Questions. Downdraft biomass gasifier. Air. Air. Blower. Air. Syngas line Filter VFD. Gas analyzer(s) (vent)

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1 Question 1 Questions Biomass gasification is a process where organic matter liberates flammable gases such as hydrogen (H 2 ) and carbon monoxide (CO) when heated to high temperatures. A gasifier is a machine designed to heat the organic matter to the necessary temperature (usually by partial combustion) in order for this to occur: Biomass inlet (wood chips, straw, etc.) Downdraft biomass gasifier Control system H L Blower (To engine) Syngas line Filter Ash collection VFD Sample pump (vent) Gas analyzer(s) Besides hydrogen and carbon monoxide, there are other gases mixed in the syngas outlet from the gasifier, which we desire to measure in order to properly control the chemical reactions happening inside the gasifier. These include nitrogen oxide (NO) and carbon dioxide (CO 2 ). Upon investigation, you discover two different gas analyzer technologies capable of accurately measuring all these gases: one is an NDIR (Nondispersive infra-red) analyzer with multiple detectors, and the other is a chromatograph. For this application, you happen to find analyzers of both types at comparable cost, which eliminates cost as a deciding factor when choosing the analyzer type. NDIR analyzers typically have measurement dead times in the range of seconds, whereas chromatographs typically exhibit measurement dead times in the order of minutes. Based on this criterion, which is the preferred analyzer technology for closed-loop control, and most importantly why? Explain why the differential pressure transmitter has its low-pressure port connected to the gasifier (with the high-pressure port vented), rather than the other way around. Explain why a VFD is a good choice for a final control element on the syngas line, as opposed to a constant-speed blower and a control valve. Identify the most significant safety hazard(s) inherent to this process, as well as any PPE (Personal Protective Equipment) operators and technicians could use while working near it. file i

2 Question 2 When an internal combustion engine starts up cold, water may be seen coming out of the exhaust pipe, at least until the exhaust pipes become hot enough to vaporize the water so you can t see it anymore. Explain why water is a byproduct of any hydrocarbon fuel (e.g. gasoline or diesel) combustion, based on principles of chemistry. When coal is partially combusted (i.e. burned in a low-oxygen environment), one of the byproduct gases is the flammable and toxic gas carbon monoxide (CO). This is also one of the major products of biomass gasification, where carbon-containing biomass fuels are heated in low-oxygen environments. Explain why an engine running on carbon monoxide gas as a fuel will not produce water as a byproduct of combustion. Is it possible to eliminate water from the exhaust gases of a car s engine operating on gasoline? Identify how we may identify fuels producing water vapor when burned, strictly by examining their chemical formulae. Identify how we may identify fuels which inherently produce no water vapor when burned, strictly by examining their chemical formulae. file i

3 Question 3 A PLC is used to sequence the operation of a lock hopper used to feed pulverized coal into a gasification reactor where the coal is heated to release flammable gases useful as fuels. Many details on the reactor have been omitted from this diagram for simplicity: Coal feed Lock valve #1 Steam LSH S PT S Lock-hopper Lock valve #2 Gas PT Gasifier reactor The purpose of the lock hopper is to prepare a charge of coal for injection into the reactor while maintaining gas pressure in the reactor (i.e. not letting the flammable coal gases escape) and also not letting any air into the reactor where it could support an explosion. The basic sequence of the lock hopper is as follows: (1) wait for command signal from production control system to inject more coal into gasifier reactor; (2) wait for proof of valve #2 closure, then open lock valve #1 to introduce coal into lock hopper vessel; (3) wait for high level switch (LSH) to trip, then close lock valve #1; (4) wait for proof of valve #1 closure, then open steam inlet and outlet valves to purge lock-hopper of air for a set time period; (5) close steam outlet valve to allow lock-hopper to pressurize until its pressure is equal to the pressure measured inside the gasifier reactor; (6) open lock valve #2 for a set time period; (7) close steam inlet valve and close lock valve #2. Suppose one day the proof-of-closure switch on lock valve #2 fails in such a way that it reports the valve as being open all the time even when it is closed tight. Explain how this switch failure will affect the operation of the lock hopper as controlled by the PLC. Furthermore, suppose an instrument technician is called to investigate this problem, and decides to fix it by forcing the input bit of the PLC so that it sees the lock valve #2 as being closed despite the broken switch s status. This allows the process to continue running while the technician then repairs the switch. Explain why this might not be a good way to proceed, and be specific in your answer! file i

4 Question 4 An NDIR gas analyzer is going to be used to measure the concentration of carbon monoxide (CO) in synthesis gas produced by a biomass gasification process. This process converts dry organic matter into a flammable gas stream which may be used to run an internal combustion engine: Biomass inlet (wood chips, straw, etc.) Downdraft biomass gasifier H L Syngas line Blower (To engine) Ash collection Filter Sample pump (vent) NDIR analyzer Other gases known to be in this stream in large quantity include nitrogen (N 2 ) and hydrogen (H 2 ). The infrared absorption characteristics of carbon monoxide are shown in the following plot. Neither nitrogen nor hydrogen gas absorbs infrared light to any appreciable degree: Carbon Monoxide (CO) Wavenumber (cm -1 ) Identify which gases the reference cell and detector chambers of the NDIR analyzer should be filled with, and whether or not this analyzer will require filter cells. If filter cells are required, identify the gas(es) they should be filled with as well. Explain why the differential pressure transmitter has its low-pressure port connected to the gasifier (with the high-pressure port vented), rather than the other way around. A gasifier s operation should be such that the production of flammable gases such as H 2 and CO are maximized. Explain how too much air admitted into the gasifier would cause a decrease in production of these fuel gases. file i

5 Question 5 This simplified PFD shows the fluid catalytic cracking or FCC process, used extensively in American oil refineries. FCC processes employ finely-powdered catalyst to accelerate chemical reactions where heavy liquid hydrocarbon molecules are split ( cracked ) into lighter molecules, producing petroleum liquids with greater market value. This is not unlike the chemical process of biomass gasification, where solid fuel materials are broken down by intense heat into simpler, flammable gases with more flexible application as fuels: Exhaust (CO 2 + N 2 gases) Steam generator CO boiler CO + CO 2 + N 2 gases Regenerator Reactor Cyclone separators Overhead Light gas oil Spent catalyst Stripping steam Fractionator Heavy gas oil Regenerated catalyst Riser Blower Clarified slurry N 2 + O 2 gases Dispersion steam Heavy feed hydrocarbons Settler Recycle slurry The cracking reactions begin in the riser and continue in the reactor, with the catalyst powder carried along by the steam and hydrocarbon fluids. These reactions leave much of the catalyst powder covered with coke (solid carbon deposits) which limits its effectiveness as a catalyst. This spent catalyst falls by gravity into the regenerator, where it encounters a blast of air entering the bottom of the vessel, converting the carbon deposits into CO and CO 2 gases and fluidizing the catalyst powder once again so it flows freely back to the riser. The hot gases leaving the regenerator pass through a heat exchanger to boil water into useful steam, then pass to a burner where more air is introduced to convert the CO gas into CO 2 gas and generate more steam with the heat. Vapors leaving the top of the reactor vessel are distilled into their constituent compounds in the fractionator vessel, with the heaviest of them recycled back to the reactor for re-processing. Identify whether the major chemical reactions are exothermic or endothermic in each of the following process vessels, explaining your rationale for each case: Reactor Regenerator CO boiler file i

6 Question 6 Read the report Improving Thermocouple Service Life in Slagging Gasifiers written by James Bennett of the US Department of Energy (DOE) Albany Research Center (document DOE/ARC ), and answer the following questions: What exactly does a gasifier do? What feedstocks does it input, and what products does it output? How do these facts make gasification a technology of interest for future energy production? Identify the typical pressures and temperatures encountered in the interior of a slagging gasifier. Express these pressure and temperature ranges in bar and o F, respectively. What type of thermocouple has traditionally been applied to this service? Explain why this choice is more appropriate than some other thermocouple types. How long have thermocouples typically lasted in this type of severe service, and how much does each one cost to replace? One of the alternative temperature-measurement technologies cited in this report is the use of sapphire process probes with fiber-optic cables to conduct light to a remote location where it may be sensed. Explain how this novel temperature-measurement technology works, based on the description alone (and/or your own independent research on the topic). An interesting style of thermocouple experimented with for gasifier applications is the so-called open thermocouple. Explain how this sensing device is constructed, and how it differs from a regular thermocouple. Identify some of the specific modes of thermocouple failure (i.e. how exactly is it that they are failing?). Explain how bridging inside a gasifier may lead to incorrect thermocouple EMFs (millivolt signals). Examine the graph of thermocouple readings over a 3-hour test period as shown in figure 10. Which slag thermocouple performed better as the sample warmed up? Which slag thermocouple performed better after the sample had reached 880 o C? Explain how syngas analysis may be alternatively used to infer temperatures inside the gasifier, and why this technique is not suitable for all phases of gasifier operation. Explain chemical interactions between thermocouple wires and compounds inside the gasifier can lead to measurement errors, even before complete failure of the thermocouple. Explain what it means to sequester carbon dioxide gas from a process, and why this is important from the perspective of environmental impact. Under what operating conditions may a gasifier be carbon-neutral (i.e. no net release of carbon dioxide gas to the atmosphere)? Under what operating conditions may a gasifier be carbon-negative (i.e. it removes more carbon dioxide gas from the atmosphere than it releases)? file i

7 Answer 1 Answers The NDIR analyzer is clearly the better choice from the perspective of dead time. More dead time in a feedback loop results in slower (possible) automatic control response, because a controller response appropriate for a minimal-deadtime loop will oscillate given the presence of additional dead time. For a more complete explanation of this, refer to Lessons In Industrial Instrumentation in the section describing dead time as a process characteristic, and how its presence affects feedback control. Answer 2 Hint: gasoline fuel molecules contain lots of hydrogen atoms! Answer 3 This is a graded question no answers or hints given! Answer 4 Answer 5 This is a graded question no answers or hints given! Answer 6 7