grammar in Kaldi

Hi, dear author, setting grammar in Vosk is very useful, so I copy UpdateGrammarFst() to Kaldi and test it with open source chain model(http://kaldi-asr.org/models/m13), but the result is bad( I set faster_decode_opts_.beam = 1000, I change other decoders and models, also bad), the background noise was be recognized as words, and some extra words. For example:

The speaker said "magnets can be found on a can opener".
start end
WORDS: 0.02 0.03
WORDS: 0.12 0.24 can
WORDS: 0.24 0.52
WORDS: 1.4 2.06 magnets
WORDS: 2.06 2.33 can
WORDS: 2.33 2.45 be
WORDS: 2.45 2.5 a
WORDS: 2.5 2.93 found
WORDS: 3 3.19 on
WORDS: 3.19 3.26 a
WORDS: 3.26 3.64 can
WORDS: 3.64 4.2 opener

But I can get correct result in Vosk. Any solution to make model no such sensitive?

Or maybe setting grammar has existed in Kaldi, can you please give me some tips, Thanks!