Combined Heat and Power (CHP) with ORC
modules

The Organic Rankine Cycle

ORC modules represent an efficient and reliable way to generate electrical power with relatively
low temperatures. Since more than a decade such systems operate safely in many places worldwide
in a range form a few kW up to 2 MW.

The concept

The Organic Rankine Cycle (ORC) is a thermodynamic process similar to a conventional steam
cycle but using a different media to drive the turbine. ORC systems use high molecular organic
fluids instead of water.

The organic media is compressed and circulated in a closed loop by a pump. It will be evaporated in
a shell and tube heat exchanger by absorbing the thermal heat of the thermal oil primary circuit.

The organic vapour expands in a special designed turbine which drives the electrical generator and
will be condensed in another heat exchanger by using a cooling media like water.

The condensate will then be compressed again by the circulation pump, which closes the
thermodynamic cycle.

Neither the thermal oil of the primary circuit, nor the cooling water is in direct contact with the
organic media.

For high temperature applications it is possible to increase the efficiency by using a regenerator
behind the turbine.

In search of further optimization, Maxxtec reviewed the overall concept of primary energy input,
the thermal oil system and the heat recovery and came up with the advanced partial stream
principle.

This advance ORC system allows a much better utilization of the exhaust heat in biomass fired
combustion systems. The partial stream principle is now a standard for such applications where the
electrical power generation has priority compared to recovered heat.

Advantages

The vapour phase organic fluid allows the use of low temperatures to generate electrical power
from a few kW to 2.5 MW module.

This results in following advantages:

• High efficiency

• Extreme high efficiency of the turbine (up to 85%)

• Low mechanical stress for the turbine due to low speed design

• The low speed allows the direct coupling of the generator without the use of a gearbox

• Extraordinary start-up and partial load behaviour due to wide spectrum usage of the organic

fluid.

• Automatic start-up and shut-down operation

• Fully automatic operation with low maintenance and operational costs

• No corrosion problem due to the non-corrosive organic fluid

• No erosion problems of the turbine blades due to "dry" vapour phase

• Long lifetime

• Low noise emission

The availability of synthetic heat transfer media and the long time experiences with high
temperature thermal oil applications has enabled Maxxtec to increase the electrical power output
significant. This is now achieved by operating Maxxtec/ Adoratec ORC modules on a higher
temperature of the primary circuit.

Modularity

ORC modules up to an electrical output of 500 kW are delivered pre-assembled on a skid. All main

equipment parts of the turbo generator e.g. heat exchanger, feed pump, turbine, generator piping,
instruments/wiring and other auxiliary equipment are pre installed and allow cost effective transport
and installation at site.

Meanwhile, the whole world is aware of the fact that fossil fuel supply is limited and further
exploration will become unbearable expensive. In addition, emission of fossil fuels generates
billions of tons of CO2, which is the main cause of global warming and environmental changes. The
long term impact of these negative developments can still not be predicted in details. Many
international operating associations like Greenpeace as well as prominent scientist have highlighted
these problems since decades and meanwhile there are bonus systems in place to counteract and to
persuade the industry to invest in “renewable energy”.

Besides additional income by trading carbon credits, many countries have granted high prices for
feeding the national grid with power generated by renewable energies.

In this interrelation, also biomass combustion with the generation of electrical power is one of these
renewable energies. Due to national and international development programs the use of biomass as
renewable energy wins more and more of importance.

The cycle of usage and production of biomass as renewable energy is balanced. Biomass can be
produced almost everywhere and has several environmental advantages over fossil fuels. The main
advantage is that biomass is a renewable resource, offering a sustainable, dependable supply. Other
advantages include the fact that the amount of carbon dioxide (CO2) emitted during the combustion
process is typically 90% less than when burning fossil fuel. Biomass fuel contains minimal amounts
of sulphur and heavy metals. It is not a threat to acid rain pollution, and particulate emissions are
controllable.

With few exceptions, biomass is not subject of international trading, nor is war fought over the
exploitation of biomass resources. During the Second World War when many parts of Europe were

destroyed, wood was one of the major fuel sources to generate heat and energy for vehicles (wood
gasification). Nowadays, advanced technologies and processes have changed the perception of
biomass. Biomass is CO2 neutral, easy to produce in large quantities, often a waste product of other
processes and still available for usually significantly less costs than competing fossil fuels.

The production of electrical energy from biomass has a higher importance than producing heat from
biomass. Electricity can be supplied far distances with very little loss and can be sold on the spot,
where it is required.

In general there are 2 ways to produce electrical power from biomass in the range up to 2 MW;
biomass combustion and biomass gasification.

The biomass gasification process is the process of heating biomass in an oxygen-starved
environment until volatile pyrolysis gases (carbon monoxide and hydrogen) are released from the
biomass. The gases can be mixed with air or pure oxygen for complete combustion and the heat
produced can be transferred to a boiler for energy distribution. Otherwise, the gases can be cooled,
filtered, and purified to remove tars (a major concern for any wood gasification process) and
particulates and used as fuel for internal combustion engines (Stirling motor), micro turbines, and
gas turbines. Stirling motors have been used successfully but are limited in their available capacity
ranges.

Biomass vary a lot in particle size, moisture content, ash content, calorific value and density
depending on the source (wood, rice husk, palm oil fibres, etc.). The production and use of biomass
gas is only economic if the fuel composition is consistent and the gasifier is monitored and serviced
on a regular basis.

The second process of power generation with biomass is the combustion of it in a furnace attached
to a boiler. The combustion takes place on a static or moving grate under high temperature and
staged combustion air supply. Modern biomass combustion systems are PLC controlled, high
efficient and do not pollute the environment. These modern combustion systems are often used with
steam boilers for generating process steam or for CHP systems to operate a steam turbine with
generator. The disadvantage of steam power plants below 1 MW is the fact that these systems are
expensive in design and operation due to the complicated control systems, water treatment issue and
high steam pressures required. The normal operation, start-up and shut down of such systems need
highly qualified operators. In addition, steam systems require a super heater which is one of the
most critical parts in such combustion system. In cold countries one faces an additional problem
with the freezing point of water. If the plant has been shut down for a longer period, one needs to
drain the complete system. That's one of the reasons why ORC turbo generators has been developed
and became so popular in such a short time. The ability to generate electrical power in small power
plants (< 1 MW) with biomass combustion and thermal oil systems makes these systems so
attractive.

ORC Module - 550 kWel

The process

1. Biomass is combusted in a furnace and generates hot gases of ~ 950

deg. C, which are heating up a thermal oil circuit. The thermal oil

is heated up to 300-330 deg. C.

2. The thermal oil heats up a secondary circuit, which contains the organic

fluid for the ORC module. The organic fluid is evaporated and drives the

turbine to generate electricity in the generator.

3. The water cooled condenser will condense the vapour and the heated

cooling water can be used for various heating application such as heating

dryers or hot water heating systems in buildings.

Summary

Biomass is utilized in modern highly efficient combustion systems, which are successfully operated
in large numbers. These systems with their sophisticated fuel feeding, grate system, staged
combustion air supply, de-ashing system, filter, control and safety instrumentation are reliable and
perform flawless.

The heat transfer media of the primary circuit is thermal oil. Thermal oil offers advantages like low
system pressure, almost no freezing problems, save control and operation and long life time.

The generated heat from the biomass combustion is converted into electrical power by the ORC
turbo generator. The ORC process is highly efficient, reliable and easy to control. The turbine rotor
runs will low r.p.m. and is directly connected with the generator. All ORC components are shop-
assembled and tested. This reduces installation and testing time at site. Compared to steam, organic
fluids do not cause any corrosion problems, no erosion in system components like valves, turbine
blades and piping.

This benefits the long life time and reliability of the power plant.

The design of the turbo generator follows criteria, which allow:

• Quick start-up within minutes

• Remote controlled monitoring and load control

• Operation without permanent operator at site

• Remote data management for monitoring and trouble shooting

Further information is available here.

http://www.adoratec.com