Internal combustion (IC) engine exhaust contains nitric oxide (NO) and nitrogen dioxide (NO2) as well as volatile organic compounds (VOCs) and carbon monoxide (CO). These air pollutants have to be removed from exhaust to comply with local air quality control regulations.
Post-Combustion Emission Control for Biogas Powered Engines
Two approaches that may be capable of complying with future emission limits are Selective Catalytic Reduction (SCR) and NOXTECH®. SCR catalysts are susceptible to sulfur poisoning and require reliable and efficient sulfur removal from the biogas. The NOXTECH® system relies on the injection of urea or ammonia into the engine exhaust to react with NOx. Additional fuel and chemicals are needed and the requirement to reheat the exhaust to 1400-1500 Fo complicates installation of heat recovery equipment used in most combined heat and power applications. A small amount of ammonia slip is also reported. The NOx removal costs are very high, $16,000-$27,000/ton and $11,000-$25,000/ton for SCR and NOXTECH®, respectively.
In 1996, CHA Corporation teamed with Sacramento Municipal Utility District (SMUD) through the California Air Resources Board (CARB) ICAT program to successfully demonstrate the use of carbon adsorption with an onsite microwave regeneration and treatment to remove NOx, VOC, CO and particulate emissions from diesel engine generators at McClellan AFB. At the time, SMUD was investigating the use of emergency backup diesel generators for electric load reduction during peak periods. The 1998 demonstration reduced the NOx concentration from over 1,000 ppm to a concenration of about 1 ppm. Based on these results, and further advancements by CHA, a post combustion treatment system for use on biogas engines has been developed that has the potential to meet or exceed performance of existing technology and meet CARB standards for NOx, SOx, and VOC.
In 2009 CARB awarded SMUD and CHA Corporation another ICAT grant to remove H2S from biogas and NOx from engine exhaust from a dairy digester installation at Tolenaar Holsteins in Elk Grove, CAs. We used magnesium oxide impregnated activated carbon to remove NOx and a 6kW moving microwave applicator to regenerate the media in place.
The NOx removal system consisted of an exhaust cooling heat exchanger, condensed water knockout pots, two carbon adsorbers with microwave regenerators and an NOx destruction reactor that uses microwaves to react NOx with carbon to form CO2 and N2. When the first carbon adsorber was saturated with NOx, engine exhaust was directed to the second adsorber. The saturated adsorber was then regenerated using microwaves. The two adsorbers were alternated in an adsorption mode and regeneration and cooling mode throughout the demonstration.
Picture of NOx Removal Adsorber, NOx Destruction Reactor and Engine
The commercial NOx removal system will consist of two independent adsorbers. These two NOx adsorbers will operate in series. The saturated adsorber or adsorbent is transported to a central microwave regeneration facility for regeneration. A microwave regenerator and microwave NOx destruction reactor at the facility regenerate the spent media and then return it engine site. A 300-lb/hr microwave regeneration facility can support over 20 biogas engines within a region. The advantage of this model is that the engine operators are not burdened with equipment and operating costs of a regeneration facility.
Pre-Combustion NOx Control
Because hydrogen has wide limits of flammability, it is used to ignite ultra lean air -fuel mixtures and accelerate the burn rate, thereby extending the lean engine-operating limit. Consequently, many groups have studied Hydrogen Assisted Lean Operation (HALO) for natural gas fired engines over the past 25 years. TIAX demonstrated that HALO allows stable engine operations to be achieved at ultra-lean (A/F = 2) conditions. NOx emission of 10-ppm (0.07g/bhp-hr) was obtained using 8% H2 supplementation at an exhaust oxygen level of 10%. They assessed the feasibility of using HALO to control NOx in the exhaust of engines running on landfill biogas and of a 90% NOx reduction solution by landfill gas fed to an on-site reformer to produce H2. It was concluded that the on-site steam-methane reforming for H2 production is more economical than trucking H2.
Under California Energy Commission EISG Grant #56938A/10-05 CHA Corporation developed microwave steam reforming technology to produce H2 from biogas to implement HALO for pre-combustion NOx control for biogas engines. The microwave process has potential to overcome the principal disadvantages of conventional steam-methane reforming process, and facilitates H2 production on smaller scales. The H2 production cost for the pre-combustion NOx control was estimated at $6.06 per 1,000 standard cubic feet (scf). Trucking H2 to the dairy for the pre-combustion NOx control has been considered by Energix Research with an estimated cost of $28.83/1,000 ft3 scf.
Combined Pre-and Post-Combustion NOx Control
Pre-combustion NOx control using HALO followed by post-combustion NOx removal with carbon adsorption is the best approach to ensure that the CARB 2007 NOx emission standards will be consistently met. HALO, may not meet the CARB 2007 NOx emission standards alone, but will reduce the NOx concentration in the engine exhaust significantly to minimize the amount of NOx that has to be removed by adsorption.
The pre-combustion NOx control system uses our Microwave Steam Reforming technology. It has a microwave H2S removal unit followed by a microwave steam reformer. Approximately 10% of the biogas used to fuel the engine is processed to generate hydrogen. The hydrogen is injected back into the fuel stream prior to combustion to create a hydrogen enriched fuel gas.
The post-combustion NOx removal system consists of an exhaust cooler and two carbon adsorbers in series. These adsorbers are filled with activated carbon to remove NOx from the exhaust. The beds are operated in a standard Lead-Lag cycle. When the average NOx concentration in the exhaust leaving the second-stage adsorber reaches 5 ppm, the saturated adsorbent is replaced with regenerated adsorbent and it is switched to the Lead bed. The saturated carbon is transported to the microwave regeneration facility for processing. We expect the adsorbent will be replaced and regenerated once every 3 months. An additional benefit is that the adsorbent also removes SO2 and VOC from exhaust.
The following presents a list of potential advantages for microwave-based pre-and post-combustion NOx control:
- The pre-and post-combustion NOx control with microwave energy will meet CARB 2007 NOx emission standards for biogas-powered engines.
- Catalyst poisoning by H2S and siloxanes is not a major issue for the microwave-based pre-and post-combustion NOx control system.
- Extension of the technology facilitates converted all of the biogas into H2 to allow use of fuel cells for power generation from biomass/biomethane with no air pollutants.
- No water treatment is required for steam generation if biogas water is used.
- Onsite H2 production by microwave steam reforming is much cheaper than trucking H2 for pre-combustion NOx control.
- The post-combustion NOx adsorber also removes SO2 and VOC from the engine exhaust.
The following figure presents a process flow diagram (PFD) for pre-and post-combustion NOx control system for the 212kW engine at the Tollenaar Holsteins Dairy digester as an example. Such an engine uses a biogas flow of about 60 standard cubic feet per minute (scfm) with a typical composition being 60% CH4 and 40% CO2. Under the grant awarded by California Energy Commission CHA Corporation will build and field-test this pre-and post-combustion NOx control system at Tollenaar Holsteins Dairy Farm in Elk Grove, California in 2015.