Emissions and deposit properties from combustion of wood pellet with magnesium additives

文章编号:0253?2409(2013)05?0530?10

Emissions and deposit properties from

combustion of wood pellet with magnesium additives

Tomas Persson1,Jochen Riedel1,Jonas Berghel2,Ulf Bexell1,Kaung Myat Win1

(1.School of Technology and Business Studies,Dalarna University,79188Falun,Sweden

2.Department of Energy,Environmental and Building Technology,Karlstad University,65188Karlstad,Sweden) Abstract:This work studies the amount of gaseous and particle emissions and deposits on heat exchanger surfaces in a boiler fired with commercially available pellets and with pellets primed with magnesium oxide and magnesium hydroxide.The combustion experiments were performed on a residential boiler of20kW.Substrates placed in the heat exchanger was analysed with SEM?EDX?mapping to evaluate the chemical composition of the deposits.The results show that particle emissions(PM2.5)using the additives increased by about50%and the mass of the deposits in the flue gas heat exchanger(excluding loose fly ash)increased by about 25%compared to the combustion of pellets without additives.The amount of additives was found to be eight times higher than the amount of the main alkali metals potassium(K)and sodium(Na)which leads to the assumption that the additives were overdosed and therefore caused the problems reported.The SEM analysis of the substrates placed in the flue gas heat exchanger indicate that the deposits of sodium(Na),potassium(K),chlorine(Cl)and sulphur(S)decrease using the additives.If this was due to the expected chemical reactions or due to the loose fly ash covering the substrates after the test,could not be determined in this study. Keywords:softwood pellets;magnesium;additives;particle emissions;deposit formation

CLC number:TK6 Document code:A

Deposit formation in boilers causing harmful particle emissions[1]and require regular cleaning, which is time consuming and costly.In addition deposits on the boiler convection surfaces decrease heat transfer and efficiency of boilers.In case of corrosive deposits including chloride and sulphur,also a reduced life time of the boiler is expected[2]. During combustion,alkali metals in the fuel such as potassium(K)and sodium(Na)are released and they may condensate on heat exchanger surfaces and build sticky layers together with chlorine(Cl), silicon(Si)and sulphur(S)which will develop more deposits and may also enhance corrosion[2~5]. B?fver et al[6]and Carvalho[7]describe in what principal way additives are reacting with alkali compounds during combustion.The additives can react instead of the alkali and form substances with higher melting temperatures that do not build sticky layers;they can prevent the release of gaseous KCl or react with KCl into less corrosive components and a combination of these effects are also possible.

Though the use of additives in combustion has been thoroughly investigated,experiments with magnesium additives in biomass fuels are scarce.This paper investigates the effect on emissions and deposit formation using softwood pellet with magnesium additive in a20kW boiler.It is known from combustion of diesel[8]and different biomass like oat[4,5]that metal?based additives can reduce deposit formation as well as particle emissions.Magnesium hydroxide is currently used to control slagging, fouling and tube cracking in coal fired boilers[9]. Steenari et al[10]point out the importance of magnesium in reactions with kaolin for anti?sintering effects.It is expected that also the addition of magnesium oxide will form mixtures with much higher melting temperatures,which will decrease the amount of deposits in the boiler convection surfaces[11,12].Khullar[12]reports experience based results by using magnesium additive,such as higher melting temperatures of ashes and improved combustion(less soot).

1　Experimental

Three different types of wood pellet were produced,one reference pellet without additives,one pellet with magnesium oxide(MgO),and one with magnesium hydroxide(Mg(OH)2).The pellet with additives and the reference pellet without the additives were produced from the same raw material.In addition a commercial pellet was tested.Firing experiments at stationary operation for about six hours were performed in a pellet boiler with nominal power of20kW.Measurements of emissions(CO,NO, TOC,and PM2.5)were performed and related to the fuel consumption and flue gas flow rate. Gravimetric measurements of ash deposition were conducted using sample rings in the flue gas heat exchanger tubes.Substrates in the heat exchanger

第41卷第5期

2013年5月燃　料　化　学　学　报

Journal of Fuel Chemistry and Technology Vol.41No.5 May2013

tubes were also analysed with scanning electron microscopes (SEM )and X?ray analysis to evaluate the different spices that glomerates on the heat exchanger surfaces.EDX?mapping was used to estimate the amount of the different spices.1.1　Pellet production and fuel analysis

Fresh sawdust of Norway spruce (Picea Abies )produced at a local sawmill with frame saws was used as raw material for the production of pellets.

The pellets were produced in a production unit located at the Department of Energy ,Environmental and Building Technology at Karlstad University ,Sweden (see Figure 1).It consists of :a diagonal mixer ,a conveyor screw ,an inlet feeder where conditioning takes place if needed ,an

Amandus Kahl C 33?390pelletizing press with a flat die and a maximum output of 300kg /h ,and a volumetric feeder for additives and a cooling tower.

The wet sawdust was first conditioned in the diagonal mixer by adding water up to the point when the appropriate moisture content was obtained.The additive was supplied to the inlet screw feeder using a volumetric feeder.

The flat die has nine holes radius with 52holes in each row ,totalling 468holes.The die has a working width of 75mm ,a hole diameter of 8mm ,an effective compression length of 30mm and a total thickness of 50mm.The open area of the die is 64%of the working

area.

Figure 1　Pellet production line at Karlstad University 1:diagonal mixer ;2:conveyor screw ;3:inlet feeder ;

4:pelletzing press with a flat die and a maximum output ;5:volumetric feeder

The pellet production plant was operated until stationary conditions were obtained.Before a sample was collected ,there was a break?in period of 5min with the current additive to insure stationary conditions.MgO and Mg (OH )2in powder form were

used as a pellet additive.The additive flow rate through the volumetric feeder was held constant at 0.8%based on weight produced pellets.The producing conditions can be seen from Table 1.

Table 1Producing conditions for the fuel producing lin

Additive /%

Pressure p /MPa Temperature t /℃

Humidity w b /%Mass flow rate /(kg 四min -1)

Reference 0.010.1110412.41.5

MgO 0.8a 9.3810612.41.5Mg (OH )2

0.8a

9.65

108

12.4

1.5

a

analysed amount of added magnesium is given in Table 2

compositions were analysed and the results are 1

35第5期Tomas Persson et al:Emissions and deposit properties from

the combustion tests were collected in a plastic bag and the moisture content was analysed according to EN 14774?1[13].The actual humidity ratio was used to recalculate the heating value of the fuels depending on the humidity level.Analysis of the remaining ash according to EN 14775?1[14]was used to determine the amount of unburned carbon which was left in the ash box after each experiment.1.2　Boiler setup

Figure 2shows the experimental setup.The combustion took place in a 20kW overfed horizontal pellet burner which was connected to a boiler with a horizontal two?pass?principle convective pass.An external?fuel?storage with a transport screw was used to feed the pellets into the combustion chamber.The fuel falls down through a shaft with a hose directly into the combustion chamber where the combustion is supported by a fan.The boiler was used for several months before these experiments took place.

The boiler was connected to a heat exchanger and a three way mixing valve to obtain a constant inlet temperature of about 75℃.The fuel feeding ratio was adjusted to a combustion power of about 12kW and the air flow rate was set as low as possible trying to keep the CO emission as low as possible.

Each combustion experiment (totally four different fuels )was run for two times 3h including a 25min break from the beginning of the stop sequence to the next start.The burner could be run for a maximum of 3h due to a limit in the control unit.During each period stable conditions at a constant heat output were

achieved.

Figure 2　Measurement set up

1.3　Energy balance and fuel consumption

Water flow rate through the boiler are measured by flow sensor (VFS 1)and temperature sensors (TS 1,TS 2)connected at the outlet and inlet of the boiler (Figure 2).Heat rate is calculated using temperature dependent density and heat capacity of water.In order to calculate the average boiler efficiency during the measurement period ,the boiler was conditioned to 20℃before and after each test.Air temperature in the room was measured by TS 3and TS 4in Figure 2.Temperature was measured at the inlet and the middle of the heat exchanger (TS 5,a scale WS 1(Figure 2)in order to measure the pellet consumption instantaneously.An external scale was also used to measure the fuel filled to double check the measurements.The store and the fuel feeder were completely emptied between the measurements of the different fuels.The volume flow rate of the flue gas was continuously measured using a multi?port averaging pitot tube (VFS 2)calibrated in combination with pressure transducers specifically to the installed chimney.Based on the measured flue gas flow rate ,flue gas temperature ,O 2concentration and the fuel properties ,the instant and total fuel 2

35　燃　料　化　学　学　报第41卷

1.4　Emission measurements

Continuous measurement of gaseous emissions ;oxygen (O 2),carbon dioxide (CO 2),carbon monoxide (CO ),nitrogen oxide (NO )and total organic carbon (TOC )are carried out by extracting a flue gas sample from the chimney through a heated filter and a heated tube (180℃).The analytical principles of the gas analysers are non?dispersive infrared (CO 2,CO ,NO ),paramagnetic gas analyser for O 2and flame ionization detection (TOC ,propane equivalent ).The measured concentrations are related to the measured flue gas flow rate (VFS 2in Figure 2)to calculate the amount (mass flow rate )of the gaseous emissions.

Continuous samples from particle emissions are taken from the dilution duct (non?isokinetic sample )and a second stage dilution with clean and dry air of 20℃is applied before measurements with an Electrical Low Pressure Impactor (ELPI ).Number concentration and size distribution of particulate matters are measured in the range of 7nm to 10μm and the flow rate of particle emissions (characterized for PM 2.5)are calculated based on the total dilution ratio and the volume flow rate in the dilution duct.1.5　Gravimetric measurements of deposited particles

To collect deposits in the flue gas heat exchanger of the boiler ,four stainless steel rings (material number :1.4432)made of X 2CrNiMo 17?12?3were used.Two of the rings were placed in the beginning of the flue gas heat exchanger tubes (position A in Figure 2)and two of them in the middle of the flue gas heat exchanger (position B in Figure 2).The rings were cleaned in an ultrasonic bath with isopropanol and then placed in an oven according to EN 14775[14]at 550℃to remove all combustible particles.The rings were weighted before and after the measurement to the nearest 0.01mg to measure the amount of deposits.As the rings were placed in a horizontal tube ,fly ash that was not fixated to the surface was removed by carefully blowing the substrates.The amount of deposits is given as a specific value by dividing the amount of deposits by the time of combustion and the area of the measured ring.Finally an average value of the weight based on all four measured samples was calculated and distributed by the measurement time and exposed surface to get the unit kg /(m 2四h ).All the sample surfaces were exposed for 6.37h except of the Mg (OH )2?case due to an unexpected boiler shut down that occurred during the measurement.Therefore ,the Mg (OH )2sample was exposed only for 5.42h.

1.6　SEM analysis determining the chemical composition of the deposits

Four substrates (15mm by 15mm )from the same material used in the gravimetric measurements were placed close to the rings for gravimetric measurements (Figure 3).The samples were analysed with a SEM?instrument Jeol 820with an ISIS EDS?system to determine the chemical composition of the deposits and to estimate the amount of the surface covered with each

substance.

Figure 3　Stainless steel rings for measuring of deposited particles in the flue gas heat exchanger (middle part )

(a ):stainless steel ring for gravimetric measurement of deposits and substrate for SEM /EDS?analysis ;

(b ):position of the stainless steel rings in the flue gas heat exchanger

2　Results and discussion

2.1　Fuel and ash analysis

The elementary analysis and the heating values of the tested pellets are given in Table 2.The main difference in composition between the four samples calorimetric value.The amount of silicon and nitrogen were highest for the commercial pellets and the amount of magnesium in the pellets with additives was about eight times higher than the main alkali metals potassium (K )and sodium (Na ).The 3

35第5期Tomas Persson et al:Emissions and deposit properties from

Table2　Elementary analysis and heating values of the used fuels analysed by an accredited laboratory

Commercial pellet Reference?pellet MgO additive Mg(OH)2additive Carbon(C)50.950.750.350.0%

Hydrogen(H)6.16.26.26.1%

Nitrogen(N)0.270.070.090.07%

Oxygen(O)42.242.742.542.8%

Sulphur(S)0.0070.0090.0100.008%

Chlorine(Cl)0.010.010.010.01%

Net cal.value(dry)19.2618.9418.9618.83MJ/kg

Moisture,delivered6.87.18.67.7% Moisture,test cond.a6.36.98.27.4%

Net cal.value,test cond.17.8917.4717.2217.26MJ/kg

Ash content0.40.41.11.0%

Silicon(Si)308178140136mg/kg

Aluminium(Al)55.530.328.526.4mg/kg

Calcium(Ca)7731020907910mg/kg

Iron(Fe)48.866.646.642.4mg/kg

Potassium(K)433422406450mg/kg

Magnesium(Mg)17011235803850mg/kg

Manganese(Mn)100119114116mg/kg

Sodium(Na)35.537.641.438.9mg/kg

Phosphorus(P)28.950.658.559.5mg/kg

Titanium(Ti)22.41.381.561.5mg/kg

Arsenic(As)0.146<0.1<0.1<0.09mg/kg

Barium(Ba)9.3519.31817.3mg/kg

Lead(Pb)0.2840.2320.2060.208mg/kg

Boron(B)1.551.771.821.8mg/kg

Cadmium(Cd)0.1480.08020.07250.0736mg/kg

Cobalt(Co)0.06060.04880.04570.0424mg/kg

Copper(Cu)1.321.080.9430.856mg/kg

Chromium(Cr)0.9540.3540.7040.253mg/kg

Mercury(Hg)<0.01<0.010.0142<0.01mg/kg

Molybdenum(Mo)0.04180.0396<0.05<0.05mg/kg

Nickel(Ni)0.1320.2140.2050.207mg/kg

Vanadium(V)0.03920.03620.0420.0364mg/kg

Zink(Zn)25.414.914.514.0mg/kg

a sample from test day analysed according to EN14774?1[13]

2.2　Emissions

During stationary combustion CO emissions were slightly lower for the pellets with the Mg(OH)2?additive compared to the reference pellets,but lowest

for the commercial pellets(Figure4).However,the differences are almost within the uncertainty of the measurements.The TOC emissions show the same tendency as the CO?emissions.The NO?emissions were the same for the three laboratory manufactured pellets and much higher for the commercial pellets. This is related to the higher nitrogen content of the commercial fuel(Table2).The particle emissions were about50%higher for the fuels with additives compared to the reference pellets.The particle emissions from the commercial pellets were about Figure5shows the average number?size

distribution of particulate matter as a function of

particle diameter during stationary periods for the four

different pellet samples.Most particles occurred

below1μm with a local peak at around0.1μm for

all four cases.The highest number of small particles

was found for the fuel with Mg(OH)2?additive,

followed by the commercial pellets and the reference

pellets.The number of particles larger than0.1μm

was smallest for the commercial pellets but quite

similar for the other pellet samples.To summarize it

can be seen that the additive of MgO reduce the

number of the smallest particles,and the Mg(OH)2?

additive increase the number of the smallest particles,

however,the total mass of particles increase using the 435　燃　料　化　学　学　报第41卷

Figure 4　Gaseous and particle emissions from combustion of the different fuels (stationary periods )

TOC?emissions were presented as methane equivalents (C 3H 8

)

Figure 5　

Number size distribution of particles ,normalized to 10%O 2

2.3　Gravimetric measurements

Figure 6shows the average collected deposits on the four sample rings.The amount of deposits was about 25%higher for the pellets with additives compared to the reference pellets and the commercial pellets.The fly ash production was much higher in the cases with additives compared to the reference case ,as the heat exchanger tubes were covered with a white layer of fly ash particles (Figure 7)

.

The increased amount of deposits on the surface is the reverse to the desired results and may reduce the efficiency of the boiler ;however ,the measurement time was too short to evaluate the influence on boiler efficiency.The measured boiler efficiency was slightly reduced in the additive cases compared to the reference case ;however ,it can as well be explained by a slightly higher air factor in the reference case and the measurement uncertainty.

The amount of magnesium in the tested pellets was almost eight times higher than the amount of the main alkali metals potassium (K )and sodium (Na )(Table 2).The magnesium content was also much higher than the dosage recommended by [12]which is roughly between 0.02%and 0.05%by weight.This leads to the conclusion that the additives were overdosed and therefore caused the problems reported ,however ,it is possible that for fuels with such low amount of alkali as soft wood pellets ,even with a proper amount of additive ,they may cause more problems than they solve.2.4　SEM analyses

Figures 8and 9show the SEM analyses with EDX mapping for the different fuels and the two different positions (A and B )in the boiler where the temperature in position A was around 460℃and the temperature in position B was around 260℃.The appearance of a specific substance will appear as white spots and a dark surface indicates no appearance of the substance.The elements iron ,chromium and nickel in steel are shown in the first three maps in the upper row of the EDX mappings.If there is a high intensity of these elements it means that the area has little ash deposit.The last two maps on the first row and all the maps in the lower row show the ash elements sodium (Na ),potassium (K ),chlorine (Cl ),sulphur (S ),silicon (Si )and in the additive 5

35第5期Tomas Persson et al:Emissions and deposit properties from

Figure 7　Flue gas heat exchanger tubes after the experiment with the reference pellets and the pellets with additives of MgO

(a ):reference pellets ;(b ):

MgO?pellets

6

35　燃　料　化　学　学　报第41卷

In general it can be seen from the figures that the deposited particles from the samples placed at the high temperature position A ”was bigger and more irregular than in the colder part of the flue gas heat exchanger B ”,where the deposits was smaller and more punctual shaped.It is likely that the bigger irregular shaped particles are coarse fly ash and the smaller particles are inorganic aerosols.

The samples showing the highest amount of particle deposits is the reference pellets ,especially in position A ”(Figure 8),where the intensity of the substrate ,Fe ,Cr and Ni was low.From the SEM analysis it seems that there are fewer deposits for the pellet with additives compared to the reference pellet.

This is especially visible in the low temperature position B (Figure 9).The deposits of sodium (Na ),potassium (K ),chlorine (Cl )and sulphur (S )decrease using the additives.The high amount of loose fly ash (that was removed before the analysis )may protect the substrates from further particles being accumulated on the substrate ;however ,this may not be the case for the gravimetric measurements ,where at least half of the rings also cover the upper part of the heat exchanger ,where less fly ash was accumulated.The gravimetric measurements in Figure 6indicate increased amount of deposits using the magnesium additives.Thus reliable conclusions from the SEM?analysis cannot be

drawn.

7

35第5期Tomas Persson et al:Emissions and deposit properties from

Figure10　Total amount of collected ash from the ash box, the amount of unburned carbon in the ash and

the amount of ash that was not collected

On all four substrates the main anticipating particles are potassium,chlorine and sulphates,which indicate that there are potassium chlorides and potassium sulphates.Small amounts of silicon indicate that there may also be potassium silicates.

2.5　Ash in the ash box

Figure10shows the total amount of collected ash and the amount of unburned carbon for each type of fuel.While the total amount of ash was increased by adding MgO and Mg(OH)2,the fraction of unburned carbon was more than halved compared to the reference cases.The ash from the combustion of the reference and the commercial pellets was black, whereas the ash from the additive pellets was white.

Using the assumption that all unburned particles are carbon the amount of energy remaining in the ash box is0.6%and0.7%for the commercial and the reference pellets respectively.For the ash from pellet with additive of MgO and Mg(OH)2,the amount of energy corresponds to0.3%and0.2%respectively. It seems that the additive improve the combustion and reduce the amount of unburned carbon in the ash, however,the energy saving potential is small as the remaining energy in the ash box was below1%in all cases.

Figure10also gives the estimated amount of ash that was not collected.This is based on the amount of pure ash collected and the calculated amount based on the used fuel and the elementary analysis of the fuels. The amount of ash that was not collected was highest for the commercial pellets,indicating that more ash was leaving the boiler to the chimney or the ambient.

A majority of these particles will be much larger than PM2.5and therefore not registered by the particle measurements.

3　Conclusions

The additives of MgO and Mg(OH)2increased the amount(mass)of particle emissions by about 50%and the mass of the deposits in the flue gas heat exchanger excluding loose fly ash increased by about 25%compared to the combustion of pellets without additives.The amount of loose fly ash caught in the flue gas heat exchanger was also much higher using the additives.The amount of additives was almost eight times higher than the amount of the main alkali metals potassium(K)and sodium(Na)which leads to the assumption that the additives were overdosed and therefore caused the problems reported.

The addition of Mg(OH)2increase the emissions of small particles below0.1μm and the addition of MgO reduce the number of small particles,however, this has not much influence on the total mass of PM

2.5particle emissions.Both types of additives caused

a small increase in particle numbers in the range from

0.1to2.5μm.

The SEM analysis of the substrates placed in the flue gas heat exchanger indicate that the deposits of sodium(Na),potassium(K),chlorine(Cl)and sulphur(S)decrease using the additives.If this was due to the expected chemical reactions or due to the loose fly ash covering the substrates after the test, could not be determined in this study.

The additives reduce the amount of carbon in the ash box,however,the energy saving potential is small as the remaining amount of energy in the ash box was below1%for all tested pellets. Acknowledgements

This work was performed within the project SWX?Energi and financed by the European Union,Region Dalarna,Region G?vleborg,and Dalarna University.

References

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[3]　KAUFMANN H,NUSSBAUMER T,BAXTER L,YANG N.Deposit formation on a single cylinder during combustion of herbaceous biomass 835　燃　料　化　学　学　报第41卷

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[8]　LYYR?NEN J ,JOKINIEMI J ,KAUPPINEN E.The effect of Mg?based additive on aerosol characteristics in medium?speed diesel engines

operating with residual fuel oils [J ].Aerosol Science ,2002,33(7):967?981.

[9]　HARE N ,RASUL M G ,MOAZZEM S.A review on boiler deposition /foulage prevention and removal techniques for power plant [C ].

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[10]　STEENARI B M ,LUNDBERG A ,PETTERSSON H ,WILEWSKA?BIEN M ,ANDERSSON D.Investigation of ash sintering during

combustion of agricultural residues and the effect of additives [J ].Energy Fuels ,2009,23(11):5655?5662.

[11]　SIMS R E H.Bioenergy options for a cleaner environment [M ].Elsevier ,2004.

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Energy Abstracts ,1997,38(2):169?178.

[13]　EN 14774?1.Solid biofuels?determination of moisture content?oven dry method?Part 1:Total moisture?reference method [S ].European

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[14]　EN 14775?1.Solid biofuels?determination of ash content [S ].European Committee for Standardization ,Brussels ,Belgium ,2009

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35第5期Tomas Persson et al:Emissions and deposit properties from

第三次上机内容

单元测试阶段Mock Object的用法 使用的开发工具： Eclipse、JUnit、eclemma和EasyMock 实际系统可能会有复杂的体系结构，例如某些类会使用底层基础类提供的功能或服务，这些被使用的类称为合作者。 当对这种复杂类进行单元测试时，实例化这些合作者通常不合理，例如：合作者类还未实现，无法提供相关功能。但目前又需要进行单元测试，一种解决方案是采用模仿对象（Mock Object）技术。 例如实现了Purchase类，该类需要调用DBAccess类的getPriceFromDB（）方法。目前需要对Purchase类进行测试，但DBAccess类可能还尚未实现，可以采用Mock Object技术（即MockDBAccess类）。 如图1所示实例，假设要测试Purchase类，但Purchase类调用的DBAccess类并未实现，可以派生出一个MockDBAccess类。 图1：类图 相关类的代码如下： Purchase类： public class Purchase { public double getTotalPrice(DBAccess dbAccess){ return dbAccess.getPriceFromDB(); } } DBAccess类： public class DBAccess { public double getPriceFromDB(){ //假设还未实现完整 double price = 180.0; return price; }

} 实际的测试代码： PurchaseWithRealDBAccessTest类 import static org.junit.Assert.*; import org.junit.Test; public class PurchaseWithRealDBAccessTest { @Test public void test() { Purchase purchase = new Purchase(); DBAccess dbAccess = new DBAccess(); assertEquals(180.0, purchase.getTotalPrice(dbAccess),00001); } } 采用Mock Object方法 MockDBAccess类： public class MockDBAccess extends DBAccess { public double getPriceFromDB(){ double price = 180.0; return price; } } 修改相应的测试代码： PurchaseWithMockDBAccess类 import static org.junit.Assert.*; import org.junit.Test; public class PurchaseWithMockDBAccess { @Test public void test() { Purchase purchase = new Purchase(); MockDBAccess mockDBAccess = new MockDBAccess(); assertEquals(180.0, purchase.getTotalPrice(mockDBAccess),0.00001); } }

as such 和 therefore 的用法区别

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C++ #pragma code_seg用法

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操作系统文件管理_答案

第六部分文件管理 1、文件系统的主要目的就是( )。 A、实现对文件的按名存取 B、实现虚拟存储 C、提供外存的读写速度 D、用于存储系统文件 2、文件系统就是指( )。 A、文件的集合 B、文件的目录集合 C、实现文件管理的一组软件 D、文件、管理文件的软件及数据结构的总体 3、文件管理实际上就是管理( )。 A、主存空间 B、辅助存储空间 C、逻辑地址空间 D、物理地址空间 4、下列文件的物理结构中,不利于文件长度动态增长的文件物理结构就是( )。 A、顺序文件 B、链接文件 C、索引文件 D、系统文件 5、下列描述不就是文件系统功能的就是( )。 A、建立文件目录 B、提供一组文件操作 C、实现对磁盘的驱动调度 D、实现从逻辑文件到物理文件间的转换 6、文件系统在创建一个文件时,为它建立一个( )。 A、文件目录 B、目录文件 C、逻辑结构 D、逻辑空间 7、索引式(随机)文件组织的一个主要优点就是( )。 A、不需要链接指针 B、能实现物理块的动态分配 C、回收实现比较简单 D、用户存取方便 8、面向用户的文件组织机构属于( )。 A、虚拟结构 B、实际结构 C、逻辑结构 D、物理结构 9、按文件用途来分,编译程序就是( )。 A、用户文件 B、档案文件 C、系统文件 D、库文件 10、将信息加工形成具有保留价值的文件就是( )。 A、库文件 B、档案文件 C、系统文件 D、临时文件 11、文件目录的主要作用就是( )。 A、按名存取 B、提高速度 C、节省空间 D、提高外存利用率 12、如果文件系统中有两个文件重名,不应采用( )。 A、一级目录结构 B、树型目录结构 C、二级目录结构 D、A与C 13、文件系统采用树型目录结构后,对于不同用户的文件,其文件名( )。 A、应该相同 B、应该不同 C、可以不同,也可以相同 D、受系统约束 14、文件系统采用二级文件目录可以( )。 A、缩短访问存储器的时间 B、实现文件共享 C、节省内存空间 D、解决不同用户间的文件命名冲突

much的比较级和用法例句_用法辨析 英语语法.doc

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Dinosaurs. 在纪录片《与恐龙同行》中可以看到最新的、更加生动的电脑特效技术。 6. More people would be attracted to cycling if conditions were right. 如果天气适宜，会有更多的人被吸引到骑自行车的行列中来。 7. I shivered and pulled my scarf more tightly round my neck. 我打了个寒战，用围巾把脖子围得更紧了。 8. Their replies were no more than grunts of acknowledgement. 他们所谓的回答不过是表示承认的咕哝罢了。 9. Then something seemed to snap in me. I couldn’t endure any more. 这时候，我的心里像有个东西突然绷断了我再也忍受不了啦。 10. If a baby is thirsty, it feeds more often. 婴儿要是渴了，就会吃得更频繁。 11. It would be difficult to find two men who were more dissimilar. 很难找到彼此间差异更大的人了。 12. Jovial ladies chivvy you into ordering more than you can eat! 热情的女招待会一再推荐，最后点的餐多到吃不下! 13. One more question and I’ll leave you in peace. 再问一个问题，我就不打扰你了。 14. This is more like a downpayment, or a deposit, if you like. 这更像是首付，也可以说是定金。 15. It was time now to show more political realism.

智能化系统建设方案

精品文档 一.背景描述： 江南海岸总体规划和设计均体现了传统中国居家理想和现代生 活方式的有机融合，是依照21世纪人居标准精心打造的高级住宅小区。 整个小区无不营造一个舒适休闲的生活空间，是一所环境优雅，闹中 取静的花园式住宅小区，满足住户对高品质生活的追求。 二.工程说明： 江南海岸位于三明市列东区，由14栋高层住宅小区组成，总建 筑面积29.7627万平方米，其中包括4栋27层，6栋25层，4栋29 层，会所1间，负一层，一层。住户总数为1182户。 项目要求: 江南海岸，是集住宅、花园、会所于一体的高级住宅小区。小区智能化系统的工程建设具有投资大、工程复杂、专业性强等特点。小区要求建设成具有国内先进水平的，既具有自身特点，又具有时代潮流特色的高尚住宅楼宇。 整个工程规划、设计、实施上要求充分体现技术的先进性、系统的复杂性、严密的安防集控管理。注重整体功能强大，中心设备完善，系统配置科学合 理，真正体现高技术、高标准、高水平的现代化智能小区。 四.需求分析: 4.1分析与评估：

本方案以江南海岸小区住宅智能化管理及安全防范为设计目标为将力求建设成为高水平、高质量、高标准的信息化智能小区。我方提出以下见解，请发包方领导参考。 ①小区建设要求基于系统可靠、稳定、先进的基础上，既能满足用户住宅 的实际需求，同时又力求经济、实用、合理。 ②整个系统的结构要求清晰合理，小区实现全封闭管理，各个子系统既 相互关联又相对独立，形成一个全方位智能安防管理系统。 ③要求考虑未来系统扩展的需求，为小区以后系统功能的增加、升级，提 供良好的环境空间。 因此，考虑江南海岸属于大型的综合住宅小区，建筑规模庞大、结构复杂，小区各项功能模块齐备，因此在智能化建设方面，产品的集成度、统一化、高效管理方面尤为重要，同时，还必须考虑小区规模的不断扩大，智能化产品必须具备高度的扩展及冗余，顺应小区的发展。 我方进行多项分析与评估，结合小区建筑结构的分布特点、规模发展，以及对小区各功能模块的深层了解，建议江南海岸智能化系统工

linux下各目录作用和功能

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重点固定短语搭配详细用法(12月24日)

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C++ #pragma预处理命令

#pragma预处理命令 #pragma可以说是C++中最复杂的预处理指令了，下面是最常用的几个#pragma 指令： #pragma comment(lib,"XXX.lib") 表示链接XXX.lib这个库，和在工程设置里写上XXX.lib的效果一样。 #pragma comment(linker,"/ENTRY:main_function") 表示指定链接器选项/ENTRY:main_function #pragma once 表示这个文件只被包含一次 #pragma warning(disable:4705) 表示屏蔽警告4705 C和C++程序的每次执行都支持其所在的主机或操作系统所具有的一些独特的特点。例如，有些程序需要精确控制数据存放的内存区域或控制某个函数接收的参数。#pragma为编译器提供了一种在不同机器和操作系统上编译以保持C和C++完全兼容的方法。#pragma是由机器和相关的操作系统定义的，通常对每个编译器来说是不同的。 如果编译器遇到不认识的pragma指令，将给出警告信息，然后继续编译。Microsoft C and C++ 的编译器可识别以下指令：alloc_text，auto_inline，bss_seg，check_stack，code_seg，comment，component，conform，const_seg，data_seg，deprecated，fenv_access，float_control，fp_contract，function，hdrstop，include_alias，init_seg，inline_depth，inline_recursion，intrinsic，make_public，managed，message，omp，once，optimize，pack，pointers_to_members，pop_macro，push_macro，region, endregion，runtime_checks，section，setlocale，strict_gs_check，unmanaged，vtordisp，warning。其中conform，init_seg, pointers_to_members，vtordisp仅被C++编译器支持。 以下是常用的pragma指令的详细解释。 1.#pragma once。保证所在文件只会被包含一次，它是基于磁盘文件的，而#ifndef 则是基于宏的。

告诉你C盘里面每个文件夹是什么作用

Documents and Settings是什么文件？ 答案： 是系统用户设置文件夹，包括各个用户的文档、收藏夹、上网浏览信息、配置文件等。补：这里面的东西不要随便删除，这保存着所有用户的文档和账户设置，如果删除就会重新启动不能登陆的情况，尤其是里面的default user、all users、administrator和以你当前登陆用户名的文件夹。 Favorites是什么文件？ 答案： 是收藏夹，存放你喜欢的网址。可以在其中放网址快捷方式和文件夹快捷方式，可以新建类别（文件夹）。 Program Files是什么文件？ 答案： 应用软件文件夹装软件的默认路径一般是这里！当然里面也有些系统自身的一些应用程序Common Files是什么文件？ 答案： Common Files. 这个文件夹中包含了应用程序用来共享的文件,很重要，不能乱删除Co mmon Files这个文件是操作系统包扩系统程序和应用程序Common Files是应用程序运行库文件数据库覆盖了大约1000多个最流行的应用程序的插件,补丁等等文件夹com mon files里很多都是系统文件，不能随意删除，除非确定知道是干什么用的，没用的可以删掉。不过就算删掉了有用的东西，也没大的关系，顶多是某些软件用不了，不会造成系统崩溃。 ComPlus Applications是什么文件？ 答案： ComPlus Applications：微软COM+ 组件使用的文件夹，删除后可能引起COM+ 组件不能运行 DIFX是什么文件？ 答案： 不可以删除，已有的XML数据索引方法从实现思想上可分为两类:结构归纳法和节点定位法.这两种方法都存在一定的问题,结构归纳法的缺点是索引规模较大而且难以有效支持较复杂的查询,而节点定位法的主要缺点是容易形成过多的连接操作.针对这些问题,提出了一种新的动态的XML索引体系DifX,它扩展了已有的动态索引方法,采用一种动态的Bisimil arity的概念,可以根据实际查询需求以及最优化的要求动态决定索引中保存的结构信息,以实现对各种形式的查询最有效的支持.实验结果证明DifX是一种有效而且高效的XML索引方法,其可以获得比已有的XML索引方法更高的查询执行效率. Internet Explorer是什么文件？ 答案： 不用说了，肯定不能删除，IE，浏览网页的！ Kaspersky Lab是什么文件？ 答案：卡巴斯基的文件包，这个是卡巴的报告，在C:\Documents and Settings\All Users\Application Data\Kaspersky Lab\AVP6\Report 的更新文件中有很多repor t文件很占地

情态动词的用法小结

情态动词的用法小结 I、can的主要用法 1.表示能力(indicating ability) I can run very fast. 2.表示获得的知识或技能（indicating acquired knowledge or skill） They can speak French. 3.与感官动词连用(used with verbs of perception) I can hear music. 4.表示许可(indicating permission) Can I read your newspapers? 5.表示要求（indicating requests） Can you feed the cat? 6.表示可能性（indicating possibility） He can’t have slept through all that noise. 7.用以表示疑惑或怀疑（used to express bewilderment or incredulity） Can she be serious? 8.用以描述特有的行为或情形（used to describe typical behavior or state） It can be quite windy on the hills 山上的风就是那么大。 9.用以提出建议（used to make suggestions） We can eat in a restaurant, if you like. II. may的主要用法 1.表示允许、许可（indicating permission） You may come, if you wish. 2.表示可能（indicating possibility） This coat may be Peter’s. 3.表示目的（indicating purpose） I will write today so that he may know when to meet us. 4. 用以表示愿望和希望（used to express wishes and hopes） May you both be very happy. can, could, may, might的用法比较 在表示“许可”这层意思时，英国人一般用can来表示准许或请求许可。could一词更客气些，但是指用于疑问句中。May是比较郑重的词，但是美国人常用may，英国人常用can。不过英美都使用could和might表示怀疑、羞怯等。 在表示可能性的时候，could、might比may更含怀疑或犹豫之意。比较这两个句子：

智能化系统配置项目及要求

智能化系统初步配置方案 一、设计原则 1、智能化系统设计，应综合考虑项目投资额度的可控性、设备选型的灵活性、工程施工的可行性、系统功能的可扩展性、系统运行维护的便利性和物业管理 的规范性等要求。 2、智能化系统的设计应参照本要求，其配置标准不得低于”初步配置方案” 的要求，同时应有一定的升级和扩展能力，并预留相应的接口。 3、智能化系统设计除了满足国家标准与规范的相关规定，以及本标准基本配 置要求外，还应满足建筑、结构以及与智能化系统存在设计相关联的其他专业 的设计规范和要求，特别需要注意符合项目当地现行有关标准和规范的特殊要求。 4、智能化系统设计与工程项目建设地点的实际情况相适应。 5、智能化系统所采用的设备及线路材料等均应符合国家现行的规定，并具有 产品合格证、质量检验证书和产品须通过国家的CCC认证。 二、智能化系统基本配置要求 1、安全防范系统 a、安全防范系统包括闭路电视监控、防盗报警系统、门禁系统、电子巡更系统及无线对讲系统。 b、闭路电视监控系统主要设置重要出入口，如公共场所、重要房间、楼梯通道、楼层电梯通道、电梯轿厢、室外主干道及交叉路口等处，电梯轿厢安装电 梯专用监控镜头并加装抗干扰器，户外监控镜头须带有红外夜视功能。所有的 监控镜头全部采用网络彩色摄像机，采用矩阵主机切换至电视墙，录像以全天 实时高清图像质量保存至少30天（具体保存时间可按照当地派出所要求确定）。 c、防盗报警系统主要设置在财务室、档案室等重要房间，并设置手动报警开关或脚挑报警开关，要求闭路电视监控系统同时显示出报警地点画面。 d、重要设备机房、财务室、档案室等重要房间设置门禁系统。 e、电子巡更系统要求采用离线式，设置于重要机房、楼层楼梯口、电梯厅。 f、无线对讲系统要求具有可进行信道改写和信道加密功能，满足内部通讯需要。 g、各系统设备品牌须选用技术成熟，性能稳定可靠的产品。 2、通讯网络系统

#pragma data code ICCAVR的使用

#pragma data:code 在Keil中为了节省数据存储器的空间，通过“code”关键字来定义一个数组或字符串将被存储在程序存储器中： uchar code buffer[]={0,1,2,3,4,5}; uchar code string[]="Armoric" ; 而这类代码移值到ICCAVR上时是不能编译通过的。我们可以通过"const" 限定词来实现对存储器的分配： #pragma data:code const unsigned char buffer[]={0,1,2,3,4,5}; const unsigned char string[]="Armoric"; #pragma data:data 注意： 《1》使用ICCAVR6.31时，#pragma data :code ;#pragma data:data ; 这些语法时在"data:cod"、"data:data"字符串中间不能加空格，否则编译不能通过。 《2》const 在ICCAVR是一个扩展关键词，它与ANSIC标准有冲突，移值到其它的编译器使用时也需要修改相关的地方。 在ICCAVR中对数组和字符串的五种不同空间分配： const unsigned char buffer[]={0,1,2,3,4,5}; //buffer数组被分配在程序存储区中 const unsigned char string[]="Armoric" ; //stringp字符串被分配在程序存储区中 const unsigned char *pt //指针变量pt被分配在数据存储区中，指向程序存储区中的字符类型数据 unsigned char *const pt //指针变量pt被分配在程序存储区中，指向数据存储区中的字符类型数据 const unsigned char *const pt //指针变量pt被分配在程序存储区，指向程序存储区中的字符类型数据 unsigned char *pt //指针变量pt被分配在数据存储区中，指向数据存储区中的数据 请问#pragma data:code和#pragma data:data是什么意思？ 前者表示：随后的数据将存贮在程序区，即FLASH区，此区只能存贮常量，比如表格之类。

文件夹的作用

All Users文件夹： 『Win9x/ME』所有用户文件夹，里面里面包括系统缺省登录时的桌面文件和开始菜单的内容。『Win2000』在Win2000的系统目录中没有这个文件夹，Win2000将用户的信息放在根目录下的Documents and Settings文件夹中，每个用户对应一个目录，包括开始菜单、桌面、收藏夹、我的文档等等。 Application Data文件夹： 『Win9x/ME』应用程序数据任务栏中的快捷方式，输入法的一些文件等等。根据你系统中使用不同的软件，该目录中的内容也有所不同。 『Win2000』在Documents and Settings文件夹中，每个用户都对应一个Application Data 文件夹，同样每个用户由于使用的软件不同，目录内容也相同。 Applog文件夹： 『Win9x/ME』应用程序逻辑文件目录。逻辑文件是用来记录应用软件在运行时，需要调用的文件、使用的地址等信息的文件。要查看这些文件，用记事本打开即可。 Catroot文件夹： 『Win9x』计算机启动测试信息目录，目录中包括的文件大多是关于计算机启动时检测的硬软件信息。 『WinME』该文件夹位于系统目录的system目录中。 『Win2000』该文件夹位于系统目录的system32目录中。 Command文件夹： 『Win9x/ME』DOS命令目录。包括很多DOS下的外部命令，虽说都是些小工具，但真的很好用，特别是对于系统崩溃时。 『Win2000』这些DOS命令位于系统目录的system32目录中。 Config文件夹： 『Win9x/ME/2000』配置文件夹，目录中包括一些MIDI乐器的定义文件。 Cookies文件夹： 『Win9x/ME』Cookies又叫小甜饼，是你在浏览某些网站时，留在你硬盘上的一些资料，包括用户名、用户资料、网址等等。 『Win2000』每个用户都有一个Cookies文件夹，位于Documents and Settings文件夹的每个用户目录中。 Cursors文件夹： 『Win9x/ME/2000』鼠标动画文件夹。目录中包括鼠标在不同状态下的动画文件。 Desktop文件夹： 『Win9x/ME』桌面文件夹。包括桌面上的一些图标。 『Win2000』这个文件夹在系统目录中也存在，同时在Documents and Settings文件夹的每个用户目录中还有“桌面”文件夹。

实验9自定义异常的使用

1）在定义一个银行类Bank 时, 有deposit、withdraw、showBalance 等方 法。若取钱数大于余额则作为异常处理(InsufficientFundsException)。产生异常的条件是余额少于取额，因此是否抛出异常要判断条件，要定义好自己的异常类。package ex91; public class Bank { public double total=0; Bank(double d) { total=d; } public void deposit(double d) { total+=d; } public void withdraw(double d) { total-=d; } public void showBalance() { System.out.println(total); } } package ex91; public class BankException extends Exception { String message; BankException(double d) { message=d+"非法取款余额"; } public String getMessage() { return message; } } package ex91; import java.util.Scanner; public class BankTest { /** * @param args */ public static void main(String[] args) throws BankException { // TODO Auto-generated method stub Bank bk=new Bank(1236.123); bk.showBalance(); Scanner sc = new Scanner(System.in); int i=sc.nextInt(); bk.withdraw(i); if(bk.total<0) { BankException be=new BankException(i); throw(be); } else bk.showBalance();

pragma的用法

#pragma的用法 在所有的预处理指令中，#Pragma 指令可能是最复杂的了，它的作用是设定编译器的状态或者是指示编译器完成一些特定的动作。#pragma指令对每个编译器给出了一个方法，在保持与C和C++语言完全兼容的情况下，给出主机或操作系统专有的特征。依据定义， 编译指示是机器或操作系统专有的，且对于每个编译器都是不同的。 其格式一般为: #pragma para。其中para为参数，下面来看一些常用的参数。 1)message 参数 message参数是我最喜欢的一个参数，它能够在编译信息输出窗口中输出相应的信息，这对于源代码信息的控制是非常重要的。其使用方法为： #pragma message("消息文本") 当编译器遇到这条指令时就在编译输出窗口中将消息文本打印出来。 当我们在程序中定义了许多宏来控制源代码版本的时候，我们自己有可能都会忘记有 没有正确的设置这些宏， 此时我们可以用这条指令在编译的时候就进行检查。假设我们希望判断自己有没有在源代码的什么地方定义了_X86这个宏, 可以用下面的方法: #ifdef _X86 #pragma message("_X86 macro activated!") #endif 我们定义了_X86这个宏以后，应用程序在编译时就会在编译输出窗口里显示"_86 macro activated!"。 我们就不会因为不记得自己定义的一些特定的宏而抓耳挠腮了。 (2)另一个使用得比较多的pragma参数是code_seg 格式如： #pragma code_seg( ["section-name" [, "section-class"] ] ) 它能够设置程序中函数代码存放的代码段，当我们开发驱动程序的时候就会使用到 它。 (3)#pragma once (比较常用) 只要在头文件的最开始加入这条指令就能够保证头文件被编译一次，这条指令实际上 在VC6中就已经有了， 但是考虑到兼容性并没有太多的使用它。 (4)#pragma hdrstop 表示预编译头文件到此为止，后面的头文件不进行预编译。BCB可以预编译头文件以 加快链接的速度， 但如果所有头文件都进行预编译又可能占太多磁盘空间，所以使用这个选项排除一些头文

linux下各文件夹的结构说明及用途详细介绍解析

linux下各文件夹的结构说明及用途介绍: /bin:二进制可执行命令。 /dev:设备特殊文件。 /etc:系统管理和配置文件。 /etc/rc.d:启动的配置文件和脚本。 /home:用户主目录的基点,比如用户user的主目录就是/home/user,可以用~user 表示。 /lib:标准程序设计库,又叫动态链接共享库,作用类似windows里的.dll文件。 /sbin:系统管理命令,这里存放的是系统管理员使用的管理程序。 /tmp:公用的临时文件存储点。 /root:系统管理员的主目录。 /mnt:系统提供这个目录是让用户临时挂载其他的文件系统。 /lost+found:这个目录平时是空的,系统非正常关机而留下“无家可归”的文件就在这里。 /proc:虚拟的目录,是系统内存的映射。可直接访问这个目录来获取系统信息。 /var:某些大文件的溢出区,比方说各种服务的日志文件。 /usr:最庞大的目录,要用到的应用程序和文件几乎都在这个目录。其中包含: /usr/x11r6:存放x window的目录。 /usr/bin:众多的应用程序。

/usr/sbin:超级用户的一些管理程序。 /usr/doc:linux文档。 /usr/include:linux下开发和编译应用程序所需要的头文件。 /usr/lib:常用的动态链接库和软件包的配置文件。 /usr/man:帮助文档。 /usr/src:源代码,linux内核的源代码就放在/usr/src/linux 里。 /usr/local/bin:本地增加的命令。 /usr/local/lib:本地增加的库根文件系统。 通常情况下,根文件系统所占空间一般应该比较小,因为其中的绝大部分文件都不需要经常改动,而且包括严格的文件和一个小的不经常改变的文件系统不容易损坏。除了可能的一个叫/vmlinuz标准的系统引导映像之外,根目录一般不含任何文件。所有其他文件在根文件系统的子目录中。 1. /bin目录 /bin目录包含了引导启动所需的命令或普通用户可能用的命令(可能在引导启动后。这些命令都是二进制文件的可执行程序(bin是binary的简称,多是系统中重要的系统文件。 2. /sbin目录 /sbin目录类似/bin,也用于存储二进制文件。因为其中的大部分文件多是系统管理员使用的基本的系统程序,所以虽然普通用户必要且允许时可以使用,但一般不给普通用户使用。 3. /etc目录