WHY WOULD A NATURAL GAS ENGINE NEED A PARTICLE FILTER? Gordon McTaggart-Cowan

Transcription

1 WHY WOULD A NATURAL GAS ENGINE NEED A PARTICLE FILTER? Gordon McTaggart-Cowan

2 Outline» NG vehicles context (3 min)» Why NG? (5 min)» NG engine technologies (7 min)» Particulate matter emissions HPDI focus (12 min)» Summary / Conclusions (3 min) Create-AAP Symposium 2

3 Global growth in natural gas vehicles» Number of natural gas vehicles increasing exponentially:» In 2013, was about 1.6% of all vehicles» 93% of these are light-duty most of which are bi-fuel (i.e. can still run on gasoline) Global total natural gas vehicles 18,000,000 16,000,000 14,000,000 12,000,000 10,000,000 8,000,000 6,000,000 4,000,000 2,000, Source: Create-AAP Symposium 3

4 Significant Growth in Natural Gas Heavy Duty Vehicles in North America 45,000 40,000 35,000 30,000 Cumulative Natural Gas Engine Sales Truck Refuse Specialty Bus 25,000 20,000 15,000 10,000 5, % of all refuse trucks 18% of all transit buses Create-AAP Symposium 4

5 Outline» NG vehicles context» Why NG?» NG engine technologies» Particulate matter emissions HPDI focus» Summary / Conclusions Create-AAP Symposium 5

6 Source: BGR, 2009 Global gas reserves (TCM) Conv. Unconv. Unconv, recoverable Conventional Unconventional» Unconventional include: shale, dense rocks; methane hydrates; coalbed methane.» Recoverable unconventional reserves (economically viable with current technology) - ~250 years supply at current consumption rates Create-AAP Symposium 6

7 Retail fuel prices - USA GGE gasoline gallon equivalent Source: DOE, AFDC Source: Create-AAP Symposium 7

8 Natural gas climate benefits» Of the abundant fossil fuels, NG has the lowest carbon energy ratio Results in ~25% less CO 2 per unit energy compared to diesel H:C ratio CO 2 per unit energy (kg / kj) Diesel 2.1: Gasoline 2.3: Natural Gas 3.8: CH 4, however, is a potent GHG: 30x the GWP of CO 2 for 100-year time horizon (latest IPCC synthesis report) - increases to 34 when climate carbon feedback is included Create-AAP Symposium 8

9 Renewable (landfill) natural gas» Landfill CNG offers the lowest life-cycle carbon intensity of any of the mainstream alternative fuels (10% of gasoline)» Production: 1. anaerobic digestion of organic 2. Contaminant (H 2 S, H 2 O) removal 3. Purification/upgrading End product up to 99% CH 4» Potential sources: Landfill Farms (organic waste, manure) Wastewater treatment» In US, estimate that about 7.8 MT/yr of CH 4 could be produced ~14x current CH 4 use in transportation Manure-fed biogas system in Abbotsford, BC Source: Create-AAP Symposium 9

10 Ambient air pollution ( criteria pollutants)» Parts of the US still fail to meet air quality standards» Species include O 3, PM (10 and 2.5), NO 2, CO, lead, SO 2» Vehicle emissions remain a contributor (especially for O 3 / NO 2 )» NG vehicles have, historically, had lower emissions than diesel Source: Create-AAP Symposium 10

11 Natural gas combustion - pollutants» As with any fuel, burning natural gas generates pollutants. Of particular importance for on-road applications are: Oxides of Nitrogen (NOx) Partial combustion by-products: CO, HCs Particulate matter (PM), predominantly: Black carbon (soot) Volatile species (adsorbed onto soot or independently nucleated) Greenhouse gases (CO 2, CH 4 )» As this symposium is focused on aerosols, will focus on PM emissions today Create-AAP Symposium 11

12 Outline» NG vehicles context» Why NG?» NG engine technologies» Particulate matter emissions HPDI focus» Summary / Conclusions Create-AAP Symposium 12

13 NG combustion technologies» Generally, natural gas engines can be divided into three classes: SI - Premixed charge / spark ignited Can be dedicated NG or bi-fuel Dual-fuel - Premixed charge / pilot ignition Can run on full diesel or primarily premixed NG HPDI - Direct injection / pilot ignition Typically limited to part load on diesel only operation» Historically, SI has been the dominant combustion strategy Easier, cheaper to implement Load limitations Create-AAP Symposium 13

14 Spark ignition» Natural gas is added to charge before intake port» Can run either stoich or lean Pre 2007, lean preferred Stoich + EGR needed to meet NOx standards in US / Europe» For MD/HD, take a base diesel engine and modify for NG For light duty, typically use base gasoline engine Benefits Three-way catalyst (simple) Premixed charge, so only PM is from lubricating oil Challenges Efficiency is 8-15% lower than diesel, reducing GHG benefit of NG Create-AAP Symposium 14

15 Premixed charge, pilot ignition Dual Fuel» Natural gas added to charge at intake port / manifold Ignition from diesel injection» Runs lean typically no throttle Often too lean to sustain propagating flame, leading to high CH 4» For on-road transient operation, often runs at ~50% diesel substitution» At low, high load, reverts to 100% diesel High load is where diesel generates most PM so PM benefits are limited Benefits Retain diesel system Simple/low cost Challenges High CH4, still uses lots of diesel Requires full diesel aftertreatment system Create-AAP Symposium 15

16 Direct injection, pilot ignition HPDI» Diesel pilot injection followed by direct injection of natural gas» Retains non-premixed combustion Diesel-like efficiency, performance (can match power, torque)» Lean-burn: needs standard diesel aftertreatment system to meet emissions standards» Low CH 4 due to non-premixed combustion compared to other NG technologies Benefits Retains diesel performance & efficiency, low CH4 Challenges System cost / complexity Requires full diesel aftertreatment system Create-AAP Symposium 16

17 Example of premixed SI: Medium duty CWI ISL-G» Cummins-Westport joint venture, production started in 2008» Dedicated NG engine» 8.9 litre stoichiometric, spark ignition with cooled EGR Up to 320 hp / 1000 ft-lb torque» Low Emissions <0.20 g/bhp-hr NOx (now also provides 0.02 g option) <0.01 g/bhp-hr PM» Three Way Catalyst aftertreatment» Target market is local distribution, transit, and refuse trucks Create-AAP Symposium 17

18 Case study refuse trucks Natural Gas Refuse Vehicle Market Penetration 100% % of new refuse trucks 0% » More than 50% of refuse trucks sold in 2014 had natural gas SI engines» Natural gas fuelling offers significant benefits: Lower operating cost (save $27,000/yr), 1.2 yr payback with tax incentives (~2 yrs without) [at 2014 fuel prices] Quieter, low emissions» Duty cycle is particularly conducive to natural gas: Return to base / central fuelling, overnight fuelling Access to free fuel methane from decomposition 80% 60% 40% 20% Create-AAP Symposium 18

19 Outline» NG vehicles context» Why NG?» NG engine technologies» Particulate matter emissions HPDI focus» Summary / Conclusions Create-AAP Symposium 19

20 Particulate matter (PM)» Requirements for PM precursor formation: High equivalence ratio (> ~2) to avoid oxidation Temperature high enough to dissociate CH 4» Natural gas engines generate PM through various routes: Directly from CH 4 (more difficult) C 2 H 6, C 3 H 8 easier, as they don t require recombination to form C 2 + species Diesel pilot (if present), lubricating oil contribute to PM Only half as much air as needed to fully oxidize CH4 to CO2 and H2O rich Soot precursor lean Create-AAP Symposium 20

21 PM formation local conditions» To form PM precursors, need rich, high temperature conditions (local)» Equivalence ratio (EQR):» If the charge is fully premixed, then too lean to form soot precursors» For direct injection (non-premixed): Local charge conditions will vary from very rich (jet core) to lean Local temperature will generally be close to adiabatic flame temperature So will pass through high-soot forming zone LOCAL EQR High soot precursor area Premixed combustion LOCAL Temperature Adiabatic flame temperature for non-premixed combustion Create-AAP Symposium 21

22 High pressure direction injection of NG» Diesel pilot with late-cycle DI of natural gas Pilot diesel injection before natural gas Diesel auto-ignites, then ignites gas jets» Diesel pilot contributes 5-10% of total energy» From combustion viewpoint, very similar to full diesel operation Except gas jet doesn t atomize, evaporate (harder to entrain air) So jet core can be rich during combustion Create-AAP Symposium 22

23 HPDI PM isn t it just all from diesel?» Black Carbon content in PM measured using filters, lightscattering instruments» BC concentrations increase with load, addition of EGR Source: Jones et al. SAE » Contribution of NG to BC PM evaluated using C14/C12 isotope ratio» Majority of the BC is from NG» With higher PM (higher load, EGR) more PM is from NG Create-AAP Symposium 23

24 What is it then? Source: Proc. IMechE-Part D JAuto 705, vol 222 (2008)» Just like diesel PM, HPDI PM is a mixture of volatile species and black carbon» Particles are qualitatively equivalent to diesel PM (agglomerates of spherical primary particles) dn/dlndp (#/cm 3 rawexhaust) ' 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 Engine-out PM size distribution from HPDI single-cylinder engine 10 o ATDC Mobility Diameter (nm) NG 20% N 2 40% N 2 75% reduction in total PM; BC virtually eliminated» Size distributions similar to modern diesel engines Techniques that reduce PM dramatically reduce size and number count Create-AAP Symposium 24

25 Source: SAE How to control PM?» HPDI base uses same particulate filter (DPF) as diesel base Achieves 90% reduction in total PM DPF particularly effective at reducing BC (oxidation catalyst can effectively reduce volatile species)» Partial-flow filters offer smaller size/cost, but reduced effectiveness Create-AAP Symposium 25

26 Effects of increased pressure» Better to reduce PM at source one option is to increase injection pressure» Faster combustion, better mixing lead to lower PM emissions (measured using FSN) Gravimetric filters showed ~75% reduction, effectively no BC Base pressure Source: SAE Create-AAP Symposium 26

27 Source: SAE CFD results at base vs. high pressure Simulation of 1500 RPM, 2400 N.m Mixture Fraction: Acetylene concentration Create-AAP Symposium 27

28 » PM predicted using Hiroyasu 2-step mechanism» Mixture fraction in rich mixture Zrich represents fraction of total fuel that is richer than stoichiometric» Model suggests significant increase in oxidation at higher pressures Maximum mixture richness not significantly reduced but time spent in rich conditions dramatically reduced Higher temperatures also tend to enhance oxidation (but lead to higher NOx emissions) Source: SAE CFD interpretation of experiments Create-AAP Symposium 28

29 Summary / Conclusion» Natural gas engines are seeing increased market uptake Recent changes in energy prices have impacted economics Environmental proposition is now focused on GHGs, with 20-25% reduction offered relative to diesel» Combustion technologies are based on two strategies: Premixed no significant formation of PM from fuel, as local EQR too lean to form precursors Non-premixed PM can be significant as gas jet will pass through soot-forming regions» With direct injection of natural gas with diesel pilot (HPDI) PM is volatile at low load, but BC at high load, mainly from NG Current technology uses DPF for PM control Techniques can substantially reduce or eliminate BC volatiles can be addressed using an oxidation catalyst Create-AAP Symposium 29