Hydrogen and its competitors, 2004

40Risø Energy Report 3Hydrogen conversion technologies5.3anode compartment of the PEMFC. The PEMFC itself iscooled by water, from which heat is recovered.Off-gas from both the anode and the cathode is firstcooled to condense out pure water (boiler feed water,BFW) for re-use. The offgas, which contains unburnedhydrogen, is then burned in the reformer heater.Such a system has an electrical efficiency of 37% and atotal efficiency of 82%.Homogeneous charge compression ignitionHomogeneous charge compression ignition (HCCI) is anemerging technology for internal combustion enginesthat can provide high efficiency (up to 43%) usingvarious fuels, including hydrogen. HCCI creates verylittle pollution, with almost no NOx or particulates. Thetechnology is of interest for both transport and electricitygeneration, and is being followed closely by automotivemanufacturers. The basic advantage of HCCI is tocombine the very low emissions of an SI engine using athree-way catalyst and the very high efficiency of a diesel(CI) engine.HCCI combines the characteristics of spark-ignition (SI,or Otto) engines, such as standard gasoline engines, andcompression-ignition (CI or diesel) engines. Fuel and airare mixed before combustion, as in a conventional SIengine. As in a diesel engine, however, there is no sparkplug, and combustion is started by autoignition instead.To ensure that the autoignition temperature is reachedjust as the piston reaches top dead centre, HCCI enginescontrol the inlet temperature, the compression ratio, orboth.Because the fuel/air mixture is close to homogeneouswhen the charge is ignited, there is no differencesbetween local and global combustion rates, in contrast tothe situation in a SI or CI engine. This means that toprevent excessive combustion rates, the mixture must bevery lean. A too rich mixture will burn the entire chargein a small fraction of the power stroke, producing veryhigh combustion pressures, high stresses and high noiselevels. A too lean mixture produces slow and incompletecombustion.Lean burning means that the HCCI engine cannot createparticulates, and the low combustion temperature limitsthermal NOx to 0.1-2 ppm. On the other hand, aproblem with current HCCI engines running on hydrocarbonfuels is higher levels of unburned hydrocarbonsand carbon monoxide in the exhaust.An HCCI engine running on hydrogen, however, cannotform carbon monoxide or emit unburned hydrocarbons– in fact its only exhaust product is steam. Researchers atLund Institute of Technology in Sweden are developinghydrogen-fuelled HCCI engines.The main problem with HCCI is to control the combustionprocess, balancing the inlet conditions to getcombustion at the correct point in the cycle. Changingthe inlet temperature by just a few degrees, for instance,can be enough to stop the engine. This degree of controlcannot be achieved with a direct link between the acceleratorpedal and the throttle. Modern electronic enginemanagement systems, however, are capable of theclosed-loop control that HCCI requires.Gas turbinesHydrogen as a fuel for gas turbines (GTs) has been investigatedfor several decades. Producing exhaust gasescontaining only water vapour, oxygen, nitrogen andsome unburned hydrogen, gas turbines running onhydrogen offer the prospect of environment-friendlyand CO 2 -free electricity generation.Burning hydrogen in gas turbines raises issues including:• high flame speed can cause the flame to move upstreamand stabilise on the fuel injector, resulting indamage to the burner;• increased risk of flashback in premix mode;• high flame temperatures increase NOx emissions; and• hydrogen can cause embrittlement of metal componentsin the fuel system.Hybrid cycles with fuel cells and HCCI/GTBecause of their high electrical conversion efficiencies,SOFCs will probably form the basis of the best futureCHP plants. Adding a heat engine such as a HCCI or a gasturbine can improve the operation of a SOFC plant evenfurther.A stand-alone atmospheric-pressure SOFC plant uses acatalytic incinerator with waste heat recovery to destroyunburned fuel in the anode exhaust gas. Replacing theincinerator with an HCCI engine improves overall efficiency,increases the proportion of shaft power availablefrom the plant, reduces exhaust gas volumes andprovides greater flexibility during load following, startupand shutdown.Running the SOFC under pressure reduces the volume ofthe fuel cell stack and improves performance by allowingthe electrochemical reactions to operate more effectively.If we use a gas turbine instead of an HCCI engine toreplace the incinerator, there is the further advantagethat compressed air from the gas turbine's compressorcan be used to supply the SOFC.Calculations show that small hybrid SOFC/GT plants ofaround 350 kW e could achieve electrical efficiencies of60-70%, which is considerably better than competingtechnologies of this size. Burning hydrogen instead ofnatural gas would further increase the efficiency by a fewpercentage points.Disadvantages of hybrid systems include:• the need to develop new gas turbines specifically forSOFC/GT hybrid systems. Off-the-shelf GTs would notbe suitable;• lack of operating flexibility, especially at part load,because of the strong interdependency between theSOFC and the GT; and• complications during start-up, shutdown and loadchange, because of the risk of compressor surge.