MORE EFFICIENT CONVERSION OF BIOMASS IN COGENERATION OR FUEL PRODUCTION

SELECTED PATHWAYS FOR A MORE EFFICIENT CONVERSION OF BIOMASS IN COGENERATION OR FUEL PRODUCTION

Daniel Favrat[1], François Marechal, Jan van Herle, Stefan Heyne, Martin Gassner

Industrial Energy Systems Lab., Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland 

ABSTRACT

Among renewable energy sources biomass plays a major role. Several paths for improving the conversion of biomass with minimum burden on the environment are being pursued worldwide. This paper reviews some of the most recent results from research at the Swiss Federal Institute of Technology of Lausanne.

On the theoretical and experimental sides it covers the improvement of biogas combustion engines using combustion prechambers both with spark ignition or autoignition in order to reduce the emissions while keeping a high efficiency level, the integration of ORC heat recovery cycles on biogas cogeneration engines and tests with real biogas in Solid Oxide planar Fuel Cells.

On the process modelling side the paper reviews the environomic (environment, economic and energetic) optimisation of the design of biofuel production by means of gasification and fuel reforming. Thermo-economic process modelling and integration techniques are coupled with a multi-objective optimisation algorithm to target the best process technology and operating conditions for the trigeneration of fuels, heat and power.

Keywords: biomass conversion, process optimisation, Organic Rankine Cycles, biogas engine, Solid Oxide Fuel Cell.

1.     INTRODUCTION

Among renewable energy sources biomass always played a major role and this role is bound to be maintained or even increased in the future in light of the closed CO₂ cycle which is made possible. Among renewables, biomass or at least solid biomass has the major advantage of being easily storable. While biomass has been for centuries primarily used for low exergy services like heating and cooking from direct combustion the future will likely be characterised by more advanced conversion paths to higher exergy value products like electricity, combined heat and power or fuels [1].

Several paths for improving the conversion of biomass with minimum burden on the environment are being pursued worldwide. This paper reviews some of the pathways and most recent results from research at the Swiss Federal Institute of Technology of Lausanne.

2. BIOGAS ENGINES CLEAN COMBUSTION

One of the most convenient form of biomass is biogas extracted from sewage water treatment plants. This biogas consists primarily of methane and CO₂, which in many areas is converted in-situ into electricity and heat by a cogeneration internal combustion engine. However, emissions from gas engines can be relatively high and contaminants present in the biogas often prevent the use of catalytic post treatments. Therefore, in countries with tough environmental legislation like Switzerland, one common way has been to lower the compression ratio of the engine in order to keep primarily NOx and CO within acceptable limits. As the mechanical efficiency of internal combustion engines is a function of the compression ratio this implies a net exergy loss.

One way to circumvent the above drawback is to improve the combustion. For lean burn engines this can be achieved by displacing the spark plug to the bottom of an unscavenged ignition prechamber as shown in figure 1.

Figure 1: 150 kW biogas engine with unscavenged ignition prechamber [2,3]

Figure 2: For a 150kW engine, comparison between (a) prechamber ignition and direct ignition of natural gas and (b) prechamber ignition with natural gas and biogas [2,3]

As shown in Figure 2a related to tests with natural gas, prechamber ignition allows a reduction of CO emissions of 40% compared to direct ignition and this for the same range of NOx emissions. Results published in [2] show that even larger reductions (55%) are achieved for the unburned hydrocarbons. Note that in Figure 2, the red rectangle corresponds to the Swiss regulation for stationary gas engines.

Figure 2b shows that, for the same conditions, a further reduction of the CO emissions can be achieved when going from natural gas to biogas, leaving a margin for an increase of the compression ration with biogas.

Figure 3: For a 150 kW engine, comparison of shaft efficiency between natural gas and biogas (ST-8CA indicates that tests have been made at the same spark timing relative to the crank angle)

Figure 3 shows that for the same conditions of compression ratio, prechamber ignition and spark timing, biogas efficiency results are only slightly lower than with natural gas. The level of efficiency with biogas, which is higher than 36% while still meeting the Swiss norms, is significantly higher than the efficiency of direct ignition biogas engines presently operated in Switzerland.

At a time of growing environmental concerns particularly in large cities worldwide the above technical improvements allowing a better use of the biomass resource with low emissions are promising.

A next step currently under investigation is the potential replacement of the spark plug ignition by auto ignition in temperature controlled prechambers. This would potentially eliminate one key maintenance concern and allow a reduction of the volume of the prechamber as a result of the faster generation of hot jets from the prechamber nozzles. The proof of concept is being investigated for natural gas both with a theoretical approach (new reaction mechanism and coupling between the new reaction mechanism [4] and Navier-Stokes calculations) and with tests in a single cylinder engine [5, 6].

Figure 4a shows a schematic view of the hot jets from the prechambers, which are to ignite the mixture of the main chamber. Note that the cylinder head is conical in this particular case. Autoignition has been demonstrated but with instabilities which need further analyses. Calculations on the right (Figure 4b) show that, in certain conditions, parasitic auto ignition could take place inside the main chamber, which is one the problems which are being faced in the on-going investigations.

Figure 4: Prechamber autoignition system, (a) representation of the hot gas jets ignition in the main chamber [5], (b) heated autoignition prechamber with Navier-Stokes calculation on the right [6].

3. BIOGAS COGENERATION UNITS WITH ORC

Figure 5 shows the efficiency of natural gas engines resulting from an extensive data base of installed cogeneration units in Europe. It has been shown above that efficiency improvements at the engine and combustion level can be made and the red mark shows the level which can be achieved with unscavenged prechambers. However, in many of the applications, recovering the heat from the jacket cooling or from the exhaust gases is only partial and often seasonal. Therefore there is a reasonable potential for electric efficiency improvements by converting the waste heat using Organic Rankine Cycles (ORC). This, of course, applies to biogas engines as well.

The green zone in figure 5 illustrates the gain of efficiency which can be expected. One of the obstacle to the use of ORCs in these applications has been the unavailability of reliable expander-generator at small scale (5 to 20 kWel). Therefore one research effort at EPFL has been the demonstration of the feasibility of small hermetic expander-generators, so far mainly derived from existing scroll compressors. Published demonstration results include the heat conversion in a hybrid solar thermal power plant coupled with a biofuel Diesel engine [5] and the initial tests of an ORC unit as an add-on to biogas engines installed near Geneva [6]. In the latter case biogas is produced in fermenters using greens collected separately from city wastes. For conservative reasons only the heat from jacket cooling is converted so far in this plant and with a net efficiency of 6% (4.8 kWe) and an exergy efficiency of 38%. Significantly better efficiencies could be expected in the case of ORC units directly assembled with the engine cogeneration units, which would limit the parasitic losses linked to long pipe connections imposed in the retrofit case.

The present work with ORC equipped with scroll expander generators is being marketed by a startup company from EPFL (LENI) and new concepts of scroll expanders are being studied jointly.

Figure 5 Gas engines with ORC unit: overall expected efficiency (left) and measured ORC efficiency in a retrofit case [6].

4. BIOGAS FED SOLID OXIDE FUEL CELLS (SOFC)

Figure 5 shows that the electrical efficiency of present cogenerator units falls in the small power range. On the other hand new technologies and in particular SOFC, which are quite tolerant to a variety of fuels are emerging. SOFC technology is one of them and can be considered as a serious candidate in particular for the small cogeneration market (1 to 200 kW). For this market planar SOFC concepts are considered because of their compactness and potentially low specific price.

Present work at EPFL(LENI) includes [9,10,11]:

-          Experimental and theoretical investigations of various planar anode supported SOFC cells and stacks in collaboration with an industrial company. The experimental investigation includes feeding a cell or a stack with hydrogen or methane or simulated biogas. Investigation also include segmented electrode tests. In addition small sample material tests in particular for cathodes and anodes are done in collaboration with other universities and European research centers.

-          The assistance to field tests of SOFC units with farm biogas [12,13].

-          System studies for the best integration of SOFC fuel cells in complete systems including hybrid fuel cell – gas turbine cycles.

With biogas several reforming aspects are to be mentioned in comparison with natural gas. One is the absence of non-methane hydrocarbons (except for landfill gas) and the other is the presence of a significant proportion of carbon dioxide. These two features represent an advantage for the fuel processing upstream of the SOFC. Higher hydrocarbons compared to methane are more prone to carbon deposition because of their lower decomposition temperature, while CO₂ can be used as a reforming agent to partly convert methane into syngas. However the amount of CO₂ is far from sufficient to operate in a thermodynamically safe region to avoid carbon deposition above 600°C and another reforming agent (either water vapor or oxygen from air) still has to be added. A sensitivity analysis and a comparison between reforming approaches can be found in [9, 10]

Figure  6 (a) Assembly of a SOFC stack (collaboration with SOFCPower)  and (b) CFD flow calculation in a cell

Figure 6 (a) shows a cocurrent stack being assembled and instrumented. Figure 6(b) shows a CFD result of the predicted fuel concentration along the anode of a planar SOFC fuel cell.

So far prototypes up to 1 kWel with efficiencies above 30% and a power density of the order of 1.5 kW/dm³ have been realised. Research key topics include the improvement of the sealing elements, which are crucial for planar fuel cells, trends towards bigger active area per cell, and improved life time.

5. BIOGAS FROM WOOD

Among the various sources of biomass wood is the one, which is not in direct competition with food and is available in many areas around the world. Although there are many discussions about future networks of hydrogen with tremendous challenges in terms of safety and overall energy efficiency, one path to be seriously considered is the conversion of wood to methane for reinjection in existing natural gas networks. It can then be used in high value applications like decentralized cogeneration or for transportation feeding compressed natural gas (CNG) vehicles, which are known to be less polluting.

The challenges to convert wood to methane are both economical and physical (gasification). Modern methods of process integration including superstructure based multi-objective optimisation are being developed at EPFL (LENI).

Currently different process designs are under investigation. From the atomic composition of wood they all have in common that the gas produced by gasification lacks hydrogen for completely reforming the carbon into methane, which implies a by-production of CO₂. However the thermochemical production processes are exothermic implying that the co-production of electricity is possible. Hence one idea consisting in integrating an electrolyser in the system to increase the methane yield has been conceptually studied and some of the results partly reported here from [14]. This is also favoured by the fact that oxygen from the electrolyser can also be profitable to the gasification process. Alternatively if electricity is imported from renewable sources, the process could represent a way of storing green electricity in the form of synthetic natural gas.

The investigated technological options can be described in a global superstructure flow diagram represented in figure 7. It includes a sub-process of wood drying, followed by the sub-processes of gasification, gas cleanup to prevent methanation catalyst damages, reforming with steam addition, gas post-treatment to finally get the quality required for reinjection in the gas network (Wobbe index between 13.3 and 15.7 kWh/Nm³).

Figure 7 Process superstructure (dashed boxes include competing sub-processes and dotted boxes include optional sub-processes) [14]

Figure 8 shows the two alternatives for the integration of electrolysis to the gasification. Detailed analysis using a multi-objective optimisation shows that indirectly heated system are preferable and figure 9 illustrates the influence in terms of total production costs for this option. Further details can be found in [14]

Figure 8 Pathways from electrolysis through directly or indirectly heated systems [14].

The exergy efficiency of figure 9 is defined as follows:

In which  designates the exergy value per unit of mass while  refers to the overall produced power and  refers to the overall consumed power (nomenclature and definitions are based on [15].

The results of figure 9 are based on a nominal power plant of 20 MWth which is under planning in Switzerland and a 50%wt humidity has been assumed. They indicate a potential increase of the exergy efficiency by 2 to 3 % as a result of the introduction of an electrolyser. For the given economic environment the optimum exergy efficiency could be achieved with an extra cost of about 20%.

Although the specific production cost of SNG tends to increase with the addition of hydrogen by electrolysis, the profit for a given amount of wood might increase as a result of the increased production of additional gas. Future implementation of such technology will be highly influenced by future governmental measures to favor renewable or not.

Figure 9 Exergy efficiency and production costs in the case of indirectly heated gasification [14]

Table 1 Assumptions for the economic analysis

Parameter	Value
Marshall & Swift index (2004)	1197
Dollar exchange rate	1 euro/US$
Interest rate	6%
Expected lifetime	15 years
Plant availability	90%
Operators	4p./shift
Operator salary	60kEuro/year
Maintenance costs	5%/year
Oxygen price	70 Euro/ton
Wood price (at 50%wt humidity)	16.7 Euro/MWh
Electricity price (import)	88.9 Euro/MWh
Electricity price (export)	26.4 Euro/MWh

6. CONCLUSIONS

Improvements for a more efficient energy conversion of biomass into high exergy products either in cogeneration of heat and power or of trigeneration of power, heat  and fuel is important for biomass to play a major role in solving present resource and environmental challenges.

While internal combustion engines play a dominant role for cogeneration units fed by biogas, less polluting combustion can be achieved by using prechambers. At small scale particularly advanced cogeneration technologies based on SOFC fuel cells are emerging with a large potential both in terms of energy efficiency, pollution and convenience of use. For both of the above technologies the integration of small ORC based on scroll expander-generators will further improve the conversion efficiency. Moreover converting wooden products into fuels, and in particular methane to be reinjected into gas networks when available, is a promising way to favor a better use of these important resources.

Finally process improvements using more advanced design and planning techniques already at the conceptual level are emerging. In particular tools taking advantage of modern information technology approaches like pinch technology and multi-objective optimisation can significantly contribute to a more rational use of energy and financial resources. Information sharing with model and technology exchanges between groups of developing, emerging and developed countries are in the present context to be further developed.

7. REFERENCES

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[2]        Roubaud A., Roethlisberger R.P., Favrat D., Lean-burn cogeneration biogas engine with unscavenged combustion prechambers: comparison with natural gas, Int J. Applied Thermodynamics, Vol.5(No4)pp169-175,Dec 2002

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[4]        Heyne S., Roubaud A., Ribaucour M., Vanhove G., Minetti R., Favrat D. Development of a natural gas reaction mechanism for engine simulations based on rapid compression machine experiments using a multi-objective optimisation strategy. Submitted to Fuel (2007)

[5]        Meier M. Etude expérimentale de l’auto-inflammation d’une préchambre de moteur monocylindre ; Perfectionnement des diagnostiques et implémentation d’un système de démarrage sans bougie. EPFL Master thesis 2007

[6]        Wunsch D. Numerical flow simulation of a natural gas engine equipped with an unscavanged auto-ignition prechamber. EPFL Master thesis 2006

[7]        Kane M., Larrain D., Favrat D., Allani Y. Small hybrid solar power system. Energy:The International Journal 28/14 pp1427-1443, 2003

[8]        Kane M., Favrat D., Gay B., Andres O. Scroll expander Organic Rankine Cycle (ORC) efficiency boost of biogas engines. Proc. ofECOS 2007, ed Mirandola A, June, Padova, Italy, pp1017-1024

[9]        Van Herle J, Membrez Y, Bucheli O., Biogas as a fuel source for SOFC co-generators. J. of Power Sources 127(2004)300-312

[10]      Van herle J, Marechal F, Leuenberger S, Membrez Y, Bucheli O, Favrat D Process flow model of solid oxide fuel cell system supplied with sewage biogas. J. of Power Sources 131 (1-2): 127-141, 2004

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[12]      Van Herle J., Final Report on Analysis of Biogas for Solid Oxide Fuel Cells for the Swiss Federal Energy Office, May 2003, CH-3003 Bern, Switzerland.

[13]      Jenne M., et al. Sulzer HEXIS SOFC systems for biogas and heating oil, in: U. Bossel (Ed.), Proceedings of the 5th European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, July 2002, European Forum Secretariat, CH 5452-Oberrohrdorf, Switzerland, pp. 460–466

[14]      Gassner M., Marechal F. Thermoeconomic optimisation of the integration of electrolysis in a wood to methane process. Proc of ECOS2006, ed Frangopoulos C.

[15]      Borel L., Favrat D. Thermodynamique et énergétique. Presses Polytechniques et Universitaires Romandes, EPFL Lausanne, Switzerland 2005 (in the process of being translated in English at EPFL Press)

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[1] Industrial Energy Systems Lab., Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland. Tel: +41 21693 2511, Fax: +41 21 693 3502.daniel.favrat@epfl.ch