Cogeneration & combined heat and power

Heat rate is the whole business case

A CHP plant sells the gap between fuel cost and the combined value of electricity and recovered heat — the spark spread. Heat rate (kJ or BTU of fuel per kWh generated) is the denominator of that business. Total fuel utilization in a well-integrated CHP installation can exceed 80%, but the electrical fraction is the high-value product, and it is set almost entirely by how well the prime mover burns its fuel at the loads the plant actually runs — which are rarely the ISO full-load point the nameplate was quoted at.

Lean-premix combustion: performance on a knife edge

Modern low-emissions gas turbines use dry low-emissions (DLE / DLN) lean-premixed combustors: fuel and air are premixed and burned deliberately lean, holding peak flame temperature down to suppress thermal NOx without water or steam injection. The physics that makes this work also makes it fragile. The design point sits close to the lean blowout (LBO) limit, and combustor performance is characterized by the CO/NOx tradeoff: drift hotter and NOx climbs; drift cooler and CO rises from incomplete combustion. Both emissions are permitted; the compliant window between them is narrow — and it moves with fuel composition, ambient conditions and load.

At part load the combustor must run leaner still. OEM control systems protect stability by adding pilot fuel — a small diffusion flame that anchors combustion but raises NOx and CO, eroding the very emissions margin the DLE hardware was bought for. Fuel-staging schedules (which nozzles are fuelled, at which loads, with what splits and ramp rates) determine the plant's real turndown: how low it can run while staying inside its permit. Poorly matched staging also excites thermoacoustic dynamics — pressure oscillations that couple with the flame and destroy combustor hardware.

Recip CHP: the lambda window

Lean-burn reciprocating engines — the workhorses of mid-scale CHP — run λ ≈ 1.7–2.0, boxed between lean misfire on one side and knock and NOx on the other. Fuel-quality drift moves the operating point inside that window without any control input: a Wobbe Index swing on a fixed-orifice fuel system directly re-tunes lambda, and with it exhaust temperature, NOx, knock margin and efficiency. Composition-aware feed-forward, combined with closed-loop trim on exhaust O₂ and combustion feedback, holds the engine at its efficiency optimum instead of oscillating around a safety margin.

What site calibration does for CHP

A CHP plant is the ideal calibration candidate: fixed installation, characterized fuel supply, known thermal host, and a measurable duty cycle. We calibrate to that reality:

• Load-profile optimization — lowest achievable heat rate at the loads the plant actually runs, not just at nameplate. Staging schedules, fuel splits and pilot usage tuned for the real duty cycle.
• Emissions-margin recovery — mapping the true CO/NOx window against measured fuel and ambient data instead of fleet-conservative schedules, extending compliant turndown.
• Seasonal scheduling — dense winter air and thin summer air are different operating regimes; the calibration should know the difference.
• Fuel-drift immunity — feed-forward rescheduling when the delivered gas moves (renewable gas blending, pipeline composition drift), before the excursion instead of after the trip.