Find. The maximum size (mass) of a rubber waste batch the incinerator can process without exceeding its design heat-release rate.
Approach. The incinerator's design heat-release capacity is a fixed thermal-input LIMIT, so switching to a much higher-heating-value fuel (rubber, 12,000 Btu/lb, versus its normal 700–1,000 Btu/lb waste) forces a correspondingly LOWER allowable mass feed rate. The mass held in the combustion chamber at steady state (the "batch" it can hold and still process effectively within the design residence time) follows from that reduced feed rate and the stated residence time.
Maximum allowable mass feed rate. To stay within the fixed heat-release capacity while burning the higher-heating-value rubber waste:
$$\dot{m}_{max} = \frac{2\times10^7\ \text{Btu/hr}}{12{,}000\ \text{Btu/lb}} = \boxed{1667\ \text{lb/hr}}$$
Maximum batch size. At this feed rate, over the stated 6-minute (0.1-hour) residence time, the mass held in the chamber at any instant is:
$$m_{batch} = \dot{m}_{max}\times t = 1667\ \text{lb/hr}\times0.1\ \text{hr} = \boxed{166.7\ \text{lb}\ (\approx76\ \text{kg})}$$
Final results
Quantity
Value
Maximum allowable feed rate
1667 lb/hr
Maximum batch size
166.7 lb (≈ 76 kg)
Check: assumes the incinerator's normal $700$–$1{,}000$ Btu/lb operating range is used only to confirm that $2\times10^7$ Btu/hr represents the FIXED design heat-release capacity (not itself a variable in the calculation) — the maximum feed rate/batch size is governed entirely by that capacity and the rubber waste's own (much higher) heating value.
