Rice-husk gasifier CHP for a rice mill
A rice mill installs a 500 kg/h downdraft gasifier using rice husk at 30% moisture and an as-received lower heating value of 13 MJ/kg. The gasifier has a cold-gas efficiency of 70%. The producer gas fuels an engine with 32% electrical efficiency, and 45% of the gas energy is recoverable as hot water at 90°C. Parasitic electric load is 8% of gross generation. The mill needs 350 kW of electricity and 400 kW of process heat. In summer, only 250 kW of the recoverable heat can be used because the drying demand falls. The mill is considering the gasifier as a replacement for grid electricity and a natural-gas boiler, and the operator wants to know whether the unit is adequately sized and how sensitive it is to fuel moisture.
1. Calculate the net electrical output and recoverable heat at 500 kg/h, and check both demands.
Read the worked answer
Feedstock energy input = 500 kg/h × 13 MJ/kg = 6,500 MJ/h = 1,806 kW.
Producer-gas energy = 0.70 × 1,806 = 1,264 kW.
Gross electrical output = 0.32 × 1,264 = 404.5 kW.
Parasitic load = 0.08 × 404.5 = 32.4 kW.
Net electrical output = 404.5 − 32.4 = 372.1 kW.
Recoverable heat = 0.45 × 1,264 = 568.8 kW.
Both demands are met: 372 kW > 350 kW and 569 kW > 400 kW. The electrical margin is only about 6%, so any degradation in gas quality or increased parasitic load will erode it. The heat margin is larger, but only 400/569 = 70% of recoverable heat is needed in the base case.
2. In summer only 250 kW of heat can be used. Calculate the overall efficiency and the wasted heat, and recommend a response.
Read the worked answer
Useful output = net electricity + useful heat = 372.1 + 250 = 622.1 kW.
Overall efficiency = 622.1/1,806 = 34.4%.
Recoverable heat wasted = 568.8 − 250 = 318.8 kW, or 56% of the recoverable heat. This is too large to ignore: the plant is effectively operating close to power-only efficiency in summer.
Options include using an absorption chiller for cold storage or space cooling, adding a drying load that can absorb low-grade heat, preheating boiler feedwater, or exporting heat to a nearby process. If no year-round heat sink exists, the CHP justification is weak and the mill should compare a power-only gasifier plus grid/boiler heat, or consider an organic Rankine cycle bottoming cycle. The design should not assume the winter heat demand all year.
3. Fuel moisture rises and cold-gas efficiency falls to 60%. Recalculate net electrical output at 500 kg/h and state whether 350 kW is still met; propose two measures.
Read the worked answer
Gas energy = 0.60 × 1,806 = 1,083.6 kW.
Gross electricity = 0.32 × 1,083.6 = 346.8 kW.
Parasitic load = 0.08 × 346.8 = 27.7 kW.
Net electricity = 346.8 − 27.7 = 319.0 kW.
This is 31 kW, or about 9%, below the 350 kW requirement. The unit is no longer adequately sized under the wetter fuel case.
Two measures: first, dry the husk to a stable moisture level, since 30% is already high for a downdraft gasifier and the efficiency loss is moisture-driven. Second, increase feedstock throughput: to deliver 350 kW net, gross must be 350/0.92 = 380.4 kW, gas energy must be 380.4/0.32 = 1,188.8 kW, and feed energy must be 1,188.8/0.60 = 1,981 kW, equivalent to about 549 kg/h. A backup boiler or grid import can cover the shortfall, but the operating plan must not assume 500 kg/h always delivers 350 kW.