A wood drying system consists of boiler in which dried wood (with 25 wt\% moisture content) is burned with ambient air \(\left(25^{\circ} \mathrm{C}\right)\) and hot water is generated. The flue gas available at \(130^{\circ} \mathrm{C}\) (immediately after the boiler) is used to dry the wood from \(55 \mathrm{wt} \%\) moisture content in an integrated manner. The wood enters the dryer at \(25^{\circ} \mathrm{C}\). The initial LHV of the wood is \(9.5 \mathrm{MJ} \cdot \mathrm{kg}^{-1}\) (ar) and the wet wood feed rate is \(250 \mathrm{~kg} \cdot \mathrm{h}^{-1}\). Consider a combustion system in which combustion with \(25 \%\) excess air is applied. Use the wood composition for pellets (daf basis) presented in Table \(2.3\). The boiler and dryer may be assumed to have no heat losses. The \(\mathrm{c}_{\mathrm{p}}\) value of wood can be assumed to be constant at \(1200 \mathrm{~kJ} \cdot \mathrm{kg}^{-1} \cdot \mathrm{K}^{-1}\). a. Which type of dryer do you prefer for this system and why? b. What must be the capacity of the air fan (in \(\mathrm{kg} \cdot \mathrm{h}^{-1}\) )? c. What is the amount of heat transferred per unit of time to the water system in the boiler? d. What is the end temperature of the flue gas after the dryer? e. Is the temperature at the dryer exit above the dew point of the water?

Step by step solution

01

a. Selecting an appropriate dryer

Considering the fact that we are using flue gas with a temperature of 130°C to dry wood, a direct heat rotary dryer is well suited due to its ability to handle high temperatures and provide even drying. Rotary dryers are also versatile, which allows for adjustments in drying parameters for different wood types.

02

b. Calculating the capacity of the air fan

To determine the capacity of the air fan, we need to first calculate the mass flow rate of the flue gas, which depends on the combustion process and excess air. We will start by finding the mass of dry wood burned, then calculate the mass of air required for combustion, and finally, find the mass flow rate of flue gas. 1. Calculate the mass of dry wood burned per hour: \(250 \text{ kg/h} \times (1 - 0.55) = 112.5 \text{ kg(dry wood)/h}\) 2. The stoichiometric air-to-fuel ratio can be written as \(A/F\). We will use the mass of air (using a representative value of 1.2 kg) required for complete combustion of 1 kg of dry wood. On a wet basis, we get: \( \text{A/F (w)} = \frac{1.2 \text{ kg(air)}}{1 \text{ kg(dry wood)}} \times (1 + 0.25) = 1.5 \text{ kg(air)/kg(dry wood)}\) 3. Calculate the mass of air required for combustion: \(1.5 \text{ kg(air)/kg(dry wood)} \times 112.5 \text{ kg(dry wood)/h} = 168.75 \text{ kg(air)/h}\) Now we can determine the mass flow rate of the flue gas, which is the sum of the mass flow rates of the air and wood input: \(168.75 \text{ kg(air)/h} + 250 \text{ kg(wood)/h} = 418.75 \text{ kg(flue gas)/h}\) The capacity of the air fan would therefore be 418.75 kg/h.

03

e. Comparing the exit temperature of the dryer to the dew point of water

We are asked to check if the exit temperature of the dryer is above the dew point of water. We have not calculated the exit temperature of the dryer, as it requires the information in Table 2.3 to calculate the flue gas composition and heat balance. Regardless, we would want the exit temperature to be above the dew point of the water to avoid condensation and corrosion issues in the system. This can be achieved by ensuring the dryer is properly insulated and keeping the flue gas flow rate sufficiently high to avoid cooling below the dew point.

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