; many studies have been published on both subjects and therefore the modeling of the behavior of these kind of materials can be implemented into the modeling approach at their thermochemical conversion. On the other hand, very few studies have been done on the thermochemical behavior of RDF. Literature seems to be insufficient on this topic, especially regarding the behavior of RDF blended with others materials and processed in facilities of pilot size. A scheme of the general flow chart, adopted to describe the gasification process of a biomass/RDF blend in an updraft gasifier, is reported in Figure 3. The process modeling has been implemented by using the commercial software ChemCad 5.2 Process Simulation (Chemstations Inc). In accordance with the ChemCad flow chart, the modeling of the gasification of a biomass/RDF blend was carried out under the considerations listed below.
Figure 3. ChemCAD flow sheet. 1: Dried biomass, 2: Char, 3: Pyrolysis vapors.
The elemental chemical composition of the biomass and RDF adopted in the present modelling are reported in Table 1. Table 1. Elemental composition of biomass and RDF. wt % a)
Elements C H O N Biomass 49.32 7.03 43.65 52.37 7.18 38.54 1.00 RDF b) a) Minor elements, such as sulphur and chlorine, have been neglected. b) In the RDF composition, Dolomite and CaCO3 were also present. With respect to the thermochemical process considered, due to the low temperature, they have been considered inert.
In the mixture biomass/RDF considered, the RDF content represented up to 50% of the total fuel weight. 3.1 Dryer The biomass/RDF mixture is supposed to be fed at a 15 wet %. At the exit of the dryer, the biomass humidity is supposed to be halved. 3.2 Pyrolysis For the biomass and RDF devolatilization the following global reaction was used, where the fractions of gas, tar and char produced should be assigned: Fuel ----> x1C + x2CO + x3CO2 + x4 H2 + x5CH4 + x6H2O + x7C6H6O + x8C7H8 +x9C7H8O2 The approach used for the devolatilization simulation of the mixture biomass/RDF is based on the sum of the different contributions of the various components (V. Cozzani & al, 1995). Two pyrolysers, modeled as stoichiometric reactors, were used in our model: one for the biomass and one for RDF. Pyrolysis is supposed to be carried out at 800 K and 1 bar. The stoichiometric coefficients (Table 2) of the reaction are normally derived on the base of literature experimental results and mass balance. Moreover, taken into account data from literature, unitary CO/CO2 ratio was adopted for both biomass and RDF (C. Di Blasi & al, 2004, S.M. Andersen & al, 2005), while for CH4 values of 3 % vol. (P. Hasler & al, 1999, T. Chmielniak & al, 2007) and 8 % vol. (Jae Ik Na & al, 2003, S. Galvagno & al, 2006) were used, respectively. For char, approximated in the simulation as carbon (C), values of 22.4 % wt (S.M. Andersen & al, 2005) for biomass and 35 % wt for RDF (W. K. Buah & al, 2007) were used; the tar yield was fixed at 70 g/Nmc (T. Chmielniak & al, 2007, T.A. Milne & al, 1998) and 40 g/Nm3 for biomass and RDF respectively. In fact, the tar content in gas generated from RDF pellets was reported to be about 45% less than the tar content in the gas generated from wood chips (M.S. Rao & al, 2004). Experimental data, carried out by chromatographic analysis, showed that the tar from updraft gasifiers is composed primarily of phenol, toluene and guaiacol among which has been allocated the whole content by weighted sum method.
Table 2. Stoichiometric coefficients for the pyrolysis reactions. Biomass C6H10O4 Char X1 2.730 CO X2 0.969 CO2 X3 0.969 H2 X4 2.790 CH4 X5 0.391 X6 0.903 H2O Toluene X7 0.0291 0.0297 Phenol X8 Guaiacol X9 0.0795
RDF C44H72O24N 29.108 2.938 2.938 8.807 5.346 14.451 0.114 0.116 0.310
3.3 Gasification The reduction and oxidation processes are modeled by a Gibbs free energy minimization reactor. The processing parameters of the gasification were 1200 K and 1 bar. In the model, the excess char formed by the pyrolysis is burned with a limited amount of air at an equivalence ratio of about 0.25 (T. B. Reed, 1981), while the total amount of steam in the process is calculated according to
steam + moisture ≅ 0 .4 (biomass + RDF) dry
4. MODEL RESULTS AND DISCUSSION The model was used to simulate the gasification of biomass and mixtures of biomass and RDF varying the gasification medium. To this end, air and a mix air/steam were used as gasification medium to evaluate how the gas composition changes. Preliminary stability tests of such fixed beds, carried out at our laboratories, provided guidance on operational limits. Then, in the simulations the content of RDF was limited to 50% taking into account the problems of the sintering bed due to the plastic melting that in any case should not exceed 15% of total feedstock. Using the mix air/steam, the heat value (LHV) at the scrubber ranged from 4.94 to 5.79 MJ/kg increasing the RDF content (Figure 4), while using only air LHV was lower and ranged from 4.43 to 5.46 MJ/kg. Regarding the gas composition, with the increase of RDF the CO and CH4 contents gradually increased while H2 and CO2 concentrations gradually decreased (Figure 5). The same trend was observed when using only air as gasification medium, but the relative amount were different. Indeed the concentration of H2 and CO2 is decreased while CO increased. The overall energy efficiency of gasification expressed as:
η=
gas flow ·LHVgas (biomass + RDF) flow ⋅ LHV(biomass+ RDF)
was about 0.8 in the case of the mix air/steam independently from the mixture fuel, while was on average 0.65 by using only air. The increase of the gas heat value with the content of RDF
resulted from the higher heat value of RDF. The tar removal efficiency in the biodiesel scrubber was estimated as 99%. In the case of air the model results fit with experimental data that Hasler and Bridgwater reported for the gasification of biomass, with counter current blown air in a fixed bed. Unfortunately, it is difficult to find in the literature experimental data which could be directly compared to the model results in the case of the mix air/steam. Indeed, the combined use of air/steam mix as a gasification medium and a mixture of biomass/RDF as feedstock in an updraft gasifier is a very specific case. In our work, the use of a solid fuel with an higher LHV and of superheated steam resulted in an increase of the hydrogen content and decrease of nitrogenun content. Therefore, the produced gas is characterised by an higher LHV. 6
LHV (MJ/kg)
5,5 air air/steam
5
4,5
4 0
20
40
60
% RDF
Figure 4. LHV of the syngas vs fuel composition. 0,3 0,25
mole fraction
CO (air+steam) CO air
0,2
CO2 (air+steam) CO2 air
0,15
H2 (air+steam) H2 air
0,1
CH4 (air+steam) CH4 air
0,05 0 0
10
20
30
40
50
% RDF
Figure 5. Syngas composition vs fuel composition
5. CONCLUSIONS This work is an attempt to model a complete process of gasification of biomass/RDF mixture
using air and air/steam mix as a gasification medium. The use of RDF increased the LHV of the produced gas by an higher content of CO, whereas the use of steam in the gasification medium increased the H2 content and the overall energy efficiency of gasification. The scarcity of bibliographical references for modelling an updraft reactor, compared for example to downdraft reactors, is evidence of the difficulties of simulating such systems. This simplified model has been used as guide to design a 50 kg/h pilot plant which is under construction at the ENEA Trisaia Research Center, and should be completed by the year 2008. The resulting experimental data from each equipment (gasifier, biodiesel scrubber and coalescer filters) will be a review and a feedback of the model.
ACKNOWLEDGEMENTS This work has been realized in the MIUR project n.13569 in collaboration with Ansaldo Ricerche SpA, Tecnoparco Valbasento SpA, SO.ME.CO. Srl, II Università Napoli, Università Roma Tor Vergata, Politecnico Milano, Politecnico Torino.
REFERENCES T.B. Reed, Biomass Gasification (1981) Principles and Technology C. Branca, C. Di Blasi, C. Russo (2005) Devolatilization in the temperature range 300-600K of liquids derived from wood pyrolysis and gasification. Fuel, 84, pp.37-45. C. Di Blasi (2004) Modeling wood gasification in a countercurrent fixed bed reactor. AIChE Journal vol.50, pp.2306-2319. V. Cozzani, L. Petarca, L. Tognotti (1995) Devolatilization and pyrolysis of refuse derived fuels: characterization and kinetic modeling by a thermogravimetric and calorimetric approach. Fuel, 74, pp.903-912. V. Cozzani, C. Nicolella, M. Rovatti, L. Tognotti (1996) Modeling and experimental verification of phisical and chemical processes durino pyrolysis of a refuse derived fuel. Ind. Eng. Chem. Res, 35, pp.90-98. S.M. Andersen, B.Gobel, S.T. Pedersen, N.Houbak, U.Henriksen (2005) Pyrolysis of thermally thick wood particles, experiments and mathematical modelling. Proceedings of ECOS pp. 1657-1664. P. Hasler, Th. Nussbaumer (1999) Gas cleaning for IC engine application from fixed bed biomass gasification. Biomass & Energy, 16, pp.385-395. T. Chmielniak1, M. Sciazko1 (2007) Gasification of biomass and wastes – pilot scale tests of fixed bed reactor of 3.5 MWth capacity. Jae Ik Na & al (2003) Characteristics of oxygen-blown gasification for combustible waste in a fixed bed gasifier. Applied Energy, 75, pp.275-285. S. Galvagno, S. Casu, G. Casciaro, M. Martino, A. Russo, S. Portofino (2006) Steam gasification of refuse derived fuel (RDF): influence of process temperature on yield product composition. Energy & Fuels, 20, pp.2284-2288. W. K. Buah, A.M. Cunliffe, P.T. Williams (2007) Characterization of products from the pyrolysis of municipal solid waste. IChemE vol. 85 pp.450-457. T.A. Milne, R.J. Evans (1998) Biomass gasifier “tar”: their nature, formation and conversion. M.S. Rao, S.P. Singh & al (2004) Stoichiometric mass, energy and exergy balance analysis of countercurrent fixed bed gasification of post consumer residues. Biomass & Energy, 27 pp.155-171.
