Dr Jacob Klimstra visits two exemplary cogeneration installations.
It is generally recognised that the best solution for decreasing primary energy consumption and reducing emissions is found in process integration so that no valuable energy is wasted. Electricity, heating and cooling all require energy, and integrated solutions certainly offer ways to reduce fuel consumption. Cogeneration of electricity and heat is a good example.
Two companies in the Netherlands are seen as global benchmarks for the industry in the context of energy savings and emission reductions. Both companies operate in the field of recycling and waste reduction and are paragons based on their performance.
|Smurfitt Kappa Roermond Papier’s 32-year-old generator sets still look as good as new
Credit: Smurfitt Kappa
Smurfit Kappa Roermond Papier
Smurfit Kappa in Roermond, the Netherlands, processes 650,000 tonnes of recovered paper per year and turns it into corrugated cardboard for the packaging sector. This is about a quarter of all recovered paper in the Netherlands. Some 100 truckloads with recovered paper arrive at the factory gates each day. The one million bales of recycled paper are processed in the factory 360 days per year.
The raw paper flow that enters the factory is first analysed for composition. The bales are subsequently shredded and then cleaned in a watery solution of 50°C to release the paper fibres. Further, dirt and other undesirable components are removed from the slurry. The rejected material still has value, however, since it is separated into substrates for market garden applications and absorption material, and also into a combustible material, the so-called rofire. Application of this rofire results in a CO2 emission reduction of 20,000 tonnes per year.
The cleaned solution of 1% of paper fibre in water has to be substantially dehydrated. The first step in this process is based on gravity and vacuum processing and therefore does not require much energy – nor does the next step. pressing of the fibre sludge. Heat is needed for the removal of the final amount of water from the fibre. To this end, the moist fibre passes two trains of large cylinders that are heated by steam. The final product, large three-tonne rolls of paper, is transported to the customer by truck.
|Installation of one of the two NEM 60 bar boilers at the SNB facility
The three paper machines running in parallel each have a length of 120 metres. The paper of the most modern machine passes through the machine in 30 seconds, at an amazing speed of 60 km/hour
Optimising the process
The cleaning of the water stream from the pulping process releases biogas. The energy in this biogas is used to cover 5% of the heating needs of the drying process. Four cogeneration installations in parallel, identical Centrax CX 350 KB5 units, provide the bulk of the electricity and heat required for the paper process.
The electric power capacity of each of the four gas turbine units is 3.6 MW. The gas turbines are from Allison, type 501 KB5. The 1500 rpm generators are from Holec and Brush. The first three units have a heat recovery only boiler. The last unit has a boiler that facilitates supplementary firing for creating some flexibility in the production of steam. The boilers are from Standard Fasel Lentjes and the supplementary firing burner system is from Rodenhuis & Verloop. The electrical efficiency of each cogeneration unit is close to 29% and the total efficiency including heat utilisation is 88%. The first three units were commissioned in 1983 and the fourth unit in October 1987.
On 4 February, Centrax Gas Turbines and Smurfit Kappa celebrated a total of one million operational hours. These units were installed in the early days of cogeneration and market conditions were very favourable for this in the Netherlands, with discounted gas prices and government grants. Guy West, Director of Centrax Gas Turbines, mentioned during the ceremony that ‘Smurfit Kappa were pioneers in industrial cogeneration and helped launch the 501 engine as the workhorse in the 3 MW-5MW cogeneration sector. One million hours at a single installation demonstrates the longevity and reliability of the Centrax 501 product and maintenance package. We are pleased to have supported Smurfit Kappa since the units were first installed 32 years ago and look forward to many more years of this relationship.’
|Comparing the old control panel (green) with the new digital one
Initially, many specialists showed some scepticism about the life of the relatively light aero-derivative gas turbines. However, the units have been very reliable and have each undergone only six overhauls. If necessary, a turbine can be replaced in one day. Despite the fact that the turbines have together accumulated a million hours, Smurfit Kappa has recently invested in upgrading the genset controls to the latest digital version, replacing the original analog controls. They clearly believe there is plenty of life left in the 501 packages.
The turbines have already been equipped with water injection to meet the increasingly stringent emissions limits for NOx. The only problem currently is the relatively high natural gas price in the Netherlands compared with the price of electricity from the grid. Nevertheless, the units are an integral part of a showcase process that resulted in a Plant of The Year Award in 2013.
Sewage sludge incinerating facility Noord-Brabant in Moerdijk, also in the Netherlands, is another example of a successful site using on-site generation. However, the plant’s 3.5 MW steam turbine-based generator is completely new. It has been integrated into the existing sludge processing plant that began operation in 1997. Sewage sludge is the end product of treatment installations that clean up the waste streams from city and municipal sewage systems. This sewage sludge has to be incinerated in order to remove polluting elements, such as heavy metals and leftovers from medicines, hormonal material and toxic organic elements. The sewage of the Netherlands’ close to 17 million inhabitants results in some 1.5 million tonnes of sludge per year. SNB processes 30% of this sludge, making it the largest operating facility of its kind in the world.
|The steam-based generator set at SNB
Credit: Monique Smulders
The SNB plant receives 50 truckloads of sludge every day. The sludge consists of 75% water, since the sewage treatment facilities are not able to remove more water with their centrifugal systems. Before the sludge enters one of SNB’s four incinerators, it is dried to a slurry with 40% solid material. The calorific value of this slurry is sufficiently high so that it can burn without an external fuel supply. The 24/7 incineration process releases so much heat that enough steam can be produced for the pre-drying process and for removal of the ammonia from the vapour emanating from the dryers.
Some of the steam was initially used to feed a Spilling reciprocating steam engine driving an electric generator with a power capacity of about 450 kW. However, process analysis revealed that it would be possible to generate much more electricity, considering the energy available in the exhaust of the incinerators. With two incinerators equipped with 60 bar steam boilers instead of the initial 10 bar boilers, some 3.5 MW could be produced. That would cover the bulk of the electricity demand of the pumps, ventilators and electrostatic dust filters, and means that the renovated facility would become 95% self-sufficient with respect to energy.
A big challenge was the limited time allowed for the tie-in of the two new boilers. The amount of incoming sludge is such that the plant should operate continuously, with only one incinerator allowed to be stopped for a while at a time. The facility has storage capacity for only one week’s worth of incoming sludge. Therefore, the total renovation had to be finished within 70 days and could be compared to open-heart surgery.
SNB found an excellent partner in NEM, the world-renowned boiler manufacturer with its headquarters based in Zoeterwoude, the Netherlands. Manufacturing a steam boiler is common practice for NEM, but here the issue was to match new elements with a running installation that was already 18 years old. Some of the piping and instrumentation drawings had not been properly updated and the real dimensions sometimes appeared to deviate from the as-built information. Hands-on skills had to be combined with clever computer-based engineering and a thorough understanding of the processes in an installation that could not be stopped.
On 2 April, Marcel Lefferts, managing director of SNB and Gerard van Dijk, CEO of NEM could lead the celebration of a very successful project, together with a happy team. Excellent engineering skills, perfect project management and close co-operation between supplier and customer were the key elements that enabled a successful installation. Although, on some occasions, 250 people were working at the site, no accidents had happened and the Siemens steam turbine coupled to a 3.5 MW AvK Stamford generator was running smoothly.
The end result is a process requiring almost no external energy. It is an example of clever integration of heat and electricity demand. Actually, when taking the whole process facility into account, it might be stated that the energy efficiency of the plant is over 100%.
The ammonia removed from the condensate resulting from the drying process is used for NOx reduction in the exhaust gases. Part of the exhaust gas containing much CO2 is transported via a 700-metre pipeline to a neighbouring factory that uses it for the production of lime. A recent innovation is phosphor recovery from the incinerated ash. The annual 36,000 tonnes of ash contain 20% phosphates, an important element in agricultural fertilisers. Global phosphor resources are rapidly diminishing, and therefore maximum phosphor recovery becomes crucial.
Local generation of electricity in combination with heat use is a crucial aspect of both factories discussed here. During a conference organised by the European Commission in February in Brussels, the delegates agreed that the integration of heat and electricity use was seen as the best opportunity for reducing the use of fuels.
In both cases presented here, there has been excellent co-operation between the equipment supplier and the factory. Engineering skills, communication and trust appear to be crucial elements in making such projects successful. Two well-known suppliers of equipment for local generation have shown that they have the necessary skills.
Next to that, it is vision that characterises the owners and managers of the two factories. In both cases, there is a desire to be as environmentally friendly as possible by trying to close the circle of raw material use including that of energy.
Dr Jacob Klimstra is Managing Editor of COSPP