Moisture upset in a 500 kg/h rice-husk gasifier CHP
A 400 kW-class downdraft gasifier–engine CHP unit is being commissioned at a rice mill. Design feed is 500 kg/h of rice husk as received at 12% moisture with LHV 13.5 MJ/kg. Guaranteed cold-gas efficiency is 68%, the producer gas LHV is 5.2 MJ/Nm³, and the engine's electrical efficiency on gas energy is 32%. Parasitic load for the blower, gas cleaning and cooling is 30 kW. During the monsoon the husk arrives at 20% moisture; at that moisture the gasifier's cold-gas efficiency falls to 62% and the as-received LHV of the husk drops because of the inert water. The mill has hot engine exhaust and gas-cooler heat available, and a vendor quotes ₹25 lakh for a dryer that will take the husk from 20% down to 12%. The mill expects to run 7,000 h/yr and sells surplus power to the grid at ₹6.50/kWh under the biomass tariff.
1. Compute the producer-gas volumetric flow and the net electrical output at design conditions.
Read the worked answer
Fuel power = 500 kg/h × 13.5 MJ/kg = 6,750 MJ/h = 1,875 kW_th.
Gas energy = 0.68 × 1,875 = 1,275 kW (4,590 MJ/h).
Gas flow = 4,590 MJ/h ÷ 5.2 MJ/Nm³ = 883 Nm³/h (0.245 Nm³/s). This is the number the gas-cleaning train and flare must be sized for; a common commissioning error is to size them on 500 kg/h of fuel rather than on gas volume.
Gross electrical = 0.32 × 1,275 = 408 kW. Net = 408 − 30 = 378 kW.
Note the implied overall efficiency: 378/1,875 = 20.2%, which is normal for a small downdraft unit and leaves roughly 45–50% of the fuel energy in the hot gas and char for recovery.
2. Estimate the electrical penalty when the husk is at 20% moisture, showing how the as-received LHV changes.
Read the worked answer
Convert to dry basis using LHV_wet = LHV_dry(1 − M) − 2.443M: 13.5 = LHV_dry × 0.88 − 2.443 × 0.12 → LHV_dry = (13.5 + 0.293)/0.88 = 15.67 MJ/kg.
At M = 0.20: LHV = 15.67 × 0.80 − 2.443 × 0.20 = 12.54 − 0.49 = 12.05 MJ/kg.
At the same 500 kg/h wet feed, fuel power = 500 × 12.05 = 6,025 MJ/h = 1,674 kW, a 10.7% drop. With cold-gas efficiency at 62%, gas energy = 0.62 × 1,674 = 1,038 kW, and gross electrical = 0.32 × 1,038 = 332 kW, net 302 kW.
Penalty = 378 − 302 = 76 kW net, about 20% of rated output, compounded from three effects: lower LHV per kg, lower cold-gas efficiency, and lower dry-solids throughput per hour at fixed wet feed. If the mill must instead process a fixed annual tonnage of husk, the comparison should be made per tonne of dry matter, and the loss narrows slightly because more wet mass is fed per hour.
3. Evaluate the dryer investment: compute the evaporation duty and the payback, then recommend.
Read the worked answer
Dry solids at 500 kg/h and 20% moisture = 400 kg/h. At 12% moisture the same solids correspond to 400/0.88 = 454.5 kg/h, so water to remove = 500 − 454.5 = 45.5 kg/h.
Heat duty at about 2.8 MJ per kg of water (latent plus sensible, allowing for dryer efficiency) = 45.5 × 2.8 = 127 MJ/h = 35 kW_th. Engine exhaust and gas-cooler heat can supply this several times over, so the marginal drying cost is only fan power.
Recovered output = 76 kW net × 7,000 h = 532,000 kWh/yr × ₹6.50 = ₹34.6 lakh/yr. Against ₹25 lakh installed, simple payback is under 9 months, and the unit also avoids derating, tar formation and engine knock associated with wet gas.
Recommendation: buy the dryer, but specify it on guaranteed outlet moisture and integrate it with exhaust heat rather than a separate fuel-fired burner, otherwise the parasitic heat cancels most of the gain. Hold 5–10% of the vendor payment against a 30-day monsoon trial at 20% inlet moisture.