Lower CH4 Yield Research Articles

Effect of CeO2 Addition to Al2O3 Supports for Pt Catalysts on the Aqueous‐Phase Reforming of Glycerol

A series of Pt catalysts supported on Al₂O₃ that was doped with different amounts of CeO₂ was developed, characterized, and tested in the aqueous-phase reforming (APR) of glycerol to H₂. Catalyst 3Pt/3CeAl, which bore 3 wt% Pt on a support that contained 3 wt % CeO₂, showed the highest carbon conversion to gas (85%) and the highest H₂ yield (80%) for a feedstock of 1 wt% glycerol in water at 240 °C and 40 bar. A CeO₂/Al₂O₃ support with only 1 wt% Pt also showed high H₂ selectivity and carbon conversion to gas, as well as a much lower CH₄ yield than the benchmark 3Pt/Al catalyst, clearly demonstrating that doping the support with 3 wt% CeO2 improved the APR of glycerol. H₂ chemisorption results showed that the highest metal dispersion (58%) and active surface area (4.3 m(2)g(-1)) were achieved for the support that contained 3 wt% CeO₂, and this effect appeared to be primarily responsible for the high H₂ yield and carbon conversion to gas. No CO was observed in the product gas; therefore, this gas could potentially be used directly in proton exchange membrane fuel cells. Thus, including CeO₂ in the Al₂O₃ catalyst support enhanced both the activity and selectivity towards H₂ of a Pt catalyst for the APR of glycerol.

Effects of feed intake on enteric methane emissions from sheep fed fresh white clover (Trifolium repens) and perennial ryegrass (Lolium perenne) forages

Published analyses of enteric methane (CH4) emissions from sheep and cattle show an inverse relationship between feed intake and CH4 yield (gCH4/kg dry matter (DM) intake), which suggests opportunities for reducing CH4 emissions from feed eaten and per unit of animal production. Most relationships between feed intake and CH4 yield have been based on animals fed conserved feeds, especially silages and grains. Our research is a series of experiments with fresh white clover (Trifolium repens) and perennial ryegrass (Lolium perenne; ryegrass) forages fed to sheep at a range of feed intake levels. This study was comprised of four experiments where good quality freshly harvested white clover or ryegrass were fed to sheep over a three-fold range in DM intake, and CH4 emissions were measured in respiration chambers for two consecutive days in each experiment. Measurements were made from 16 sheep in Experiment 1 (fed at 1.6×metabolizable energy requirements for maintenance; MEm), 28 sheep in Experiment 2 (at 0.8 and 2.0×MEm), eight sheep and two measurement periods in Experiment 3 (at 1.6×MEm), and 30 sheep in Experiment 4 (fed at 0.8, 1.2, 1.6, 2.0 and 2.5×MEm). Prior to each experiment, sheep had a 10d acclimatization period to diets. Apparent digestibility was measured over 7d from sheep in Experiments 1, 3 and 4, along with collection of rumen digesta for volatile fatty acid (VFA) determination. Although CH4 yields differed when sheep were fed white clover or ryegrass at similar intakes, the differences were inconsistent and mean values similar across all experiments. This, and a similar structure of all experiments, enabled combined analysis of data from all four experiments using the restricted maximum likelihood (REML) procedure to estimate effects of feed intake level on digestibility, digestible nutrient intake, gas emissions, and VFA concentrations in the rumen. The REML analysis showed that when DM intake increased from 0.40 to 1.60kg/d, the predicted responses were an increase in CH4 production (g/d) of 187% (12.4–35.6g/d; P<0.001), and a decline in CH4 yield of 21% (25.6–20.2g/kg DM intake; P<0.001). High feed intake levels were associated with increased molar proportions (mM of total VFA) of propionate from 0.17 to 0.21 (P=0.038). Single and multiple regressions were completed on the data from all experiments, with organic matter (OM) intake predicting 0.87 of the variation in CH4 production, and molar proportion of propionate predicting 0.60 of the variation in CH4 yield. Increasing feed intakes by 1kg/d of DM reduced CH4 yield by 4.5g/kg DM intake. Plant chemical composition was weakly related to CH4 yield. High intakes of fresh forages will lower CH4 yield from fermentation, but effects of feed composition on CH4 emissions were minor. The interaction between effects of feed intake and rumen function requires further investigation to understand relationships with CH4 emissions.

A thermodynamic analysis of methanation reactions of carbon oxides for the production of synthetic natural gas

Synthetic natural gas (SNG) can be obtained via methanation of synthesis gas (syngas). Many thermodynamic reaction details involved in this process are not yet fully understood. In this paper, a comprehensive thermodynamic analysis of reactions occurring in the methanation of carbon oxides (CO and CO2) is conducted using the Gibbs free energy minimization method. The equilibrium constants of eight reactions involved in the methanation reactions were calculated at different temperatures. The effects of temperature, pressure, ratio of H-2/CO (and H-2/CO2), and the addition of other compounds (H2O, O-2, CH4, and C2H4) in the feed gas (syngas) on the conversion of CO and CO2, CH4 selectivity and yield, as well as carbon deposition, were carefully investigated. In addition, experimental data obtained on commercial Ni-based catalysts for CO methanation and three cases adopted from the literature were compared with the thermodynamic calculations. It is found that low temperature, high pressure, and a large H-2/CO (and H-2/CO2) ratio are favourable for the methanation reactions. Adding steam into the feed gas could alleviate the carbon deposition to a large extent. Trace amounts of O-2 in syngas is unfavourable for SNG generation although it can lower carbon deposition. Additional CH4 in the feed gas almost has no influence on the CO conversion and CH4 yield, but it leads to the increase of carbon formed. Introduction of a small amount of C2H4, a representative of hydrocarbons in syngas, results in low CH4 yield and serious carbon deposition although it does not affect CO conversion. CO is relatively easy to hydrogenated compared to CO2 at the same reaction conditions. The comparison of thermodynamic calculations with experimental results demonstrated that the Gibbs free energy minimization method is significantly effective for understanding the reactions occurring in methanation and helpful for the development of catalysts and processes for the production of SNG.

Genetic parameters for predicted methane production and potential for reducing enteric emissions through genomic selection

Mitigation of enteric methane (CH4) emission in ruminants has become an important area of research because accumulation of CH4 is linked to global warming. Nutritional and microbial opportunities to reduce CH4 emissions have been extensively researched, but little is known about using natural variation to breed animals with lower CH4 yield. Measuring CH4 emission rates directly from animals is difficult and hinders direct selection on reduced CH4 emission. However, improvements can be made through selection on associated traits (e.g., residual feed intake, RFI) or through selection on CH4 predicted from feed intake and diet composition. The objective was to establish phenotypic and genetic variation in predicted CH4 output, and to determine the potential of genetics to reduce methane emissions in dairy cattle. Experimental data were used and records on daily feed intake, weekly body weights, and weekly milk production were available from 548 heifers. Residual feed intake (MJ/d) is the difference between net energy intake and calculated net energy requirements for maintenance as a function of body weight and for fat- and protein-corrected milk production. Predicted methane emission (PME; g/d) is 6% of gross energy intake (Intergovernmental Panel on Climate Change methodology) corrected for energy content of methane (55.65kJ/g). The estimated heritabilities for PME and RFI were 0.35 and 0.40, respectively. The positive genetic correlation between RFI and PME indicated that cows with lower RFI have lower PME (estimates ranging from 0.18 to 0.84). Hence, it is possible to decrease the methane production of a cow by selecting more-efficient cows, and the genetic variation suggests that reductions in the order of 11 to 26% in 10 yr are theoretically possible, and could be even higher in a genomic selection program. However, several uncertainties are discussed; for example, the lack of true methane measurements (and the key assumption that methane produced per unit feed is not affected by RFI level), as well as the limitations of predicting the biological consequences of selection. To overcome these limitations, an international effort is required to bring together data on feed intake and methane emissions of dairy cows.

Effects of feeding fresh white clover (Trifolium repens) or perennial ryegrass (Lolium perenne) on enteric methane emissions from sheep

Enteric CH4 contributes about one third of New Zealand's greenhouse gas emissions. Measurements from our laboratory using the sulfur hexafluoride (SF6) technique suggested much lower CH4 yields (as gCH4/kg dry matter (DM) intake) from sheep fed fresh white clover (Trifolium repens) compared with fresh perennial ryegrass (Lolium perenne). Thus, white clover offers an opportunity to reduce CH4 emissions from pastoral based ruminant livestock systems if its lower CH4 yield can be confirmed using respiration chambers. This study was comprised of 2 experiments where good nutritional quality freshly harvested white clover and perennial ryegrass were fed to 16 sheep in Experiment 1 (8/diet) and 32 sheep in Experiment 2 (16/diet). Intakes were about 1.6 times metabolizable energy requirements for maintenance (×MEm) in Experiment 1 and 0.8×MEm and 2.0×MEm in Experiment 2 (8/diet/feeding level). In both experiments sheep had a 10 d acclimatization to their diets and CH4 emissions were measured in respiration chambers for 2 consecutive days. Methane yield (gCH4/kg DM intake) was 12% lower (P=0.04) for white clover (19.8) compared with ryegrass fed sheep (22.5) in Experiment 1. In Experiment 2, there were no effects of forage on CH4 yield (as gCH4/kg DM intake) at 0.8×MEm (27.1 and 25.5, respectively) but, at 2.0×MEm, CH4 yield (as gCH4/kg DM intake) was 7% higher (P=0.05) for white clover (23.4) compared with ryegrass (21.7). Analyses of combined data from both experiments show that there were no overall dietary effects on CH4 yield, but increased intakes reduced CH4 yield (P<0.01). Use of white clover as an alternative fresh forage to ryegrass is not an opportunity to reduce enteric CH4 emissions from sheep.This paper is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance Between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors: K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.

Within- and between-animal variance in methane emissions in non-lactating dairy cows

Several studies on methane (CH4) emissions have focussed on selecting high and low CH4-emitting animals. One challenge faced by this work is the lack of consistency, or repeatability, in animal rankings over time. Repeatability for individual animals over time needs to be high to reliably detect high and low CH4-emitting animals. A possible explanation for the lack of repeatability is a relatively high within-animal variation in daily CH4 emissions, meaning that animals could then change their ranking when compared at different points in time. An experiment was undertaken with four non-lactating dairy cattle to assess the within- and between-animal variation in CH4 emissions over time when measured using the sulfur hexafluoride (SF6) tracer technique. Two contrasting diets were fed to the cattle at maintenance energy levels: lucerne silage (diet 1) and a cereal + lucerne + straw mixed ration diet (diet 2). Daily CH4 measurements were undertaken for 23 days on diet 1 and 30 days on diet 2. There was a significant (P &lt; 0.001) difference between diet 1 and diet 2 in daily CH4 production, with mean (±s.e.) production of 124.3 (11.1) g CH4/day from diet 1 and 169.8 (±11.0) g CH4/day from diet 2. Lower CH4 yield (g CH4/kg dry matter intake) was recorded on diet 1 (22.8 ± 2.0) than diet 2 (32.0 ± 2.0). Cows differed significantly (P &lt; 0.05) from one another in daily CH4 yield (diet 1: cow 1 = 19.4 ± 0.6, cow 2 = 22.2 ± 0.8, cow 3 = 23.2 ± 0.7, cow 4 = 25.4 ± 0.6; diet 2: cow 1 = 26.0 ± 0.7, cow 2 = 36.4 ± 0.7, cow 3 = 29.3 ± 0.7, cow 4 = 36.6 ± 0.7). Variances for daily CH4 yield were smaller for diet 1 (within animal = 6.91, between animals = 6.23) than for diet 2 (within animal = 10.09, between animals = 27.79). Estimates of repeatability (variation between animals/total variation) for daily CH4 yield were 47 and 73% in diet 1 and 2, respectively. Coefficients of variation in average daily CH4 emissions in this experiment ranged from 8 to 18% despite the fact that each animal received the same quantity and quality of feed each day. While further research is required, the high within-animal variability in CH4 emissions measured using the SF6 tracer technique may explain why there has been difficulty in obtaining consistent rankings in CH4 yields when animals are measured on multiple occasions. The results also suggest that the SF6 tracer technique may exaggerate apparent between animal differences in CH4 emissions.

Effect of some physical and chemical parameters on the fermentation of cellulose to methane by a coculture system

In the fermentation of cellulose to methane by the triculture of Acetivibrio cellulolyticus – Desulfovibrio sp. – Methanosarcina barkeri, methanogenesis was the rate-limiting step. The optimal temperature was 35 °C. In the presence of initially added hydrogen, initiation of cellulose hydrolysis was delayed until most of the hydrogen was metabolized, and then the fermentation which followed was comparable with the N2: CO2 control. Increased CH4 yields and rates of formation were stoichiometrically related to the utilization of the hydrogen initially present. Added acetate had no effect on cellulolysis. The increased yields of CH4 observed could be accounted for by utilization of the added acetate. When Methanobrevibacter sp. was included in the coculture (without added H2 or acetate), no hydrogen accumulated and lower rates of acetate utilization and subsequent CH4 evolution were observed. These results suggest a requirement for hydrogen by M. barkeri for efficient acetate utilization. Controlling the pH at 6.8 increased the amount of cellulose degraded, but CH4 yields were lower and no acetate was used. Increasing sulfate levels altered the ratio of fermentation products but had no effect on cellulose degradation. Lower CH4 yields were obtained at elevated sulfate concentrations.