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Polish heat and ORC

The heat and power industry in Poland is traditionally based on coal, and while the use of biomass in the energy sector is not new, it is still considered unconventional. Until recently this was true for Miejskie Przedsiębiorstwo Energetyki Cieplnej w Lęborku Sp. z o.o. (MPEC), a municipal thermal energy company in Lębork, northern Poland.

Miejskie Przedsiębiorstwo Energetyki Cieplnej w Lęborku Sp. z o.o. (MPEC) new 7 MW biomass CHP in Lębork, Poland (photo courtesy Polytechnik).

Miejskie Przedsiębiorstwo Energetyki Cieplnej w Lęborku Sp. z o.o. (MPEC) new 7 MW biomass CHP in Lębork, Poland (photo courtesy Polytechnik).

MPEC’s district heat plant no. 1 is an old 45 MW pulverised coal (PC) plant. In June 2012 the city embarked on a ≈ EUR 9 million project to complement the old PC plant with a modern biomass-fired combined heat and power (CHP) plant. The new CHP would become the primary thermal plant for the city’s district heat network with the old PC plant used for back-up and peak-demand. It would also be electrically self-sufficient with excess supplied to the power grid.

Local biomass resources

The decision to explore the use of unutilised biomass is not accidental. Lębork is an agricultural region with plenty of agricultural residues such as straw available as well as set-aside land that could be used for short-rotation coppice. In addition there are managed forests that could provide forest residues as well as a number of wood processing plants which could supply residuals.

A biomass CHP would provide farmers and other residue owners with a revenue generating waste management solution. At the same time it would radically reduce the city’s fossil carbon footprint, improve the air quality and deliver reliable and affordable heating to its citizens.

International project

The project, 85 percent of which was financed by a grant from the Swiss Fund with the remainder put up by the municipality, was put to tender. In November 2013, a contract was signed with an international consortium led by Austria-headed biomass combustion specialists Polytechnik Luft- und Feuerungstechnik GmbH, for the turnkey delivery of a 7 MWfuel biomass-fired combined heat and power plant including a 1 000 m2 covered storage area for the biomass fuel. Tractebel Engineering was awarded the investor supervision and coordination of designing project, site works, erection works and start-up for the construction of biomass-fired CHP as a primary source in the district heating system in the city Lębork. In June this year the plant was commissioned.

The winning solution consists of a thermal oil biomass boiler paired with an Organic Rankine Cycle (ORC) unit. The plant’s total heat output is 7 MW, of which 5.5 MW is heat in the form of water with the parameters 70/90 °C and 1.2 MWe electrical energy (gross). The new plant consists of two buildings: the roofed biomass storage and the CHP building. Consisting primarily of wood processing residues from the region the biomass is supplied throughout the year. During the summer the fuel consumption is lower than the volume received but this excess is needed during the winter when the boiler operates on full-load.

Step-grate furnace

Supplied by Polytechnik, the boiler is a hydraulically fed step-grate type unit, which enables the efficient combustion of biomass with up to 60 percent moisture content. A vapour recovery system keeps nitrogen oxide and carbon monoxide emissions in check and a combination of multi-cyclone and electrostatic precipitator (ESP) remove particulates from the flue gas before discharge to the stack. According to Polytechnik the plant fulfils all requirements of stack emissions in Poland.

The ash from the plant, bottom ash from the grate and fly-ash captured from the flue gas cleaning system, is collected using an automatic ash removal system and collected in an outdoor container. The ash is made available to farmers who can use it as fertiliser.

Efficient heat transfer with a split ORC

The heat transfer from the boiler to the ORC block, a Turboden 14 CHP Split, is done using two thermal oil circuits as the heat carrier, a high temperature and a low temperature. In the former circuit the hot combustion gases in the combustion chamber heat the thermal oil from incoming 250 °C to 310 °C. In the latter circuit heat exchangers capture heat from the flue gases heating the incoming thermal oil from 130 °C to 250 °C.

Instead of using water vapour as in a conventional steam turbine, the ORC system vaporises a high-molecular-mass organic fluid, in this case silicon oil. According to Turboden this results in better electrical performance with slower turbine rotation, lower pressure, no erosion of metallic parts and blades and no water consumption.

The ORC turbogenerator uses the medium-to-high-temperature thermal oil to preheat and vaporise the silicon oil working fluid in the evaporator. The vapour turns a low-revolution turbine. Thus in the turbine, thermal energy is converted into mechanical energy which in turn is converted into electrical energy in an asynchronous generator.

The exhaust vapour flows through the regenerator, where it pre-heats the liquid silicone oil that is en route to the evaporator. The vapour is then condensed in the condenser, which is cooled by the 70 °C incoming water from the district heating network. The silicone oil, now a liquid, is then pumped into the regenerator and evaporator, thus completing the closed-cycle operation. The temperature of the outgoing water for the district heating is now 90 °C.

The thermal oil circuit is equipped with multiple safety devices which protect the oil against overheating and degradation such as carbonisation. The thermal oil emergency circulation system consists of an emergency cooler and two additional pumps: one diesel pump and one electrical pump, which is powered by an emergency generator. In the event of a power failure or any other emergency situation, the thermal oil is automatically cooled to a safe temperature.

Short-payback possible

The whole plant is controlled by an automatic control system developed entirely by Polytechnik. A single employee is able to monitor the operation of the plant while also able to manage the loading and unloading of the biomass at the same time.

As the plant is a baseload district heating plant, this two-stage “split ORC” setup enables the maximisation of electrical power output for a given thermal input. According to Turboden the net electrical output is 1.2 MW and electrical efficiency is just over 18 percent. According to Polytechnik the entire system reaches an  an overall efficiency around 86 percent.

The new CHP fulfils Polish requirements for a highly efficient cogeneration system and is thus eligible for green certificates for its electricity production representing an additional source of revenue. As a result even relatively small ORC based CHP’s, such as the Lębork installation with 5.5 MWth and 1.2 MWe, can achieve short payback times of 3-6 years according to Polytechnik. This is due to the part-load operational ability of the ORC, which according to Turboden, can maintain 90 percent of the cycle efficiency down to a 50 percent loading.

Combined heat, power and cooling

However, the higher the electrical output of the plant, the shorter the amortisation period given that the cost per MWh electricity produced is proportionally lower for higher outputs of the plant. Therefore from an economical point of view, CHP plants should be operated at the highest level possible, something that district heating plants find difficult given the seasonal heat demand.

Plants with combined heat, power and cooling typically resolve this constraint by producing the latter for air conditioning in the form of cold water. Poland’s first biomass-fired combined heat, power and cooling plant, also supplied by Polytechnik and Turbodoen, is in operation at the Arłamów hotel and sports complex. That though is another story.

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