To achieve cost-effective levels of operation – with corresponding reductions in charges for waste water – the operators of sewage treatment plants frequently optimize not only the actual process of waste water treatment but also the energy-related scope for recycling existing resources. On top of this, they strive to reduce the volume of material that requires disposal and even process sewage sludge into fertilizer products.
The opportunities for power generation are many (see Figure 1). Even before sewage enters the actual treatment plant, there is the option of tapping off the heat by extracting it from the waste water, then using a heat pump to heat residential buildings. The water which leaves the treatment plant can be used to drive a turbine, if the slope is sufficiently steep, and can therefore help to generate electricity.
But the most important provider of power is inside the treatment plant itself: sewage gas. This gas is created in the digestion tank by a fermenting process (see Figure 2) and usually has a methane content of 55%-65%. It is well suited to the operation of CHP. Every cubic metre of sewage water yields roughly the amount of sewage gas required to produce 1 kWh of electricity plus 1 kWh of heat in a CHP operation.
A classic meets its modern counterpart
In the classic case, a sewage treatment plant employs engine-driven compact cogeneration modules in a CHP application. Manufacturers offer suitable lean-burn engines for use with biogas and sewage gas that make effective use of sewage gas with a methane content of 50%-65% (see photo, next page). Compact cogeneration modules of this kind are frequently employed that have power ratings of about 150 kWe and above, provided that the sewage treatment plant produces a sufficient volume of gas.
The principle of the lean-burn engine is to counteract the formation of pollutants during the combustion process. A closed loop control system such as MDE oxygen-sensor, closed loop control helps to achieve optimized combustion, even if the methane content fluctuates.
The high-temperature fuel cell is a modern version of the CHP approach. It can be used as an alternative or an addition to the classic system. Devices such as molten carbonate fuel cells (MCFCs) process gases with a combustion value of about 3 kWh/m3 and above. The MCFC electrochemically converts the energy contained in combustion gas into heat and power at a process temperature of 650à‚°C. It can also, by virtue of this high operating temperature, bring about a kind of ‘internal reformation’. In comparison, a proton exchange membrane fuel cell – PEMFC for short – depends on an ‘external reformation’ process, which involves hydrogen being separated out from the fuel. However, for functional reasons, PEMFC is not really suitable for operation in conjunction with sewage gas.
First use of sewage gas in fuel cells
Two years ago, an MCFC system called HotModule was installed in a sewage treatment plant in Germany by CFC Solutions. Now, a further two sewage gas projects using fuel cells of this kind are at the implementation stage and are scheduled to go into operation this autumn. With power ratings of about 250 kWe and 180 kWth, the typical size of an MCFC unit is suitable for incorporation into an overall energy system rated at up to 2 MWe.
The HotModule molten carbonate fuel cell can process natural gas or scrubbed sewage gas and bio-gas without any problems. Source: CFC Solutions
Typically, the volumes of sewage gas generated by medium-sized treatment plants are sufficient to allow the use of power-generating systems, and these can include a hybrid solution that combines a fuel cell compact cogeneration module with a classic gas engine cogeneration module. Such a system is able to bring together the strengths of both of these technologies. Table 1 shows the key properties of fuel cells and engine-driven compact cogeneration modules.
A common feature of fuel cell and engine-driven classic compact cogeneration modules is their reduced output of CO2 when compared with the separate generation of heat and power. Compared with the power generated in large coal-fired power stations and separate heat generation, CHP produces about one third less CO2 per kilowatt hour of usable energy. The use of sewage gas as a primary energy source makes it possible to achieve CO2-neutral energy conversion.
CHP to provide power in-house
If gas is available in sufficient volumes, the characteristics of engine-driven and fuel cell operated compact cogeneration modules demonstrate the benefits of combining these two technologies. A hybrid system gives investors the opportunity to combine a proven technology that has good prospects with a new efficient energy converter. Such a system can serve all internal consumers in a sewage treatment plant and can generate surplus power that can be offered for sale on the open market.
An overall control system must be employed that integrates all of the closed loop control systems employed in the power generating facilities of the hybrid system. This is essential to make optimum use of the hybrid system. It must take into account the prevailing availability of sewage gas and the requirement for heat and power.
Various factors govern whether the investment in a hybrid system is economical (they also apply to the more general CHP system). Severe fluctuations in the volume of sewage gas – which is typical of treatment plants in holiday resorts, for example – can also have a bearing on an investment decision. This is also true in the case of a local waste disposal facility.
In the considerations that follow, it is assumed that we are dealing with a sewage treatment plant for a town of at least 40,000 inhabitants (in other words, a comparably large system) which has an almost constant volume of waste water passing through it. Furthermore, it is assumed that the sewage sludge can be disposed of inexpensively and that it produces fertilizer-compatible products that can be sold to generate revenue. In such a case, operators of sewage treatment plants face various attractive ways of using power for their own operations.
Typical energy sinks in a sewage treatment plant
Heat at a low temperature can be used to heat the digestion tank. At only 40à‚°C, methane generation occurs at a favourable level. The methane content can account for as much as two thirds of the volume of the sewage gas. This enables the generation of combustion gas to be maximized. In addition, this heat can be used to warm the administration buildings. The engine-powered and fuel cell compact cogeneration modules can also act as suppliers of heat to this type of consumer.
Sewage sludge can be dried using high temperature heat, thereby minimizing the volumes of residue requiring disposal. Effective drying takes place at about 150à‚°C, which means that the heat from the fuel cell can be used directly. Heat generated by the engine (90à‚°C) is not by itself sufficient and would make an additional combustion source necessary.
Electrical power produced can be used to operate equipment (such as agitators) and the buildings. Any surplus production of electrical power can be sold on. In Germany, for example, legislation concerning CHP and renewable energy means that electrical power fed into the national grid in this way receives a good price. In such a case, the high electrical efficiency of the fuel cell can make a positive and noticeable contribution when used in a sewage treatment plant. However, in those cases where a sewage treatment plant’s revenue streams from electrical power are low and where a high thermal load requirement exists, the high heat component of an engine-driven compact cogeneration module system may be viewed as an advantage.
Potential external consumers of thermal energy
Although fuel cell and engine types of CHP plant can deliver thermal energy into a local heating network, the clean waste air from fuel cells is suitable for heating nearby greenhouses. Its pollutant-free, CO2-laden waste air can also help the plants in the greenhouses to grow. The waste air does not even need to be cleaned. It is diluted with fresh air to reach a CO2 concentration of about 2% so that the plants can absorb the CO2 and respond with fast growth and, if they are vegetables, additional flavour.
Distribution of tasks in hybrid systems
The technical properties of a hybrid system that combines an MCFC and an engine mean that it is advisable for it to operate the fuel cell at base load to ensure that it can operate continuously. The engine-driven compact cogeneration module makes it possible, regardless of the level of sewage gas available, to vary nominal power flexibly between 50% and 100%. When two units with roughly equivalent power ratings are combined, scope for closed loop control of 75%-100% can be achieved – a more than adequate level, assuming virtually constant volumes of waste water.
However, if the volume of sewage gas requires a level of operation below the 75% mark, the power rating of the fuel cell can be restricted or, if necessary, the engine may need to be shut down. Whenever the engine is switched off, the fuel cell needs to operate solely on the volume of gas available until such time as the engine-driven compact cogeneration module starts to operate again.
Sewage sludge – even when dried – presents the operators of treatment plants with a disposal problem. It is often undesirable or even prohibited for it to leave the plant, so its disposal can be expensive. The drying of sewage sludge with the help of high-temperature heat from a fuel cell makes a welcome contribution here, substantially reducing the volume of waste material without further incineration.
With the additional help of pyrolysis, the disposal costs can be reduced yet further. Sewage sludge is rendered down to ash through gasification. However, this method requires a relatively large volume of sewage sludge to make it cost-effective.
The product of the pyrolysis is ash, a hygienic substance which can be sold as fertilizer. The resultant gas (typically a mixture of carbon monoxide, carbon dioxide, hydrogen and methane) can also be rendered suitable for use in CHP systems. To achieve this, it first needs to pass through a filter, after which it can be mixed into the flow of sewage gas heading to the engine-driven compact cogeneration module. A gas mixer arranged upstream of the module enables the ratio of pyrolysis gas and sewage gas to be mixed to achieve the desired combustion value for the gas mixture.
The use of pyrolysis gas in fuel cells is also conceivable but has not so far been tested. Because the pyrolysis of sewage sludge is a process with which relatively little experience has yet been accumulated, it would be preferable at this time for a hybrid system to use pyrolysis gas in an engine-driven compact cogeneration module.
The properties of engine-driven compact cogeneration modules and MCFC units complement one another and can create a flexible hybrid CHP system.
Despite the fact that prices for fuel cell plant are still high, under new framework parameters (such as legislation for renewable energy sources or Germany’s national innovation programme), cost-effectiveness is achievable. In the course of the next few years, as substantial cost savings are being achieved in the manufacture of fuel cells, an earlier return on investment looks very likely.
The hybrid system of MCFC and classical engine-driven compact cogeneration modules also presents an opportunity for retrofit solutions. Due to the fact that closed loop control technology, gas supply, scrubbing facilities and peripheral items generally need to be built separately, it would also be possible to locate an MCFC unit alongside an engine-driven compact cogeneration module.
Gas scrubbing: sulphur in particular can damage the MCFC and is therefore filtered out from sewage treatment and bio-gas applications with the help of active carbon. Source: PNR
In any event, placing primary control in the hands of control room technology assigns this equipment the task of coordinating both CHP systems and employing each in a manner which best suits its respective properties profile. The hybrid system is therefore also an interesting option for parties whose sewage treatment plants require rising amounts of energy to contend with growing volumes of incoming material and who wish to enlarge their power generating facilities to make use of the increased volumes of sewage gas. It will also be of interest to users who wish to replace an existing compact cogeneration module.
Mirko Gutemann is with Tognum AG, Friedrichshafen, Germany.
This article is on-line: www.cospp.com
The function of the HotModule molten carbonate fuel cell
The HotModule essentially comprises a cylindrical steel container with a horizontally arranged fuel cell stack, starter unit, catalytic burner and mixing chamber. Added to this are the supply of media (fuel and water) and an inverter which converts the DC current generated into AC for the mains grid. Another section of the plant is responsible for separating off thermal energy (heat).
Gases that have a high methane content, such as natural gas, bio-gas and sewage gas, can be used as fuels, as can liquid fuels such as methanol. In keeping with all fuel cells, the electro-chemical process is based on a reaction between hydrogen and oxygen which creates an electrical current and heat.
The methane in the fuel gas is directed to the anode along with water vapour. The ensuing catalytic reaction produces hydrogen. In turn, this hydrogen reacts with the carbonate ions in the electrolyte to form water and carbon dioxide. In this process, electrons are released on the anode side and flow through a consumer (such as a public power grid) to the cathode. On the cathode side, carbon dioxide and oxygen in the air react with the electrons released by the anode reaction to form carbonate ions. These then pass through electrolytes to the anode. This completes the electrochemical circuit.
The molten carbonate fuel cell is well suited to the continuous supply of heat and power. The effective operation of absorption refrigeration machines is made possible by the high temperature within the system.