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Introduction

Veritask Energy Systems, Inc has developed and is preparing to demonstrate its patented (US Patent # 5,501,162 and others pending) regenerative Enhanced Rankine Cycle (ERC) that achieves as much as a 10% (efficiency point) improvement in overall power generation cycle efficiency (coal to busbar) together with a concurrent reduction in NOx emissions of up to 90%. This new regenerative cycle achieves these goals by reducing energy losses to the flue gas and cooling water systems by reconfiguring the other heat recovery systems and using that recovered energy to add steam to the combustion zone. Steam addition results in production of sufficient carbon monoxide and hydrogen in the combustion zone to significantly reduce NOx production. The ERC approach is an innovative engineering solution that is built on the use of proven equipment and, therefore, should result in minimum risk. Unlike other solutions for NOx reductions, the implementation of ERC provides a relatively quick return on investment, usually less than three years, based on fuel savings alone. Any benefits of NOx emission reduction are in addition to the fuel saving.

Background

Fossil fuel-fired steam generation dominates today's electricity supply market worldwide. According to the Energy Information Agency, fossil steam plants supply 70% of the current power demand in the United States. However, the efficiency of most subcritical steam plants has approached the practical limit of 35-38%. Incremental improvements beyond those levels have been obtained only as a result of increasingly severe supercritical and ultra-supercritical steam conditions. Major development efforts have been undertaken during the last several decades to develop advanced fossil-fired power systems, specifically by combining steam and gas cycles. While the efficiency of these combined cycles is about 10% (efficiency points) higher than respective simple cycles, the increase comes at the expense of increased capital, operating, and maintenance costs, which together with greater complexity and reduced availability, can lead to a lower return on investment.

The Enhanced Rankine Cycle (ERC)

The 10 percentage point's improvement of fuel-to-power efficiency with the ERC is achieved through the introduction of additional regenerative duties, distributed primarily among the existing boiler equipment. The new duties supply supplemental heat input to the combustion zone of the boiler in the form of superheated steam. The energy of the superheated steam comes from specially designed heat recovery system that regenerates a portion of energy contained in the steam that passes through the turbine. The final step of superheating the steam is provided in the boiler heat recovery area. The energy of the superheated steam replaces a portion of fuel input to the boiler, therefore reducing fuel consumption and increasing efficiency.

Simultaneously, up to 90% NOx reduction is achieved by the modified, proprietary combustion process¹. The modified combustion process along with supplemental superheated steam input, as mentioned above, enhances the conversion of a parent fuel to carbon monoxide and hydrogen within the combustion zone of the furnace. The supplemental heat input from the superheated steam reduces the endothermic energy required for the conversion of fuel to carbon monoxide and hydrogen. Due to lower energy requirement for parent fuel conversion, the rate of carbon monoxide and hydrogen production is increased within the envelope of the furnace, especially under atmospheric pressure. Rich-combustion of the carbon monoxide, hydrogen and the balance of the original fuel significantly reduce NOx formation. Favorable conditions for significant reduction of nitrogen oxide formation are created along with the efficiency improvement.

To emphasize, the cornerstone of the proprietary ERC is a unique regeneration system that recovers heat from working media (steam) and, therefore, increases overall cycle efficiency. The NOx reduction is achieved without additional capital investment.

The VESI solutions for ERC implementation are modular and enable several alternate integration options. The modular approach allows VESI to tailor the ERC to specific customer needs while utilizing proven equipment. Its add-on capabilities also provide an option to improve efficiency for those plants where competitive Low-NOx systems have been installed previously. In such cases, operational costs for expensive additives will be also reduced. Modularity of the ERC configuration dramatically reduces downtime required for installation.

Depending on the selected modular solution, various degrees of fuel saving and NOx reduction may be achieved, providing various degrees of improvements. Table 1 below illustrates the advantages of ERC for NOx reduction applications in comparison to specific competitive alternatives. The Table shows NOx reduction percentages relative to uncontrolled NOx levels along with the pertinent efficiency improvement, as a function of total incremental capital investment, therefore establishing an economical justification for the investment.

The options for ERC technology as shown in Table 1 include the following modifications in the order of increasing total incremental capital cost:

Addition of steam injection to the existing burners ($3/kW)

Addition of steam heat regeneration system (Proprietary Concept 1) to the existing burners ($10/kW)

Installation of variable flame shape burners (See Figure 1) and steam heat regeneration system (Proprietary Concept 2 with no high quality condensate loss) ($20/kW)

Installation of variable flame shape burners, steam heat regeneration system (Proprietary Concept 2) and waste heat recovery system (WHRS, see Figure 2). ($60/kW)

Table 1. Technology Comparison (compared to a 400 MW-coal fired with uncontrolled NOx Emissions)

Technology

Legends
Δr - NOx reduction, %
Δe - efficiency increase, %
       (base percentage points)
Total Incremental Capital Cost, $US/kW
$3 $10 $20 $60 $80
Δr Δe Δr Δe Δr Δe Δr Δe Δr Δe
Low NOx Burner 50 0
Low NOx burner + OFA 70 0
SNCR 35 0
SCR 80 0
ERC 30 0 30 2 90 2 90 10



















It should be emphasized that the total incremental costs in Table 1 are for reference only and usually must be confirmed for specific fuel and site conditions

Figure1. Variable Flame Shape Coal Burner
This burner operates in two modes - regular LNB amd Ultra Low NOx. The later mode allows for water wallprotection under severe sub-stoichiometric conditions (below 70%) for wall-fired applications

The heat losses, as shown in the simplified ERC diagram in Figure 2, indicate a significant improvement of energy utilization by reducing condenser losses from 50% to 42% and stack
losses from 15% to 13% when compared to typical coal fired plants. As a result the overall efficiency increases from 35% to 45%.



Summary of Enhanced Rankine Cycle (ERC) Benefits

Up to 10 percentage points improvement in steam cycle efficiency due to introduction of the innovative waste energy recovery from working media

Utilization of the recovered energy in the modified combustion process to achieve up to 90% NOx emissions reduction by cycle modifications only, without post combustion treatment of flue gas

No need for expensive and/or hazardous additives to reduce NOx emissions

Multiple modular integration options

Use of proven heat recovery equipment reduces pay-back time and risk

Significant capital reduction for re-powering in comparison to combined cycles

Short return on investment for re-powering and retrofits


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