EP2842188A1

EP2842188A1 - Method and arrangement for determining enthalpy balance of a fuel cell system

Info

Publication number: EP2842188A1
Application number: EP12724348.3A
Authority: EP
Inventors: Kim ÅSTRÖM; Tuomas Hakala; Tero Hottinen; Topi KORHONEN
Original assignee: Convion Oy
Current assignee: Convion Oy
Priority date: 2012-04-24
Filing date: 2012-04-24
Publication date: 2015-03-04
Also published as: WO2013160521A1

Abstract

The object of the invention is an arrangement comprising means (120) for providing enthalpy balance information of the fuel cell system on the basis of information of enthalpy flows accomplished by utilizing enthalpy feed in flow information provided by one or more of following means: means (122) for performing air feed in measurements to form air feed in information, means (124) for performing water flow in measurements to form water flow in information, means (126) for providing exhaust feed out information, means (128) for measuring to form electrical power production information, means (130) for measuring to form heating information of the fuel cell system, and means (134) for determining heat loss information of the fuel cell system. The arrangement comprises said means (120) for determining enthalpy balance of the fuel cell system by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero, and means (136) for determining methane content information of the fuel feed on the basis of said provided enthalpy balance information.

Description

Method and arrangement for determining enthalpy balance of a fuel cell system

The field of the invention

Most of the energy of the world is produced by means of oil, coal, natural gas or nuclear power. All these production methods have their specific problems as far as, for example, availability and friendliness to environment are concerned. As far as the environment is concerned, especially oil and coal cause pollution when they are combusted. The problem with nuclear power is, at least, storage of used fuel.

Especially because of the environmental problems, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above-mentioned energy sources, have been developed.

Fuel cell's, by means of which energy of fuel, for example biogas, is directly converted to electricity via a chemical reaction in an environmentally friendly process, are promising future energy conversion devices. The state of the art

Fuel cell, as presented in fig 1, comprises an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with fuel 108 producing water and also typically carbon dioxide (C02). Between anode 100 and cathode 102 is an external electric circuit 111 comprising a load 110 for the fuel cell.

In figure 2 is presented a SOFC device as an example of a high temperature fuel cell device. SOFC device can utilize as fuel for example natural gas, bio gas, methanol or other compounds containing hydrocarbons. SOFC device in figure 2 comprises more than one, typically plural of fuel cells in stack formation 103 (SOFC stack). Each fuel cell comprises anode 100 and cathode 102 structure as presented in figure 1. Part of the used fuel can be recirculated in feedback arrangement 109 through each anode. SOFC device in fig 2 also comprises fuel heat exchanger 105 and reformer 107. Typically several heat exchangers are used for controlling thermal conditions at different locations in a fuel cell process. Reformer 107 is a device that converts the fuel such as for example natural gas to a composition suitable for fuel cells, for example to a composition containing hydrogen and methane, carbon dioxide, carbon monoxide and inert gases. Anyway in each SOFC device it is though not necessary to have a reformer.

By using measurement means 115 (such as fuel flow meter, current meter and temperature meter) necessary measurements are carried out for the operation of the SOFC device. Part of the gas used at anodes 100 may be recirculated through anodes in feedback arrangement 109 and the other part of the gas is exhausted 114 from the anodes 100. A solid oxide fuel cell (SOFC) device is an electrochemical conversion device that produces electricity directly from oxidizing fuel. Advantages of SOFC device include high efficiencies, long term stability, low emissions, and cost. The main disadvantage is the high operating temperature which results in long start up and shutdown times and in both mechanical and chemical compatibility issues.

Natural gases such as methane and gases containing higher carbon compounds are typically used as fuels in SOFCs, which gases, however, have to be preprocessed before feeding to the fuel cells to prevent coking, i.e. formation of harmful carbon compounds such as for example coke, fly dust, tar, carbonate and carbide compounds. These different forms of carbon can be in this context called as general term being harmful carbon compounds. Hydrocarbons go through a thermal or catalytic decomposition in the formation of harmful carbon compounds. The produced compound can adhere to the surfaces of the fuel cell device and adsorbs on catalysts, such as nickel particles. The harmful carbon compound produced in the coking coats some of the active surface of the fuel cell device, thus significantly deteriorating the reactivity of the fuel cell process. The harmful carbon compounds may even completely block the fuel passage.

Preventing formation of harmful carbon compounds is, therefore, important for ensuring a long service life for the fuel cells. The prevention of formation of harmful carbon compounds also saves catalysts that are the substances

(nickel, platinum, etc) used in fuel cells for accelerating chemical reactions.

Gas pre-processing requires water, which is supplied to the fuel cell device.

The water produced in combining the oxygen ion and the fuel, i.e. the gas on the anode 100 side, can also be used in the pre-processing of the gas.

The single pass SOFC fuel utilization (FU) and oxygen-to-carbon ratio (OC) are critical parameters in SOFC system control. Exceeding the limit values for FU and OC increase the degradation and/or damage immediately the SOFC system e.g. by coking. However, small safety margins are desirable for FU and OC to maximize system efficiency, reduce unnecessary high fuel recycling and reduce unnecessary steam flow to fuel system.

In determination of FU and OC accurately in process control, the

hydrocarbon (such as methane, CH4, and higher hydrocarbons)

concentration in fuel needs to be known. In biogas applications this is problematic since CH4 concentration varies, but accurate on-line CH4 concentration measurement devices are too expensive for commercial SOFC systems. Similarly in natural gases the amount of CH4 and also that of higher hydrocarbons may vary, causing a similar problem. In one prior art embodiment type, which is in use e.g. in Wartsila New Energy unit running on a landfill gas, the measured variation of CH4 fraction in the fuel has been 30-45 %. The accuracy of a suitable and adequately inexpensive CH4 measurement device is typically only 4 %-units since the calibration tends to drift during a long usage. The 4 %-unit error in CH4 concentration as such means about 10 %-unit error in calculated FU (fuel utilization).

Short description of the invention

The object of the invention is to accomplish a fuel cell system, where enthalpy balance is determined practically by utilizing preferred

measurements to achieve an advanced controlling of the fuel cell system operation process. This is achieved by an arrangement for determining enthalpy balance of a fuel cell system, each fuel cell in the fuel cell system comprising an anode side, a cathode side, and an electrolyte between the anode side and the cathode side, and the fuel cell system comprising means for feeding fuel to the fuel cell system. The arrangement comprises means for providing enthalpy balance information of the fuel cell system on the basis of information of enthalpy flows accomplished by utilizing enthalpy feed in flow information provided by one or more of following means: means for performing air feed in measurements to form air feed in information, means for performing water flow in measurements to form water flow in

information, means for providing exhaust feed out information, means for measuring to form electrical power production information, means for measuring to form heating information of the fuel cell system, and means for determining heat loss information of the fuel cell system, and the

arrangement comprises said means for determining enthalpy balance of the fuel cell system by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero, and means for determining methane content information of the fuel feed on the basis of said provided enthalpy balance information.

The focus of the invention is also a method for determining enthalpy balance of a fuel cell system, in which method is fed fuel to the fuel cell system. In the method is formed enthalpy feed in flow information from information gained by performing one or more of following method steps: is performed air feed in measurements to form air feed in information, is performed water flow in measurements to form water flow in information, is measured exhaust feed out to form exhaust feed out information, is measured electrical power to form electrical power production information, is measured temperature conditions to form heating information of the fuel cell system, and is formed heat loss information of the fuel cell system, and on the basis of the formed enthalpy feed in flow information is accomplished information of enthalpy flows to provide enthalpy balance information of the fuel cell system by summing information of enthalpy flows, and by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero, and methane content information of the fuel feed is determined on the basis of the provided enthalpy balance information.

The invention is based on accomplishing information of enthalpy flows by utilizing enthalpy feed in flow information provided by at least mainly existing means in the fuel cell system. For example means for providing at least methane content information as fuel feed in information can be used to provide enthalpy feed in flow information. The invention is further based on determining enthalpy balance of the fuel cell system by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero.

The benefit of the invention is that cost effective measurement technology can be utilized for determining a good estimate for enthalpy flows and for CH4 concentration. Furthermore most of the measurements are in practice easy as no substantially hot gas flows need to be measured in embodiments according to the present invention. Short description of figures presents a single fuel cell structure. Figure 2 presents an example of a SOFC device.

Figure 3 presents a preferred embodiment according to the present

invention.

Detailed description of the invention

Solid oxide fuel cells (SOFCs) can have multiple geometries. The planar geometry (Fig. 1) is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte 104 is sandwiched in between the electrodes, anode 100 and cathode 102. SOFCs can also be made in tubular geometries where for example either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. This can be also arranged so that the gas used as fuel is passed through the inside of the tube and air is passed along the outside of the tube. Other geometries of SOFCs include modified planar cells (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are promising, because they share the advantages of both planar cells (low resistance) and tubular cells.

The ceramics used in SOFCs do not become ionically active until they reach a very high temperature and as a consequence of this the stacks have to be heated at temperatures ranging from 600 to 1,000 °C. Reduction of oxygen 106 (Fig. 1) into oxygen ions occurs at the cathode 102. These ions can then be transferred through the solid oxide electrolyte 104 to the anode 100 where they can electrochemically oxidize the gas used as fuel 108. In this reaction, water and carbon dioxide byproducts are given off as well as two electrons. These electrons then flow through an external circuit 111 where they can be utilized. The cycle then repeats as those electrons enter the cathode material 102 again. In large solid oxide fuel cell systems typical fuels are natural gas (mainly methane and some amounts of higher hydrocarbons), different biogases (mainly nitrogen and/or carbon dioxide diluted methane), and other higher hydrocarbon containing fuels, including alcohols. Methane and higher hydrocarbons need to be reformed either in the reformer 107 (Fig 2) before entering the fuel cell stacks 103 or (partially) internally within the stacks 103. The reforming reactions require certain amount of water, and additional water is also needed to prevent possible carbon formation, i.e. coking caused by higher hydrocarbons. This water can be provided internally by circulating the anode gas exhaust flow, because water is produced in excess amounts in fuel cell reactions, and/or said water can be provided with an auxiliary water feed (e.g. direct fresh water feed or circulation of exhaust condensate). By anode recirculation arrangement also part of the unused fuel and dilutants in anode gas are fed back to the process, whereas in auxiliary water feed arrangement only additive to the process is water.

In embodiments according to the present invention is presented a method and an arrangement to calibrate methane (CH4) measurement on-line in a fuel cell system, such as for example in a biogas SOFC application. The principle of the method is to calculate the enthalpy balance of the SOFC system. The sum of all enthalpy streams has to be zero. If only one enthalpy stream is unknown it can be calculated based on others. The known enthalpy streams are calculated based on measurements which are already needed to other purposes in the SOFC unit control. These other measurements are more accurate or more stable than the CH4 measurement so their usage to calibrate CH4 measurement is justified.

The known enthalpy feeds and the utilized measurements in a SOFC anode recycle type unit can be: A) air feed in measurements, such as temperature, flow amount and humidity measurement of the air feed-in, B) water flow in measurements, such as flow amount and temperature measurements of the water flow in, C) exhaust feed out information, which preferably is a combination of exhaust temperature measurement information and process calculation of composition and flow volume. Process calculation can be performed from system in flows for example by assuming that all fuel is burned in the afterburner. D) Electrical power production information, which can be based on voltage and/or electrical current measured, E) system heating information, which can be based for example on power consumption measured from at least one electrical heater, F) fan and/or compressor heating information of gas lines, which information can be based for example on efficiency information provided for example from manufacturer and/or on power consumption measured in the at least one fan and/or compressor, and G) heat loss information, which can be calculated, for example as is described in more detail later in this description.

The remaining enthalpy is fuel feed in. From fuel the C0₂ concentration, temperature and flow are measured, so the only unknown parameter is methane CH₄ concentration in an exemplary case where fuel is assumed to contain CH₄, C0₂ and N₂, where N₂ = 1-CH₄-C0₂.

According to the present invention only relatively rough accuracy in measurements is needed to obtain a good estimate for enthalpy flows and CH4 concentration. Furthermore most of the measurements are in practice easy as no substantially hot gas flows need to be measured. By utilizing this calculation method according to the invention and by performing needed measurements with standard SOFC system measurement instrumentation will result approximately to 3-5 times higher accuracy in CH4 measurement.

When heat loss to surroundings is determined, heat loss cannot be directly measured from an on-field unit. By implementing a calibrated CH4

measurement device, the calculation method according to the present invention can be utilized in a backward mode to estimate the heat loss for the particular fuel cell unit type. The heat loss is the same in all similar units so reference values should be also available when several fuel cell units are operated on-field. Additional estimate for heat loss can be also obtained from thermal simulations. The accuracy of heat loss estimate can be further increased by developing a model for heat loss changes during unit lifetime and due changes in surrounding temperature based on laboratory tests and thermal simulations.

During a constant load operation the heat loss varies between different constant loading operation points even +/- 33 % in a substantially small fuel cell unit when error effects of the other measurements are considered.

Although the variation seems quite large, using heat loss estimate with this accuracy in embodiments according to the invention would yield still in only 0.6 % fuel cell unit error in CH4 concentration and about 1.5 % fuel cell unit error in the estimated fuel utilization (FU). During start-up or shut-down situation the thermal gradients to surroundings vary by assuming a constant heat loss is not valid. It can be considered that during start-up or shut-down situation larger safety margins are employed for methane CH4 concentration, FU and oxygen to carbon ratio (O/C). After the unit has operated near nominal operating point for example 1-2 hours and the thermal gradients have stabilized, the safety-margins can be reduced and the calibration method according to the present invention can be used with its full accuracy.

In one exemplary embodiment according to the invention the method is applied to natural gas fuel, which contains 80-98 %, or even 60-98 %, of methane CH₄ with variation depending for example on geographical location and seasonal changes. The rest of the gas can consist varying amounts of higher hydrocarbons (e.g. C2H6, C3H8, C₄H₁₀), which are, as CH₄, reformed to SOFC fuel in the SOFC system, and the gas also consists of N₂ and of small amount of C0₂. Usually long-term average composition is well known by the gas supplier but the short-term deviations from the average values can be large, for example 10-20 %-units for CH₄. Accurate on-line measurement of all gas components is practically impossible. However, good estimates for FU and O/C can be achieved with the enthalpy balance calculation if fixed amounts are assumed for higher hydrocarbons, C0₂ is assumed to zero and only CH₄ fraction is solved. The N₂ content is "1 - others". The solved CH₄ fraction is not the real CH4 fraction as it resembles also the varying amount of enthalpy from the higher hydrocarbons. In two exemplary situations CH4 fraction is solved in situations where the actual fuel consists 80 % of CH₄, 18 % of higher hydrocarbons and 2 % of nitrogen N₂ fuel utilization FU is estimated with 1.5 % accuracy and oxygen to carbon ratio O/C is estimated with 6 % accuracy, with artificial fuel consisting only CH₄ and N₂.

Next is described a preferred method according to the present invention for determining enthalpy balance of a fuel cell system, in which method is fed fuel to the fuel cell system, and is performed anode 100 side recirculation flow of reactants. Enthalpy feed in flow information is formed from

information gained by performing one or more of following method steps: is performed air feed in measurements to form air feed in information, is performed water flow in measurements to form water flow in information, is measured exhaust feed out to form exhaust feed out information, is measured electrical power to form electrical power production information, is measured temperature conditions to form heating information of the fuel cell system, and is formed heat loss information of the fuel cell system. Said measurements can be performed, at least mainly, by utilizing common and relatively cheap measurement equipment without high accuracy demands. Enthalpy feed in flow information can be provided by gaining information about gas lines for example from fan, compressor, etc. Exhaust feed out information can be provided by calculating information from fuel cell system in flows on the basis of an assumption that essentially all fuel is burnt in the afterburner. Further, in the method can be measured power consumption of at least one electrical heater to form heating information of the fuel cell system. On the basis of the formed enthalpy feed in flow information is accomplished information of enthalpy flows to provide enthalpy balance information of the fuel cell system by summing information of enthalpy flows, and by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero. Methane content information of the fuel feed is determined on the basis of said enthalpy balance information gained through summing of information of the enthalpy flows. Information of one enthalpy flow can be determined by calculating it on the basis of the summed information of enthalpy flows. By this way in the method is determined more adjusted methane content information to be utilized in controlling more accurately at least one of fuel utilization (FU) and oxygen to carbon ratio (O/C).

In one preferred embodiment of the method is calibrated at least one of the means for measurements 122, 124, 126, 128, 130, 134, 138 on the basis of the determined methane content information of the fuel feed. In the method can be also estimated heat loss characters for the fuel cell unit type, which is in use, to form a heat loss model of the fuel cell system on the basis of said estimation, said model to be utilized by the fuel cell system. The estimation of heat loss characters can also be calibrated on the basis of the determined methane content information of the fuel feed.

In figure 3 is presented a preferred arrangement according to the present invention for determining enthalpy balance of a fuel cell system by implementing the described preferred method. The fuel cell system comprises means 117 for feeding fuel to the fuel cell system. The means 117 can be arranged for example by prior art fuel feed system comprising a fuel source and a pipe line through which fuel is fed into the fuel cell system. The fuel cell system of figure 3 comprises means 109 for performing anode side 100 recirculation flow of reactants. The means 109 are not necessary in implementing the arrangement and method according to the invention, i.e. the present invention can also be implemented and utilized in a fuel cell system, which does not comprise recirculation of the anode side 100 gas. The preferred arrangement comprises means 120 for providing enthalpy balance information of the fuel cell system on the basis of information of enthalpy flows accomplished by utilizing enthalpy feed in flow information provided by one or more of following means: means 122 for performing air feed in measurements to form air feed in information, means 124 for performing water flow in measurements to form water flow in information, means 126 for providing exhaust feed out information, means 128 for measuring to form electrical power production information, means 130 for measuring to form heating information of the fuel cell system, and means 134 for determining heat loss information of the fuel cell system. Said means 122, 124, 126, 128, 130, 134 can be implemented, at least mainly, by utilizing common and relatively cheap measurement equipment without high accuracy demands. The arrangement can comprise as means for providing enthalpy feed in flow information means 138 for gaining information about gas lines for example from fan, compressor, etc. The means 126 can provide exhaust feed out information by calculating information from fuel cell system in flows on the basis of an assumption that essentially all fuel is burnt in the afterburner. The means 130 can measure power consumption of at least one electrical heater to form heating information of the fuel cell system.

The preferred arrangement comprises the means 120 to determine enthalpy balance of the fuel cell system by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero. Information of at least one enthalpy flow can be determined by calculating it on the basis of the summed information of enthalpy flows. Further, the arrangement comprises means 136 for determining methane content information of the fuel feed on the basis of said provided enthalpy balance information. The preferred function of the means 136 is to determine more adjusted methane content information to the means 120 for controlling more accurately at least one of fuel utilization (FU) and oxygen to carbon ratio (O/C). The means 120 and the means 136 are preferably implemented by a calculative program used in at least one digital processor and the means 120, 136 can be located for example in a same computer (Fig. 3), which comprises said at least one digital processor. The controlling of at least one of fuel utilization (FU) and oxygen to carbon ratio (O/C) is performed for example so that the means 120 give a control command via a control line 120 (wired or wireless), and the fuel cell system changes at least one of its operation factors (air amount, water amount, temperature, fuel cell voltage, etc) according to the control command.

One preferred arrangement can comprise means 138 for calibrating at least one of the means for measurements 122, 124, 126, 128, 130, 134, 138 on the basis of the determined methane content information of the fuel feed. The arrangement can also comprise means for estimating heat loss characters for the fuel cell unit type, which is in use, to form a heat loss model of the fuel cell system on the basis of said estimation, said model to be utilized by the fuel cell system. Also the means for estimating heat loss characters can be calibrated by said means 138 for calibrating on the basis of the determined methane content information of the fuel feed. The means for estimating heat loss characters can be arranged for example by a calibrated methane measurement device.

Although the invention has been presented in reference to the attached figures and specification, the invention is by no means limited to those as the invention is subject to variations within the scope allowed for by the claims.

Claims

1. An arrangement for determining enthalpy balance of a fuel cell system, each fuel cell in the fuel cell system comprising an anode side (100), a cathode side (102), and an electrolyte (104) between the anode side and the cathode side, and the fuel cell system comprising means (117) for feeding fuel to the fuel cell system, characterized by, that the arrangement comprises means (120) for providing enthalpy balance information of the fuel cell system on the basis of information of enthalpy flows accomplished by utilizing enthalpy feed in flow information provided by one or more of following means: means (122) for performing air feed in measurements to form air feed in information, means (124) for performing water flow in measurements to form water flow in information, means (126) for providing exhaust feed out information, means (128) for measuring to form electrical power production information, means (130) for measuring to form heating information of the fuel cell system, and means (134) for determining heat loss information of the fuel cell system, and the arrangement comprises said means (120) for determining enthalpy balance of the fuel cell system by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero, and means (136) for determining methane content information of the fuel feed on the basis of said provided enthalpy balance information.

2. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 1, characterized by, that the arrangement comprises means (138) for calibrating at least one of the means for measurements (122, 124, 126, 128, 130, 134, 138) on the basis of the accomplished methane content information of the fuel feed.

3. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 1, characterized by, that the arrangement comprises the means (136) for determining more adjusted methane content

information to the means (120) for controlling more accurately at least one of fuel utilization (FU) and oxygen to carbon ratio (O/C).

4. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 1, characterized by, that the arrangement comprises the means (120) for determining information of at least one enthalpy flow by calculating it on the basis of the summed information of enthalpy flows.

5. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 1, characterized by, that the arrangement comprises as means for providing enthalpy feed in flow information means (138) for gaining information about gas lines from at least one of fan and compressor.

6. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 2, characterized by, that the arrangement comprises means for estimating heat loss characters for the fuel cell unit type, which is in use, to form a heat loss model of the fuel cell system on the basis of said estimation, said model to be utilized by the fuel cell system.

7. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 1, characterized by, that the fuel cell system comprises means (109) for performing anode side (100) recirculation flow of reactants.

8. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 1, characterized by, that the arrangement comprises the means (126) for providing exhaust feed out information by calculating information from fuel cell system in flows on the basis of an assumption that essentially all fuel is burnt in the afterburner.

9. An arrangement for determining enthalpy balance of a fuel cell system in accordance with claim 1, characterized by, that the arrangement comprises the means (130) for measuring power consumption of at least one electrical heater to form heating information of the fuel cell system.

10. A method for determining enthalpy balance of a fuel cell system, in which method is fed fuel to the fuel cell system, characterized by, that in the method is formed enthalpy feed in flow information from information gained by performing one or more of following method steps: is performed air feed in measurements to form air feed in information, is performed water flow in measurements to form water flow in information, is measured exhaust feed out to form exhaust feed out information, is measured electrical power to form electrical power production information, is measured temperature conditions to form heating information of the fuel cell system, and is formed heat loss information of the fuel cell system, and on the basis of the formed enthalpy feed in flow information is accomplished information of enthalpy flows to provide enthalpy balance information of the fuel cell system by summing information of enthalpy flows, and by detecting that sum of summed information of enthalpy flows is zero or essentially near to zero, and methane content information of the fuel feed is determined on the basis of the provided enthalpy balance information.

11. A method in accordance with claim 10, characterized by, that in the method is calibrated at least one of the means for measurements (122, 124, 126, 128, 130, 134, 138) on the basis of the accomplished methane content information of the fuel feed.

12. A method in accordance with claim 10, characterized by, that in the method is determined more adjusted methane content information to be utilized in controlling more accurately at least one of fuel utilization (FU) and oxygen to carbon ratio (O/C).

13. A method in accordance with claim 10, characterized by, that in the method is performed anode (100) side recirculation flow of reactants.

14. A method in accordance with claim 10, characterized by, that in the method is provided enthalpy feed in flow information by gaining information about gas lines from at least one of fan and compressor.

15. A method in accordance with claim 11, characterized by, that in the method is estimated heat loss characters for the fuel cell unit type, which is in use, to form a heat loss model of the fuel cell system on the basis of said estimation, said model to be utilized by the fuel cell system.

16. A method in accordance with claim 10, characterized by, that in the method is provided exhaust feed out information by calculating information from fuel cell system in flows on the basis of an assumption that essentially all fuel is burnt in the afterburner.

17. A method in accordance with claim 10, characterized by, that in the method is determined information of at least one enthalpy flow by calculating it on the basis of the summed information of enthalpy flows.

18. A method in accordance with claim 10, characterized by, that in the method is measured power consumption of at least one electrical heater to form heating information of the fuel cell system.