Design: Biomass

District heating

The proposed area of district heating consists of 384 flats, all in close proximity to the proposed location of the CHP system.  The flats currently have a combined electric heating load of 3.96GWh per year however it is estimated that with improvements to the fabric of the building such as insulation and draft proofing the heating demand could be reduced to 1.72GWh per year.  The space heating load on the district heating system will be reduced during the summer months but this excess heat will be used in the brewing process.

Carbon Cycle

Burning biomass is considered as carbon neutral if the fuel comes from a sustainable source as the CO2 released when it is burnt was absorbed during the growing process.  A sustainable producer will replace and biomass that is burnt so biomass can be seen as a renewable fuel.  There are some CO2 emissions associated with the production and transport of biomass fuel and although they are relatively small they were taken into account in the calculations.

It is proposed that the CHP system is combined with the production of algae to form a cycle with minimal waste output.  The image below shows how the waste water from pressing the spent grain, known as press water, which is high in nutrients is used in the production of algae.  The flue gas from the biomass boiler is also used in the algae production, where the high concentration of CO2 helps to stimulate algae growth and also contributes to further reducing the CO2 emissions of the process.  As described below, the algae also helps to remove other harmful pollutants from the flue gas.  The algae produced in this process can then be used as additional fuel for the biomass boiler, further reducing the need for imported wood chips.  Remaining press water can then be used in the production of biogas which is further described in the biogas section.

This process is virtually waste free; even the ash from the biomass boiler can be used.  Ash from burning spent grain is very high in nutrients, notably nitrogen, and makes an effective fertiliser which could either be used in the community or sold to external customers.  The process is expected to produce 562 tonnes of ash per year.


the carbon cycle

Figure 2: The carbon cycle

Pollutants

Spent grain contains higher levels of sulphur and nitrogen than other types of woody biomass and as a result the flue gasses have higher levels of NOx and SO2.  Dust and soot emissions are also a serious consideration given the location of the plant, next to a residential area.  Wärtsilä, manufacturer of a biomass CHP plant running on spent grain suggest that the dust particles can be effectively filtered using cloth filters.  They also claim that the NOx emissions can be reduced by inserting additives containing NH3 into the combustion chamber.

A study at MIT found that not only can algae remove CO2 but it can also remove up to 85% of NOx emissions in flue gas.  This technology is still in the development stages but if proven successful it could provide an effective method of cleaning flue gases before they are released to the atmosphere.

Pollutant

Units

Quantity

CO

mg/m3n,
O2 =11% db.

200

NOx

mg/m3n,
O2 =11% db.

315

SO2
S ≤ 0.15%

mg/m3n,
O2 =11% db.

230

Particles

mg/m3n,
O2 =11% db.

. ≤ 10

Table 2: Flue gas pollutants

Fuel

The system is powered by a combination of spent grain, wood chips and algae. 

The brewery produces 2.2 million hectolitres of beer each year and it is estimated that this results in 44000 tonnes of wet spent grain.  The wet spent grain typically leaves the brewing process with 80% moisture content and this must be dried before it can be effectively burnt in a biomass boiler.  The water content of the spent grain must be reduced to below 58% for to make it an effective fuel and the most energy efficient way to do this was found to be through the use of a belt press which squeezes the water out of the grain.  

The energy content of spent grain varies considerably with the moisture content and the value can be calculated with the following equation:
energy content of spent grain equation

Where H is the calorific value of the spent grain in MJ/kg and w is the water content.  The graph below shows the calorific value of spent grain as moisture content varies.

Calorific value of spent grain

Graph 1: Calorific value of spent grain

At 58% moisture content the mass of spent grain has a calorific value of 7.5MJ/kg.  At this moisture content the total mass of spent grain is reduced to 20952 tonnes which has a total energy content of 53GWh.

Details of the wood chip and algae fuel are shown in the table below.

Fuel

Calorific value (MJ/kg)

Mass (tonnes)

Energy content (GWh)

Spent grain (58% moisture)

7.5

20952

43.7

Wood chips (30% moisture)

11.66

3418

11.1

Algae

25

246

1.7

Table 3: CHP fuel

Output

Electrical demand matching
The system will have an electric power of 2MWe and is designed to meet the base load electricity demand of the Brewery and will generate 17.52GWh per year.  As seen in graph 2 below there are occasions when the electricity load falls below the 2MW supply so 436MWh of electricity will be exported each year.  The brewery will also be connected to a wind turbine that will supply electricity above the base load.

The CO2 savings associated with this exported electricity has been attributed to the community, however it would be worthwhile carrying out further investigations into the use of a micro-grid in the community.  A micro-grid would aim to ensure that any low carbon electricity generated in the community is used in the community before being exported out to the wider national grid. This would help to maintain the accuracy of the CO2 saving estimates and reduce the chance of the renewable energy being lost in the national grid.

Brewery electricity demand profile and CHP supply

Graph 2: Brewery electricity demand profile and CHP supply


When combined with the proposed wind turbine there is an 85% match between electricity supply and demand, with a total of 3.27GWh of electricity exported and 1.96GWh imported from the grid.


Brewery thermal demand profile and CHP supply

Graph 3: Brewery electricity demand profile and CHP and wind turbine supply

Thermal Demand matching

The system has a thermal power of 3.3MWth and is designed to meet the base load heat demand of the brewery as well as the total heating load of the district heating.  The system will supply 29.20GWh of thermal energy per year.

Graph 4 shows the annual heat demand profile compared with the output from the CHP system.

Brewery thermal demand profile and CHP supply

Graph 4: Brewery thermal demand profile and CHP supply


 

Electricity (GWh)

Thermal (GWh)

Total energy generated

17.52

29.2

Energy used by brewery

17.08

27.48

Energy used by district heating

 

1.72

Energy exported

0.44

0

Table 4: CHP energy output

References

Incineration of Solid Food Waste: A Project About Spent Grain
Gerald Zanker, Werner Kepplinger, and Christian Pecher

A new biofuel – spent grain. In detail, WÄRTSILÄ TECHNICAL JOURNAL. Issue 2, 2008