BLOG: Making the right choice – Organic Rankine Cycle compared to Steam Cycle
The last 10 years, a large number of small and medium-sized wood combined heat and power (CHP) plants between 300 kWe and 5,000 kWe have been build. Due to optimising, repetition and learning curves the costs of the units have gone down dramatically. In some circumstances a wood-fired CHP with a price of 70 €/MWh is even economical feasible. But which technology is the right choice and what really needs your attention when comparing offers? Find it out in this article.
by Herman Klein Teeselink | Director HoSt
It can be very difficult to evaluate different offers. Which different aspects should be evaluated to make the right choice of technology? What is economically interesting? Many aspects should be taken into account. Aspects such as the differences in electrical efficiency and thermal efficiency, the influence of heat usage, differences in investment costs, availability and maintenance and safety aspects.
Comparing an Organic Rankine Cycle (ORC) to a steam cycle based technology with a cylindrical boiler and a water tube boiler, it becomes clear that a higher net electrical efficiency is obtained by a high pressure water tube steam boiler in combination with an efficient multi stage steam turbine. Moreover, twenty to thirty percent higher net electricity yield is generated than with a comparable installation based on an ORC (Organic Rankine Cycle). This is achieved by a low own electricity consumption and a high boiler efficiency. Let’s look deeper into this case.
“A higher net electrical efficiency is obtained by a high pressure water tube steam boiler in combination with an efficient multi stage steam turbine”.
THE DIFFERENT TECHNOLOGIES
Many aspects need to be taken into account when choosing a certain technology, definitely not to forget all the different circumstances. But first the basics. A wood-fired ORC is a system in which the flue gasses from the furnace flow into a thermal oil boiler. The thermal oil is heated, mostly to about 300°C and then pumped through the ORC. In the ORC a thermal fluid, instead of water, is evaporated and flows into the turbine which in turns drives the generator.
“In the ORC a thermal fluid, instead of water, is evaporated and flows into the turbine”.
A cylindrical boiler is a big water drum. In this type of boiler the shell is filled with water and the flue gasses flow through the flame tubes. Typical pressures are 14 up to 30 bar. HoSt has built in Krāslava, Latvia a 14 bar wood-fired cylindrical boiler integrated with an existing steam turbine and realised a 22 bar wood-fired cylindrical boiler in Eindhoven. This proven plant supplies the city with heat.
The last, but not the least technology is the water tube steam boiler, in which the water and steam flows through the pipes and the flue gasses pass around the pipes. The higher pressure of 52 bar and temperature of 450°C gives a higher electrical yield of the turbine.
BOILER EFFICIENCY
Comparing a steam boiler and a thermal oil boiler, the latter gets the short end of the stick. For the same thermal input the ORC needs twelve percent more fuel compared to the steam boiler. If fuel was free or at almost no costs, then of course there would not have been a problem. But with fuel costs of 16 €/MWh, meaning 35 €/ton wood chips with fifty percent moisture, the extra wood costs for a 5 MWt boiler are no less than 85.000 euros annually.
“For the same thermal input the ORC needs twelve percent more fuel compared to the steam boiler”.
Most of the thermal oil boilers have a flue gas temperature of 280 °C due to the thermal oil that runs back from the ORC at a temperature of 250 °C. To increase the boiler efficiency, a combustion air pre-heater should be installed. This allows the flue gas temperature to drop to about 230 °C. In this situation the boiler efficiency is then still mostly limited to eighty percent.
On the other hand, the steam boiler with an economiser is fed with water having a temperature of 105 °C. Usually the flue gas temperature is between 140 and 170 °C. In this case the boiler efficiency can reach up to as much as ninety percent, depending on the design of the economiser and the sulphur content in the fuel. Thus, a better choice seeing this comparison.
ELECTRICAL EFFICIENCY
To calculate the electrical efficiency – electricity from generator divided by heat input – it is not enough to only look at the efficiency of the ORC or the steam turbine. Nearly every ORC will convert the heat supplied to the ORC into electricity with an efficiency between seventeen and nineteen percent.
The steam turbine efficiency is strongly depending on the type of steam turbine, steam pressure and hot water temperatures. The influence on steam pressure, hot water temperature and the scale of the installation are very good to catch in a graph, saying more than a thousand words. Let’s use the following cases: a high pressure boiler with a water tube boiler at 52 bar and 450 °C steam and a cylindrical boiler at 30 bar and 330 °C. Compared to an ORC the 52 bar boiler is the most efficient technology, followed by the 30 bar boiler.
“The steam turbine efficiency is strongly depending on the type of steam turbine, steam pressure and hot water temperatures”.
OWN ENERGY CONSUMPTION
Besides boiler efficiency and efficiency of the turbine or ORC, the own consumption can have a major influence on the total efficiency. The big difference between an ORC and a steam cycle is the thermal oil pump. Generally, about ten percent of the produced electricity is consumed by the thermal oil pump that pumps the oil from the boiler to the ORC. This is often forgotten in evaluations, but is nonetheless a very important factor.
Also, the electricity consumption of a large air compressor to clean a thermal oil or cylindrical boiler has to be taken into account. For example, the own consumption of an ORC project in Sittard in the Netherlands has a total parasitic power of 300 kWe on a generator output, which is 25 percent of the production. A best case project in Beetgum, realised by HoSt, with a 5 MWt water tube boiler and a multistage turbine has an generator output of 1.15 MWe and an parasitic power of ten percent (116 KWe). To generate the same amount of electricity the ORC project needs 45 percent more wood per MWh electricity. It is possible to install an extra water economiser allowing an increasement to ninety percent efficiency. But in many cases the value of electricity is two to five times higher than the value of heat, concluding that the water tube boiler is much more efficient.
INVESTMENTS
ORC’s, especially smaller units, are generally cheaper. However, in practice additional costs arise for extra required components. Think of the thermal oil boiler, thermal oils pumps, the thermal oil pipes and the thermal oil safe guarding systems. For example, in a Dutch ORC project even two tanks had to be installed in a basement when the system completely burned down, assuring that the oil will flow automatically in the tanks if and when any future calamities happen.
What about ORC’s on the scale of 1 to 1.5 MWe? Compared to a cylindrical boiler with steam turbine both technologies have the same costs per MWe. If we take into account the lower parasitic power usage of the steam boiler plant the price of this technology is fifteen percent lower per MWe. An example, the technology for the 1.6 MWe CHP in Eindhoven costs less than 4,000 €/kWe including the bag house filters and the nitrogen oxides (NOx) measurements which are required in the Netherlands. The investment would drop to about 2,500 €/kWe – excluding grid connection and civil works – in the case of a power increasement to 4 MWe. HoSt expects that the costs will even further drop to 2,000 €/kWe for 3 and 4 MWe projects based on water tube boilers due to repetition and standardisation. In other words: a very competitive price level.
ORC’s, especially smaller units, are generally cheaper. However, in practice additional costs arise for extra required components. Think of the thermal oil boiler, thermal oils pumps, the thermal oil pipes and the thermal oil safe guarding systems. For example, in a Dutch ORC project even two tanks had to be installed in a basement when the system completely burned down, assuring that the oil will flow automatically in the tanks if and when any future calamities happen.
What about ORC’s on the scale of 1 to 1.5 MWe? Compared to a cylindrical boiler with steam turbine both technologies have the same costs per MWe. If we take into account the lower parasitic power usage of the steam boiler plant the price of this technology is fifteen percent lower per MWe. An example, the technology for the 1.6 MWe CHP in Eindhoven costs less than 4,000 €/kWe including the bag house filters and the nitrogen oxides (NOx) measurements which are required in the Netherlands. The investment would drop to about 2,500 €/kWe – excluding grid connection and civil works – in the case of a power increasement to 4 MWe. HoSt expects that the costs will even further drop to 2,000 €/kWe for 3 and 4 MWe projects based on water tube boilers due to repetition and standardisation. In other words: a very competitive price level.
AVAILABILITY AND MAINTENANCE
Stops for cleaning purposes have the greatest impact on availability. It is generally known that the more minerals in biomass, such as sodium and potassium, the more often the boiler needs to be cleaned. Although, cylindrical boilers and thermal boilers are equipped with compressed air cleaning systems, still periodically cleaning is required.
Another big impact on availability is poor quality wood, which nonetheless often is being used in many countries. Think of bark, topping, wood with leaves or even wood from composing installations. These types of poor quality biomass contain more sand and minerals and have a huge impact on availability.
Every system can be built in such a way that it is able to run without operators, but daily checks of course remain. Just like an ORC is able to, water tube boilers and cylindrical boiler CHP’s in the Netherlands also run unattended. How that works? In a high pressure water tube boiler the pipes are very well cleaned by steam soot blowers, which reach the dirt between the pipes. Stops are limited to one or two stops annually when combusting normal wood and maybe three stops with poor quality wood. Thermal and cylindrical boilers need mechanical cleaning stops each two months and sometimes even each month.
High boiler and power plants availability is important when evaluating different offers and an availability of more than 96 percent is absolutely feasible. It is needless to say that plant and boiler availability depends on supplier design, the type of boiler and, definitely not irrelevantly, the quality of the biomass.
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