Regenerative Thermal Oxidation of Flue Gases to Increase Efficiency of Power Production from Landfill Gas and Biogas

Transcription

1 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 1 Bauhaus-Universität Weimar Fakultät Bauingenieurwesen Weiterbildendes Studium Wasser und Umwelt M a s t e r a r b e i t im Weiterbildenden Studium Wasser und Umwelt Regenerative Thermal Oxidation of Flue Gases to Increase Efficiency of Power Production from Landfill Gas and Biogas eingereicht von Dipl.-Ing. Ludger Dinkelbach geb. am in Gelsenkirchen Reg.-Nr. WU MA 34 /04 Erstprüfer: Zweitprüfer: Prof. Dr.-Ing. E. Kraft Prof. Dr.-Ing. R. Widmann (Uni Essen) Ausgabedatum: 01. Januar 2005 Abgabedatum: 30. Juni 2005

2 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 2 Contents Contents Introduction Background Objective Approach Technology Combined Heat and Power Production from Landfill Gas or Biogas Origin and Composition of Landfill Gas and Biogas The Concept of Mobile Containerized Combined Heat and Power Plants Emission Legislation and Emission Control Regenerative Thermal Oxidation (RTO) Principle and Function of Regenerative Thermal Oxidation Examples of RTO Applications The Concept of Regenerative Thermal Oxidation of Flue Gases from Gas Engines Experimental Investigation Set-up of the Test Facility Landfill Gas Engine RTO Reactor Measuring Equipment Experimental Results Reference Measurements Variation of Excess Air Ratio and Point of Ignition Discussion of the Results Technical Aspects Economic Aspects Environmental Aspects Conclusions References List of Figures List of Tables Acknowledgement Declaration / Erklärung... 50

3 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 3 1 Introduction Chapter 1 is to introduce both subject and methodology of this thesis by outlining necessary background information (paragraph 1.1), presenting the objective (paragraph 1.2) and explaining the approach how this objective is to be achieved (paragraph 1.3). 1.1 Background The utilization of landfill gas and biogas in gas engines for power (and heat) production is state-of-the-art technology. In most cases, lean burn engines are applied that make use of a relatively high excess air ratio (λ > 1,5) to ensure low emissions of carbon monoxide (CO), hydrocarbons (C x H y ), and nitrogen oxide (NO x ). However, lean burn engines do not operate under optimum conditions in terms of efficiency. In other words: Based on state-of-the-art technology, it is not possible to achieve highest engine efficiency and low emissions simultaneously. The process of regenerative thermal oxidation (RTO) is an effective means to treat waste gas streams that contain combustible components like carbon monoxide and hydrocarbons. A typical RTO application is cleaning of ventilation air from paint shops. Contrarily, RTO has only exceptionally been applied for the treatment of flue gases from gas engines. Regenerative thermal oxidation of flue gases from gas engines offers the possibility to achieve both high engine efficiency and low emissions simultaneously. Though, additional equipment is required leading to extra investment and maintenance costs. 1.2 Objective The objective of this master thesis is to analyze the application of regenerative thermal oxidation technology for the treatment of flue gases from gas engines running on landfill gas or biogas. Based on experimental results, conclusions are to be drawn to what extent this application is technically feasible, environmentally desirable, and economically attractive.

4 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas Approach First of all, the innovative concept of regenerative thermal oxidation (RTO) of flue gases from gas engines will be defined based on the well-known technologies of combined heat and power (CHP) production on the one hand and RTO on the other hand (chapter 2). A test facility will be designed and erected to carry out experiments of flue gas treatment from a landfill gas engine making use of RTO technology. Experimental investigations will be carried out to gather experimental data on technology performance. Based on these results, the concept will be evaluated from the points of view of technology, environment, and economics (chapter 3). This will finally lead to a conclusion to what extent RTO is applicable for the treatment of flue gases from gas engines running on landfill gas or biogas (chapter 4).

5 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 5 2 Technology In this chapter, the technologies of Combined Heat and Power (CHP) production from landfill gas and biogas (paragraph 2.1) on the one hand and Regenerative Thermal Oxidation (RTO) for waste gas cleaning (paragraph 2.2) on the other hand are described. The combination of both technologies leads to the innovative concept of Regenerative Thermal Oxidation of flue gases to increase the efficiency of power production from landfill gas and biogas which is introduced in paragraph Combined Heat and Power Production from Landfill Gas or Biogas Combined Heat and Power (CHP) production from landfill gas or biogas is state-of-the-art technology. Mostly, gas engines (or dual fuel engines) are applied that drive a generator for electricity production. Heat is recovered as long as a suitable consumer is available in the neighborhood of the power plant. In Fig. 1, the entire processes of the utilization of landfill gas and biogas, respectively, are sketched [G.A.S.]. In paragraph 2.1.1, origin and composition of landfill gas and biogas are described. Then, the concept of containerized CHP modules is introduced (paragraph 2.1.2). Paragraph is dedicated to flue gas emission control and relevant legislation. Landfill Gas Collection/Compression Flare CHP Modules Power/Heat Organic Waste/Litter Biogas Reactor Gas Compressor CHP Modules Power/Heat Fig. 1 Utilization of Landfill Gas and Biogas for CHP Production [G.A.S.]

6 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas Principle and Function of Regenerative Thermal Oxidation Basically, there are three concepts of oxidation (post-combustion) as a means to reduce (or eliminate) combustible components in gas streams [MegTec]: Simple-Flow Thermal Oxidizers: A simple-flow oxidizer consists of only one reaction chamber. Once the oxidizer has been heated to a temperature high enough for oxidation, the waste gas is blown through the reaction zone in which the combustible components are converted to CO 2 and H 2 O. The technology is simple and cheaper than flow reversal technology. On the other hand, the heat o f oxidation is not being recovered within the process but leaves the reactor via the (hence relatively hot) clean gas. Due to the modest heat balance, this technology is only ad visable if waste gas streams with a high load of combustibles are to be treated so that the process can do without extra heat from external sources. Simple-flow thermal oxidation of flue gases from gas engines would lead to enhanced costs for the continuous supply of additional energy. Regenerative (Flow Reversal) Thermal Oxidizers: In Regenerative Thermal Oxidation (RTO), the majority of the heat of combustion is recovered within the process to reduce or even avoid addition of external energy. The heat balance of the process is improved by an alternating direction of the gas flow: Waste gas is being pre-heated in a regenerator (regenerator A ) before being further heated by oxidation of combustibles (and, if necessary, by an additional heat source). Before leaving the reactor, the hot gas transfers part of its thermal energy to regenerator B. During this process, regenerator A is being cooled, whereas regenerator B is being heated. As soon as a certain lower temperature limit is being achieved in regenerator A, the flow direction of the waste gas is inverted which means that the waste gas is first pre-heated in regenerator B, then heated by the combustion enthalpy, and finally cooled in regenerator A transferring part of its heat. RTO is more sophisticated and costly than simple-flow technology. Though, RTO is most commonly applied due to the favorable heat balance of the system resulting in low operating costs for additional heat supply even if waste gas with a relatively low amount of combustibles is to be treated. Catalytic Oxidizers: Catalytic oxidizers make use of catalysts to reduce the temperature that is required for (virtually) complete oxidation of the combustible components. Both simple-flow and flow reversal reactors can be equipped with catalysts. A reduction of the process temperature is advantageous for the heat balance (the lower the temperature, the lower the heat losses) and allows application of less heat-resistant materials. On the other hand, both investment and operating costs are higher. Application of catalysts is not advisable if flue gas from biogas or landfill gas engines is to be treated, since these gases might contain components like chlorine, fluorine, or sulfur that might destroy the catalyst.

7 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 18 A summary of the advantages and disadvantages of the different types of oxidizers is given in Table 4. For the treatment of flue gases from biogas or landfill gas engines, flow reversal thermal oxidation (upper right rectangle) appears most suitable. Those disadvantages that are underlined were treated as knockout-criteria against other technologies. Table 4 Types of Oxidizers, Choice for Type for the Treatment of Flue Gas from Gas Engines Type Simple-Flow Flow Reversal Thermal Advantages: simple, robust, cheap. Advantages: improved heat balance. Disadvantages: only suitable for gases Disadvantage: moving parts, more with a high amount of combustibles. costly. Catalytic Advantages: lower temperature, lower Advantages: see simple-flow catalytic material requirements. oxidizer and flow reversal thermal Disadvantages: enhanced investment oxidizer and operating costs, not suitable for Disadvantages: see simple-flow catalytic waste gases with components that might oxidizer and flow reversal thermal destroy the catalyst. oxidizer. Fig. 10 shows the principle of a flow reversal thermal oxidizer basically consisting of gas inlet and outlet, two regenerators A and B and a reaction chamber. The direction of the gas flow is being alternated regularly by changing the positions of the valves V1 and V2. Reaction chamber Reaction chamber Inlet Reg. A Reg. B Inlet Reg. A Reg. B V1 V2 V1 V2 Outlet Outlet Fig. 10 Principle of Flow Reversal Thermal Oxidation

8 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 23 3 Experimental Investigation A test facility was designed, erected, and operated to carry out experiments on RTO of flue gases from a landfill gas engine. The set-up of the test facility will be described in paragraph 3.1. Then, the results of several days of operation of the test facility will be summarized in paragraph 3.2. These results of the experimental investigation will be discussed in paragraph Set-up of the Test Facility Based on the theoretical considerations as presented in chapter 2, an RTO test facility was designed consisting of a flow reversal RTO reactor (paragraph 3.1.2) being connected to a landfill gas engine (paragraph 3.1.1) and measuring equipment to gather data on flue gas emissions and gas engine performance (paragraph 3.1.3) Landfill Gas Engine For the experiments, a gas engine was used that had been and still is in operation on a landfill site in the German province of Northrhine-Westphalia. The engine is of TBG 620 V8 type manufactured by Deutz Energy. A summary of the major characteristics of the engine is given in Table 5. A picture of a TBG 620 engine is shown in Fig. 15. Table 5 Major Characteristics of the Deutz Engine TBG 620 V8 Gas Engine [Deutz] Manufacturer Type of Engine Deutz Energy TBG 620 V8 Number of Cylinders 8 Fuel Input Electric Output kw 491 kw el Electric Efficiency 35,5 % Amount of Wet Flue Gas kg/h (2.440 m n 3 /h) Flue Gas Temperature 464 C

9 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 24 Fig. 15 Gas Engine Deutz Energy TBG 620 [Deutz] Before installing the RTO, emission control of the landfill gas engine was confined to careful adjustment of the point of ignition and the air/fuel-ratio without further measures of flue gas treatment. In order to integrate an RTO, the flue gas piping between silencer and chimney had to be modified RTO Reactor A regenerative thermal flow reversal oxidizer (RTO) was integrated in the flue gas piping of the gas engine between silencer and chimney of the landfill gas engine, see Fig. 16. The RTO had originally been used to clean waste air from a paint shop and had been retrofitted for gas engine application: New bed material had been filled into the regenerators (see Fig. 10). Due to the higher temperature of the flue gas of C compared to waste air from paint shop s ( C), inlet and outlet v alves (V1, V2, see Fig. 10) had to be replaced by new valves made of stainless steel instead of carbon steel. Insulation of the gas inlet and outlet had to be improved due to higher gas temperatures. The combustion chamber had been equipped with a propane gas burner taking into account that flue gases from gas engines contain only a small amount of combustible components and that supplementary thermal energy is required to maintain the required RTO temperature of about C. The entire RTO had been cleaned.

10 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 44 4 Conclusions Based on the experimental results and their discussion that have been presented in chapters 3.2 and 3.3, respectively, conclusions will be drawn to what extent and in what cases application of RTO for the treatment of flue gases from gas engines running on landfill gas or biogas appears sensible. Again, a distinction will be made between conclusions related to technology, economics, and environment. Conclusions Related to Technology It was shown that RTO is an effective means to reduce CO-emissions in the flue gas of a gas engine by about 90 %. This end-of-pipe reduction of CO-emissions allowed to adjust operating parameters of the engine towards enhanced efficiency without exceeding the limit of CO-emissions: The point of ignition could be adjusted towards at greater angle before the top dead center, and the air/fuel-ratio could be adjusted towards lower values. Though, this adjustment was limited by value of NO x in the flue gas that was increased by both the adjustment of engine parameters and by formation of additional thermal NO x in the reaction chamber close to the propane gas burner that is necessary to maintain the temperature of the RTO above 800 C. Despite the limitation caused by additional NO x -emissions, adjustment of engine parameters resulted in an increase of engine efficiency by 4.6 %-points. In this figure, additional energy consumption of the RTO (both electricity and heat) have already been taken into account. Increasing engine efficiency means increasing electricity production at constant fuel consumption or constant electricity production at reduced fuel consumption. In general, it is not advisable to operate a gas engine at higher load than specified by the manufacturer. Continuous overload operation results in enhanced maintenance costs and causes the risk to loose manufacturer s guarantee. In some cases, it is even favorable to continuously operate the engine at reduced capacity in order to extend the expectable lifetime of the engine and to reduce maintenance costs. Production of a certain amount of electricity at reduced fuel consumption is desirable as long as the fuel gas is not available in abundance. If the fuel gas is available for free and fuel savings in the gas engine means that part of the available gas must be flared or even simply released to the atmosphere, it does not make sense to increase the efficiency.

11 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 45 It can be assumed that RTO also effectively reduces the amounts of other combustible flue gas components like hydrocarbons and formaldehyde. However, no data was gathered on the fate of these components in the flue gas streams. When designing a full-scale commercial RTO installation for the treatment of flue gas from gas engines, the following aspects must be taken into account that had been of minor importance concerning the experimental facility: Noise Emissions: On the one hand, operation of an RTO results in additional noise emissions due to manipulation of valves and operation of the combustion air fan. On the one hand, an RTO has a noise reducing effect in the exhaust gas system and might allow to do with a simpler and cheaper conventional silencer. This must be analyzed based on local noise emission regulation. Pressure Drop: An RTO causes a pressure drop in the exhaust gas system that mainly depends on the velocity of the flue gases streaming through the RTO. When designing the RTO (especially the size of the cross-section), the maximum acceptable pressure drop has to be taken into account. This maximum pressure drop depends on engine characteristics and pressure drop due to other parts of the exhaust gas system like piping, silencer, flue gas heat exchanger. Fuel for RTO Burner: The test facility was equipped with a propane burner for startup of the RTO as well as to ensure an operating temperature above 800 C. In a full-scale commercial RTO, the burner should be fuelled by the same (type of) gas that is used in the gas engine (e.g. landfill gas) to avoid additional fuel costs. From a technical point of view, application of RTO to increase engine efficiency makes sense in the following two cases: If the gas engine(s) cannot operate at full capacity because of a lack of available fuel gas. This can especially happen at landfill sites where the amount and quality of the available landfill gas change in the course of the lifetime of the landfill site. Increasing engine efficiency can then allow the engine(s) to operate at higher capacity for a longer period. Contrarily, biogas and sewage gas plants are typically designed for a certain amount of input material (e.g. animal litter or sewage sludge) that remains more or less constant. In this case, the gas engine(s) can normally maintain their capacity during the lifetime of the installation. If CO-emissions of the gas engine cannot be kept below the valid emission limit despite adjustment of engine parameters towards optimum operation in terms of emissions. RTO or other unconventional end-of-pipe flue gas cleaning then become necessary, since application of catalytic converters is not possible due to their sensitivity towards gas contaminants like sulfur or chlorine.

12 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 46 Conclusions Related to Economics Application of RTO results in additional capital and operating expenditures. To make application of RTO economically attractive, these expenditures must be compensated by additional earnings. There are two possibilities of additional earnings: If application of RTO results in an enhanced electricity production, electricity sales will increase. If application of RTO results in a reduction of (combustible) flue gas emissions and this leads to an economic advantageous, e.g. an enhanced payback tariff for electricity that has been produced at very low emissions. This possibility is of rather theoretical nature and has not been analyzed within this thesis. Economic calculations have shown that the minimum depreciation time for the compensation of an RTO installation that is connected to a gas engines is about four years. Of course, economic figures depend on a series of circumstances and economic assumptions that individually have to be specified for each project. Though, it can be concluded that the concept of RTO to increase gas engine efficiency will hardly be economically attractive in most cases. This means that this application will probably be restricted to the cases in which RTO application is necessary to operate the gas engine within the emission limits. Conclusions Related to Environmental Concerns From an environmental point of view, application of RTO to treat flue gases from gas engines result in two advantages and one disadvantage. Reduction of Carbon monoxide (CO) and other combustible components in the flue gas is advantageous. Increase of the NO -content in the flue gas is disadvantageous. x Reduction of fuel consumption due to an enhanced engine efficiency is advantageous as long as the fuel that has been saved can be used elsewhere. It is difficult to compare the advantage of a reduction of one type of emission (here: CO) to an increase of a different type of emission (here: NO x ). Though, an increase of NO x by 5 % in 3 3 relation to the limit (10 to 30 mg/m n in relation to 500 mg/m n ) appears acceptable when CO- emissions can simultaneously be reduced by about 90 % from more than 900 mg/m 3 n to less than 100 mg/m 3 n. It is even more complex to compare a (positive) effect on fuel consumption to a simultaneous (negative) effect on flue gas emissions.

13 RTO of Flue Gases to Increase Engine Efficiency of Power Production from Landfill Gas and Biogas 47 Taking these three effects into account, it becomes obvious that the overall environmental effect of RTO application is positive. Nevertheless, RTO should not be considered state-ofthe-art technology that has to be applied in every installation of this kind, since the resulting reduction of CO-emissions cannot justify an additional investment of 20 to 40 % of the entire (combined heat and) power plant. This might cause failure of biogas or landfill gas engine (or other) projects that basically appear desirable from an environmental point of view. Overall Conclusion Based on the conclusions that have been individually discussed for the aspects of technology, economics, and environmental concerns, the following overall conclusions can be drawn: Application of Regenerative Thermal Oxidation of flue gases from gas engines to increase engine efficiency and to reduce flue gas emissions is only sensible in the following cases: The valid emission limits of combustible components cannot be met without additional (end-of-pipe) flue gas treatment, and cheaper flue gas cleaning technologies like catalytic converters cannot be applied. In this case, RTO - or other measures must be applied in order to fulfill legal requirements and to allow operation of the installation. At the location of the power plant, there is not enough fuel gas to have the engine operate at full capacity. By increasing engine efficiency, more electricity can be produced by the same amount of fuel gas. As long as it can be expected that the engine can be operate at enhanced capacity for a period that is longer than the depreciation time of the investment related to the RTO, application is advisable. The owner and/or operator of the power plant wants the installation to operate at lower emission than requested by law, which might make sense e.g. for marketing reasons. By applying additional measures (and while accepting certain additional expenditures), negative environmental effects of the installation can be reduced. These reasons why to apply RTO for gas engines appear exceptional rather than standard. Therefore, it can be expected that this concept of RTO will not be applied in a great number of gas engine power plants in the near future. Nevertheless, this thesis has shown that this RTO concept is technically feasible, environmentally desirable, and economically acceptable in some cases.

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