Nooter/Eriksen - Heat Recovery Steam Generators (HRSGs)
Heat Recovery Steam Generators (HRSGs) are used to recover waste heat from the exhaust of combustion gas turbines (CGTs) in the useful form of steam. The efficiency of the CT alone (simple cycle) can be in the 40% range. Recovery of the waste heat in a HRSG can boost cycle efficiencies to nearly 60% (some claim higher!). The steam generated can be used for process heating (cogeneration), the production of power in a steam turbine (combined cycle), or both (combined heat and power or CHP).
N/E also has many features dedicated to enhance HRSG lifetime and reduce lifecycle costs. We include spring hangers at strategic points in the hot end of the unit (HPSHTRs and RHTRs) to minimize accumulated stresses due to overall thermal growth differentials between the coldest and hottest rows of tubes (can be hundreds of degrees temperature differentials); stub all upper tube to header connections to minimize header thickness and provide superior cyclic capability; in the RHTR where pressures permit, steam side bypass attemperation is used rather than spraywater desuperheating for control of the final steam temperature thereby eliminating the possibility of liquid carryover/under that may cause quench damage to surrounding heating surface or increase impurities in the steam; and more.
HRSGs are designed based on the envelope of specific cases (heat balances) defined by the client. Any additional modes of operation such as CGT part load, CGT steam injection/dry operation, other ambient temperatures, etc. may impact the design of the HRSG and should be specified in detail. Enhanced operation with the addition of supplemental (duct) firing can improve the output from the steam cycle. Firing at base load of the CGT is assumed. Firing at partial load conditions is possible and the details of the required envelope must be specified. N/E can offer guarantees including “terminal” thermal/hydraulic performance (electrical from theoretical ST and CGT or steam flow and temperature), steam purity, emissions from components like supplemental (duct) burners as well as emissions control devices including CO and SCR catalyst systems, and noise transmission based on specified process parameters, outlet/terminal point pressures, CGT pollutant and noise emissions, etc. Flow model testing and analysis are included for all units with a duct burner or catalyst system utilizing turbine exhaust information supplied by the client.
N/E also provides innovative solutions for common industry issues. We have patented systems which replace the parasitic power consumption and maintenance problems of the recirculation pumps common to prevent cold end corrosion of the HRSG for inlet water temperature control with an external heat exchanger which requires no parasitic power load and no maintenance yet has all the benefit. N/E also offers a patented system to measure and monitor the TEG acid dewpoint.
N/E’s “typical” HRSG is in general terms a horizontal turbine exhaust gas (TEG) flow, vertical tube, top supported, natural circulation design with all welded pressure part construction and a cold external TEG pressure casing. With many hundreds of HRSGs in operation around the world, we have led the way in bringing this technology to the forefront of major HRSG supply globally. Horizontal TEG flow and top support allow the heat transfer tubes to hang in tension where they can grow freely up and down with varying temperature gradients and avoid the additional stress from the compressive loads of their own weight and the weight of additional equipment on top of the HRSG. Employing natural circulation eliminates the first cost, parasitic power consumption, and high maintenance costs associated with the evaporator circulation pumps necessary with forced circulation (sometimes euphemistically referred to as assisted circulation) systems. N/E’s natural circulation design instead use the natural buoyancy of the steam bubbles and the density differential between an all liquid external downcomer leg and the two-phase flow in the heat transfer tubes and external risers to drive the evaporator circulation. This leads to higher circulation ratios and a safer more robust design and operation. N/E`s standard internally insulated casing design utilizes a cold, gas tight outer pressure casing to virtually eliminate thermal expansion of the casing and prevent rapid thermal transients from overstressing and cracking the casing. The casing will be internally insulated and lined with a special floating liner that is free to move to accommodate thermal growth without distortion or warping. In high turbulence areas, each liner plate will be supported on its perimeter with a rigid structural system, and floating batten channels will be added to the perimeter of each liner plate for additional support.
N/E has taken the industry lead with respect to field installation as well. We have installed many of our units and have also responded to ideas from many outside erectors around the world. Our entire structural/mechanical design revolves around improving the erectability of our products in order to maintain the highest level of client satisfaction. As a standard we deliver large modules shipped with integral roof casing with sealed penetrations and upper gas bypass baffles all shop installed; shop fabricated main structural columns with integral baseplates welded to the insulated and lined casing panels and no large bore piping welds inside the casing envelope are required.
