Available online at ScienceDirect. Energy Procedia 105 (2017 )

Transcription

1 Available online at ScienceDirect Energy Procedia 105 (2017 ) The 8 th International Conference on Applied Energy ICAE2016 Comparison of gas quality from black liquor and wood pellet gasification using Modelica simulation and pilot plant results Erik Dahlquist a,*, Muhammad Naqvi a, Eva Thorin a, Jinyue Yan a,b, Konstantinos Kyprianidis a a School of Sustainable Development of Society and Technology, Mälardalen University (MDH), Västerås 72123, Sweden b Department of Chemical Engineering, Royal Institute of Technology (KTH),100 44, Stockholm, Sweden Abstract There is a potential to integrate biomass gasification with pulp & paper and CHP plants in order to complement the existing systems with production of chemicals, such as methane, hydrogen, and methanol etc. To perform system analysis of such integration, it is important to gain knowledge of relevant input data on expected synthesis gas composition by gasifying different types of feed stock. In this paper, the synthesis gas quality from wood pellets gasification (WPG) has been compared with black liquor gasification (BLG) through modeling and experimental results at pilot scale. In addition, the study develops regression models like Partial Least Squares (PLS) made from the experimental data. The regression models are then combined with dynamic models developed in Modelica for the investigation of dynamic energy and material balances for integrated plants. The data presented in this study could be used as input to relevant analysis using e.g. ASPEN plus and similar system analysis tools. Keywords: pellets; black liquor; modelica; gasification; synthesis gas; CHP 1. Introduction In the pulp and paper industry, the trend is shifting from the production of new and fine paper towards more packaging, tissue and different type of chemicals (using gasification based bio-refinery system) [1-4]. Similarly, in the power plants, the trend is to shift from base load of electric power to more compensating the deficit when there is no wind or sun. The latter is especially evident in Northern Europe, where Germany currently have a feed-in tariff system guaranteeing to purchase all electric power produced by PV and wind by anyone [5]. In Scandinavia, this is further accentuated with competition between district heating in CHP plants and use of heat pumps utilizing the electric power. As district heating becomes less needed due to better insulated buildings and also the installation of heat pumps (e.g. in Northern Europe), the heat demand over the years is reduced [6]. At the same time, the district heating demand becomes substantially high during the winter, and then the heat pumps could not provide the needed heat. For such reasons, it is techno-economically efficient to operate the CHP plants integrated with biomass gasification for chemical production all year round even though the district heating demand is low during the summer time. A number of concerted efforts have been made on modelling of biomass gasification and various modeling approaches were categorized [7,8]. The research work included the conversion of single fuel particles, char, and gas and concluded that most of the different approaches adapted fit quite well between * Corresponding author. Tel.: address: erik.dahlquist@mdh.se Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi: /j.egypro

2 Erik Dahlquist et al. / Energy Procedia 105 ( 2017 ) models and experimental results [9-12]. For the system analysis of biofuel production, CH 4 and H 2 production from biomass gasification in a CHP plant showed substantial potential with economic viability [13-16]. The previous studies concluded that there were problems to get reasonable predictions of tar formation and it is important to describe what is actually taking place inside the reactors to be able to predict the process. This shows that different results are obtained depending on the fuel feed and plant operating conditions that affect the gasification. In this paper, the synthesis gas quality from wood pellets gasification has been compared with black liquor gasification through modeling and experimental results. The experimental results are correlated into partial least squares models to predict main composition of the synthesis gas produced under different conditions. The quality prediction models are combined with physical models using Modelica for investigation of dynamic energy and material balances for integrated plants at large scale. 2. Description of experimental setup and simulation model 2.1. Experimental setup The experimental work has been performed in two circulating fluidized bed (CFB) gasifiers with the same dimensions, i.e. one for BLG and the other for WPG. The reactors had a diameter of 170 mm and a height of 10 meters connected to a cyclone with the G-valve. The synthesis gas is cooled through a heat exchanger and the gas was cleaned using bag filter and then through a scrubber. The difference between two reactors is that the BLG reactor had an electrical heating system to balance out heat losses. Whereas, the second reactor (WPG) has glass wool insulation (0.15 m) without the electric heater at the walls. Both reactors have pressurized air that is heated in an electric heater to reach the desired temperature. The WPG reactor has a combustor that heats the system first, but shut off when the operating temperature is reached. In the BLG reactor, black liquor was introduced 0.5 m above the reactor bottom, while wood pellets were introduced 0.7 m above the bottom of the WPG reactor. At BLG plant, a NIR meter was used to perform simultaneous analysis of several component gases. In the WPG plant, there is a ABB gas chromatograph (GC) used for the synthesis gas analysis. The synthesis gas is extracted continuously and the sample is introduced to the GC approximately every five minutes. The schematic lay out of the gasifiers is shown in Fig Simulation model The simulation model is developed in Modelica that can be run in both Dymola and Open Modellica. The model consists of a heat and mass balance where material and molar flows of both organics and inorganics is followed through the system with the gasifier, cyclone, G-valve, heat exchanger/cooler and the scrubber (see Fig. 1). The developed model is used for both type of fuels, wood pellets and black liquor, but with different set of parameters in the Partial Least Squares (PLS) models. The gas composition is given by the PLS-models, determined from experimental measurements, for each of the gas components H 2, CO, CO 2 and CH 4, while N 2 is assumed as a ballast from the air fed to the system. H 2O is given by the shift reaction at given conditions. The heat balance of the system is based on the partial combustion and the heat losses. The measured major gas components are correlated to the relative oxidation (relox), plant load, moisture content (MC) and the average temperature in the circulating fluidized bed (CFB) using PLS regression. The relative oxidation is defined as that is defined as the ratio of amount of oxygen introduced into the reactor to amount of oxygen required for 100% oxidation of all organic material of biomass. The gas composition, C (vol %) is calculated for major gas components with a polynom as shown in (1).

3 994 Erik Dahlquist et al. / Energy Procedia 105 ( 2017 ) Fig. 1. Schematic illustration of the CFB gasifier Table 1. Polynoms (PLS) for prediction of gas composition as a function of operating conditions with respect to average temperature in the reactor, load, relative oxidation and moisture content Vol % B 0 A 1 A 2 Relox MC % R 2 Q 2 CO H CH CO C = B 0 + A 1.T +A 2.FF + A 3.RE + A 4.MC (1) Where T is the value of the average temperature in o C of the five positions in the reactor, FF is the load in ton dry solids per m 2 of the reactor cross area per hour, RE is the relative oxidation (relox) and MC is the moisture content in % of the total fuel weight including the added steam. In addition, B 0, A 1, A 2, A 3 and A 4 are the regression constants that are shown in Table 1. R 2 is 1.0 when perfect fit of all experimental data into the model and Q 2 is the corresponding prediction power when the model is used to predict performance at any condition covered by the experiments. 3. Results & discussions 3.1. Comparative analysis of gas quality using experimental results The experimental results are presented in Table 2 as the ratio between CH 4, CO and H 2 in relation to CO 2. Here a normalization has been made comparing CH 4, CO and H 2 in relation to CO 2 since different relative oxidation result in different values of CO 2 content. From Table 2, the results show that there is a significant difference between the gas composition between the gasification of black liquor and wood pellets. There are substantially lower levels of H 2 and higher levels of CH 4 in wood pellet s synthesis gas in comparison with black liquor s synthesis gas. However, the CO levels are considerably higher in wood pellet gasification. Although the water content is higher in case of black liquors (more than 30%) than wood pellets, there is still no significant increase in H 2 content in the gas obtained from wood pellet gasification despite the addition of steam. This indicates that the high content of alkali salts in black liquor drives the increase in H 2 and decrease in CO content.

4 Erik Dahlquist et al. / Energy Procedia 105 ( 2017 ) Table 2. Results from experiments comparing the syngas quality from black liquor gasification with wood pellet gasification Avg. T ( o C) DS (%) Relox (%) kg DS/h CH 4/CO 2 CO/CO 2 H 2/CO 2 BLG Wood pellets Comparison using simulation results Modeling results for black liquor In Table 3, the summarized simulation results from black liquor gasification at different conditions are presented. Further, the calculated gas compositions with the PLS-models together with energy and material balances using the physical model are included. The physical model takes into account the endothermal and exothermal reactions such as the reduction of SO 4 with respect to oxidation of C and H. The heat transfers through the reactor walls, in the heat exchanger and the scrubber are calculated. The condensation and evaporation of water respective to H 2S is addressed. For the black liquor gasification case, it could be seen that there is still some residual carbon not converted during the gasification. The residual carbon content could be reduced significantly by recirculating the solids from the bag-house into the reactor. This results in a carbon conversion of more than % at steady state with recycled dust from the bag house to the down comer of cyclone. The H 2 content is in the range 9-13% while the CO content at only 2-4 %. The CH 4 content is in the range %, that is quite high in relation to the CO content. Since H 2S is stripped off, the concentration of H 2S is reduced to % at the SO 4 reduction of 87-93%, which is as equally effective as in a conventional recovery boiler at the pulp & paper mill. Table 3. Results from the simulation with black liquor gasification using the combined physical model for energy and material balances and PLS models for gas composition Input DS, % Capacity, tonne DS/m 2 h Relox, % Temp bottom, o C Output Theoretical possible heat prod, kw Heat consumed in reactor, kw Vol % in wet gas H 2O H CH CO CO N H 2S Heating value dry gas, kj/kg BL C-conversion, %

5 996 Erik Dahlquist et al. / Energy Procedia 105 ( 2017 ) Table 4. Results from simulation using the combined Modelica and PLS models for gas after condensation to 20 o C for wood pellets Load Relox MC inc Temp TP (kw) Heat demand H 2O H 2 CH 4 CO 2 CO N 2 tds/m 2.h % Steam o C HHV gas kw Vol % Vol % Vol % Vol % Vol % Vol % Simulation results for Wood pellets The combined Modelica and PLS model results for wood gasification are shown in Table 4. The results show that the relative oxidation together with plant loading has strong impact on the synthesis gas quality. The operational temperature also affects the gas quality but the effect is relatively lower than effects of relox. Whereas, the steam has no significant impact when increasing by 40% from a relatively high level. In the reactor, it is observed that the heat demand for driving the processes varies considerably, e.g. from 30 % to 50%. This indicates that the heat must be re-utilized in order to run the process efficiently. 4. System analysis For the synthesis gas conversion to energy products through gasification of black liquor and wood pellets, it is interesting to present brief discussion on the system analysis as the possibility to extract gas components like H 2 or CH 4 from the synthesis gas. A detailed system analysis with comprehensive estimation of biofuel (H 2 or CH 4) production potential will be presented in future study WPG system integrated with existing CHP plant for CH 4 The WPG system integrated with existing CHP plant for CH 4 production is shown in Fig. 2 (a). The gasification system could utilize dry wood, wood pellets or wood chips as feedstock. The separated CH 4 from the synthesis gas could be sold as bio-methane. Whereas, the residual gases could be used for production of other chemicals but can also be combusted in the existing combustor of the CHP plant. If the feedstock contains waste, the synthesis gas would be able to use in the existing boiler that is not designed for waste, since the hazardous components can be removed effectively before the combustion. The wood pellets are mostly used in the municipal power plants, both for combined heat and power (CHP) and for heat only applications. In both cases, it may be interesting to complement an existing CHP plant with a gasifier to utilize existing infrastructure and to extend the business possibilities by producing CH 4 for sale. In such integration, the gasifier can either be upfront a combustor or with a CFB boiler in the G-valve from the down comer BLG system integrated with pulp & paper plant for H 2 Regarding the BLG systems for H 2 production, the H 2 can effectively be separated from the synthesis gas using membrane separation as shown in Fig. 2 (b). Since H 2 is a very small molecule and thus passes through tight membranes quite effectively as compared to most other molecules except water. By condensing the water before the membrane unit, a relatively pure H 2 can be obtained in the permeate. The recently developed membranes requires only a few bar pressure difference that makes the synthesis gas upgrading system efficient from a system perspective. Example of such membranes are porous graphene [17] and PDMS composites with SiO 2 and B 2O 3 [18]. The H 2 is separated as permeate from the membrane separation while the rest of gas components are obtained as reject that can be combusted in a boiler or even in a gas turbine with an external combustion chamber, making a combined cycle possible. The heat from the steam turbine condenser then can be used for the district heating.

6 Erik Dahlquist et al. / Energy Procedia 105 ( 2017 ) Fig. 2. (a) CH 4 production from wood pellet gasification based bio-refinery system integrated with a CHP plant, (b) Black liquor gasification based H 2 production together with combined heat and power (CHP) production From a system perspective, the BLG solution with a combi-cycle gives an electric to fuel heating value efficiency of up to 38%, which is high for a process with such a poor fuel. If alternatives with biomass in a CHP plant are considered, the district heating becomes economically viable but the issue is that the heat demand varies significantly over the year. It is thus important to be flexible, e.g. the heat demand shall first be fulfilled during the winter and the chemical production may be of secondary importance. 5. Conclusions In this study, the synthesis gas quality from wood pellets gasification (WPG) has been compared with black liquor gasification (BLG) using modeling and experimental results at pilot scale. Based on the comparative analysis, it is concluded that black liquor as fuel feedstock is better for the production of H 2 while wood gasification is better for the production of methane and CO (that can be used further for methanol or DME production). Further, the CH 4 concentration has higher levels in the synthesis gas from wood pellet gasification as compared to CH 4 concentration from black liquor gasification. The relative oxidation together with plant loading has strong impact on the synthesis gas quality. In addition, the study shows how regression models such as Partial Least Squares (PLS) and similar can be made from experimental data and then combined with dynamic physical models developed in Modelica. The presented models not only be used to study different biomass gasification systems from the energy and balance perspective, but also investigate how to achieve a transition from one certain process to another efficiently. The data presented in this study could be used as input to relevant analysis using e.g. ASPEN plus and similar system analysis tools. Acknowledgements We thank Bioregional and especially Sue Riddlestone, for making their pilot plant available for the tests with wood pellets, and Swedish Energy Agency and KKS are acknowledged for the financial support. References 1. Asadullah M. Biomass gasification gas cleaning for downstream applications: A comparative critical review. Renewable and Sustainable Energy Reviews 2014; 40: Naqvi M. Analysing performance of bio-refinery systems by integrating black liquor gasification with chemical pulp mills. Doctoral dissertation. KTH Royal Institute of Technology, Stockholm. 85 p; 2012.

7 998 Erik Dahlquist et al. / Energy Procedia 105 ( 2017 ) Koppatz S, Pfeifer C, Rauch R, Hofbauer H, Marquar-Moellenstedt T, Specht M. H 2 rich product gas by steam gasification of biomass with in situ CO 2 absorbtion in a dual fluidized bed system of 8 MW fuel input. Fuel Processing Technology 2009; 90: Naqvi M, Yan J. Bio-refinery: Production of biofuel, heat, and power utilizing biomass. In: Handbook on Clean Energy Systems, John Wiley & Sons, ISBN: (2015). 5. Dvorak M, Havel P. Combined heat and power production planning under liberalized market conditions. Applied Thermal Engineering 2013; 43: Pruitt KA, Braun RJ, Newman AM. Establishing conditions for the economic viability of fuel cell-based, combined heat and power distributed generation systems. Applied Energy 2013; 111: Dipal B, Baruah DC. Modeling of biomass gasification: A review. Renewable and Sustainable Energy Reviews 2014; 39: Peduzzi E, Boissonnet G, Haarlemmer G, Dupont C, Marchal F. Torrefaction modelling for lignocellulosic biomass conversion processes. Energy 2014; 70: Naqvi M, Yan J, Dahlquist E. Synthetic natural gas (SNG) production at pulp mills from a circulating fluidized bed black liquor gasification process with direct causticization. In Proceedings: 23rd International Conference on Efficiency, Cost, Optimization, Simulation, and Environmental Impact of Energy Systems, ECOS 2010, Lausanne, Switzerland, June Larsson A, Seemann M, Neves D, Thunman H. Evaluation of performance of industrial-scale dual fluidized bed gasifiers using the Chalmers 2-4-MWth gasifier. Energy Fuels 2013; 27: Capata R, Mario Di V. Mathematical Modelling of Biomass Gasification in a Circulating Fluidized Bed CFB Reactor. Journal of Sustainable Bioenergy Systems 2012; 2: Colomba DB. Kinetic modeling of biomass gasification and combustion. Intelligent Energy Europe (PyNe). (downloaded 5 Jan, 2016) 13. Naqvi M, Dahlquist E, Yan J. Complementing existing CHP plants using biomass for production of hydrogen and burning the residual gas in a CHP boiler. Biofuels, ( 14. Yang C, Ogden J. Determining the lowest-cost hydrogen delivery mode. International Journal of Hydrogen Energy 2007; 32: Devi L, Ptasinski KJ, Janssen FJ. A review of the primary measures for tar elimination in biomass gasification processes. Biomass and Bioenergy 2003; 24: Dahlquist E, Jones A. Presentation of a dry black liquor gasification process with direct caustization. TAPPI Journal, June 2005: Huailiang D, Jingyuan L, Jing Z, Gang S, Xiaoyi L, Yuliang Z. Separation of Hydrogen and Nitrogen Gases with Porous Graphene Membrane. The Journal of Physical Chemistry 2011; 115(47): Suk HL, Hyun KL. Separation of Hydrogen-Nitrogen Gases by PDMS-SiO 2 B 2O 3 Composite Membranes DOI: /MEMBRANE_JOURNAL Biography (Erik Dahlquist) I am Professor of Energy Technology and Research Director of the School of Business, Society & Engineering. My research interests are Process Efficiency Improvements (Optimization and Automation towards process industries like pulp and paper, power plants, steel industry, power plants, biogas production and water treatment) Process Development (i.e. sensor developments, along with several new processes like biogas production, black liquor gasification and technologies for utilization of biomass and waste in different ways, among other waste gasification.) Energy and load management and energy efficient buildings (smart grids with integration between e.g. buildings and solar and wind power and smart houses with new services for the interaction between customers and energy companies)