Question

Question 3 [Total Marks- 25 Biomass, Bioenergy and Biomass Supply Chain a) Briefly describe how to use a bomb calorimeter to measure the higher heating value (HHV of a biomass fuel. 5 Marks] The data for proximate analysis and ultimate analysis of a woody biomass are given in Table 1. The higher heating value (HHV, MJ/kg) of the fuel can be estimated using the equation below: b) HHV-19.914-0.2324 x Ash, where Ash is the weight percentage on dry biomass basis. Estimate the HHV of the biomass, and calculate its lower heating value (LHV) based on the data in Table 1. Note: the latent heat of vaporisation of water is 2.44 MJ/kg at 25 °C [10 marks Table 1: Proximate analysis and ultimate analysis of a woody biomass dbb l analysis ( xygen 44.11 6.7 0.19 80.7 5.3 0.4 18.9 49.0 air dried basis. 'dry basis. "dry and ash-free basis. volatile matter. fixed carbon.by difference. In a particular part of the Wheat Belt of Western Australia, one wheat crop is grown per year c) and the wheat yield is 2.5 tonnes per hectare. The Harvest Index is defined as the ratio of the grain weight to the total plant weight. Winter cereals such as wheat typically have a Harvest Index of 0.36. Estimate the total plant material produced per hectare. Assuming that the harvested wheat has a moisture content of 50% (on the wet basis), that the energy content of the plant material is 18 GJ/tonne of oven-dried material. It is proposed to use some of the wheat residue after harvesting to fuel a nearby 10 MW power station. 1.4 green tonne/hectare of wheat stubble is to be left in the ground for reasons of soil stability and soil nutrients. If the power station is to have a 90% availability, and an efficiency of 32%, what area is required to produce sufficient fuel for such a power station? 10 marks]

Answer 1

a) A bomb calorimeter can be used to measure the heat released by any chemical reaction including combustion. The energy calculations require the mass as well as the specific heat capacity of the fuel. Once the combustion starts, we measure the maximum temperature attained using the bomb calorimeter and multiplly the mass and specific heat capacity. Then we double this, since higher heat capacity also takes into account the heat released during cooling after maximum temperature has been achieved. It is important to note that the initial temperature of the fuel should be 25°C.

b) The HHV for the woody biomass is

HHV = 19.914 - 0.2324x0.4 = 19.8214 MJ/kg.

The water released per kilogram of fuel is 0.053kg, since moistuer content is 5.3%.

Thus, lower heating value is LHV = 19.8214 - 0.053x2.44 (LHV = HHV - mC where m is mass of water released and C is specefic heat capacity of water).

Thus, LHV = 19.69208 MJ/kg

c) Since the yield is 2.5 tonnes per hectare, and this is 36% of the total produce, the total produce is 2.5/0.36 tonnes per hectare =  6.9 tonnes per hectare.

Now, out of the total 6.9 tonnes per hectare, 2.5 is used for food, and 1.4 needs to be left in the soil. Thus, the total biomass available for power consumption is 3 tonnes per hectare. Since the annual availability of the power plant is 90%, the total time where energy needs to be produced is 0.9x365x24 = 7884h.

The total energy produced by the plant over the year is thus 10x7884 = 78840 MW or 78.840GJ

The efficiency of the power plant is 32%. Thus, the total energy that needs to be input is 78.840/0.32 = 246.375 GJ.

The energy yield of the biomass is 18GJ/tonne. Thus, the total amount of biomass that needs to be burned is 246.375/18 = 13.6875 tonnes.

Thus, the total area that needs to be harvested for such a power plant is 13.6875/3 = 4.5625 hectares.